THE DIMENSIONAL BEHAVIOR OF INVAR
By
BERNARD SAMUEL LEMENT
B.Sc. (Met), M.I.T., Cambridge, Massachusetts 1938
Submitted in Partial Fulfillment of the
ReqOuirements for the Degree of
DOCTOR OF SCIENCE
from the
Massachusetts Institute of Technology
1949
Signature of Author
Department of Metallurgy
Signature of Professor
in Charge of Research
Signature of Chairman
Department Committee
on Graduate Research
Signature redacted
Signature redacted
Signature redacted
4,
1(
TABLE OF CONTENTS
Page
Number
Chapter
Number
List of Tables
................
List of Figures
. . . . . . . . .
II
III
..
viii
.....
1
...............
Introduction
Summary. . . . . . . . . . . . . . . . .
'Previous Work
x
..............
Acknowledgments.
I
vi
.
2
.
3
................
3
..
A.
Prior Investigation
B.
Possible Explanations of Invar . .
.........
. . . .
7
. . .
7
1.
Magnetic Inversion of Cementite
2.
Precipitation of Cementite, Graphite, or
Nickel Carbide.
8
.
. ....
3.
Transformation of Austenite to Ferrite
4.
Transformation of Austenite to Marten13
........
site. .o
5.
Residual Stress Formation and Stress
14
Relief ................
6.
Alteration of Magnetic State by Cold
16
Work .................
IV
V
.
17
. . .
20
.
20
Plan of Experimental Work and Materials Used
Experimental Equipment and Techniques
A.
12
. ... .
Heat Treatment
.
..
..
20
. . . . . . . . . * .
21
. . . . . . . . . . . . . . . .
21
.
1.
Quenchingo.
2.
Furnace Cooling
3.
Aging
.
.
..
. .
..
. .
Page
Number
Chapter
Number
V
VI
4.
Carburization and Homogenizatio4
5.
Decarburiz4tion
. .
.
.
.
.
.
22
. .
.
.
.
.
.
25
.
.
.
B.
Precision Length Measurements
.
.
.
25
C.
Specific Volume Measurements . . . . .
.
.
27
D.
Drop Tests . . . .
E.
Heyn Analysis for Residual Stress
F.
X-ray Diffraction Measurements
.
. .
.
.
.
.
.
.
.
.
.
.
.
27
.
.
.
.
27
.
.
.
.
29
1.
Precision Lattice Parameter Measurements
29
2.
Identification of Phases Present
.
31
G.
Electrolytic Extraction. . . . . . . . . .
31
H.
Hardness Tests
I.
Magnetic Measurements
J.
Measurement of Thermal Expansion Coefficient
.
.
.
.
.
.
.
.
. . . ...
32
.
.
32
32
34
.
34
Invar (0.07 C, 0.44 Mu, 36.8 Ni) . . .
34
Dimensional Charges on Aging
1.
.
.............
Experimental Results .
A.
.
a.
.
.
.
.
.
.
Aging Following Quenching from
8300 C . . . . . . . . . . . . . .
b.
Aging at 700 C Following Quenching
from Temperatures up to 540" C
c.
34
35
.
Re-Aging at 500 C Following Aging
.
35
e.
Hot Quenching from 205* C to 70* C
39
f.
Aging Following Cold Working
40
at 700 C . . . . . . . .
d.
.
. .
.
Aging Following Quenching from
2050 C . ...
. . .
...
36
.
.
.
-iii-
Page
Chapter
Number
Number
g.
Aging Following Furnace Cooling
h.
2.
. .- - -
. . . . .
. .
41
42
Quenching in Water from 5400 C
Decarburized Invar (0.01 C, 0.44 Mn,
43
. . . . . . . . . . . .
36.8 Ni). .
a.
Aging Following Quenching from 830 0 C
b.
Aging Following Furnace Cooling from
C
8300
c.
3.
40
Rate of Cooling from 8300 C and
3150 C
i.
. . . . . . .
.
from 8300 C . ..
43
. . . . . . . . . . . . .
44
.
45
Summary . . . . .
.
.
.
. .
.
.
Invars of Varying Carbon Content (0.02 to
0.58 C, 0.10 Mn, 36.0 Ni)
a.
45
45
Aging Following Quenching in Water
C...
48
........
Aging Following Quenching from 8300 C
andl2 0
C.
.
.
.
....
Summary
.
.
.
.
.
.
. .
.
.
.
.
.
.
. -
.
.
.
48
...
.
.
.
.
. .
50
51
Drop Tests
C.
Residual Stress . . . . . . . . . . .
52
. .
.
52
Surface Stress Values . . . . . .
X-ray Measurements. . . . . .
1.
. .
.
.
2.
.
Complete Stress Distribution
53
55
Determination of Solid Solubility of
Carbon
. . . . . . . .
. . . .
.
D.
1.
.
B.
.
d.
.
.
.
.
. . . . . . . . . . . . .
from 83 0
c.
.
Aging Following Furnace Cooling from
$300 C
b.
.
.
vi
55
-
- iv
Chapter
Number
Page
Number
2.
VI
Changes in Lattice Parameter due to
Aging at 20 0 C to 2050 C.
3.
Debye Patterns of Electrolytic Ex-
..............
tractions
4.
Existence of Martensite or Ferrite
E.
Metallographic Examination
F.
Hardness Tests
1.
-2.
3.
.
.
.
.
.
60
.
63
65
.
. ..
.
.
.
.
65
Temperature and Time of Aging
.
.
.
.
65
Aging Following Quenching from 8300 C
C.
Magnetic Tests
.
.
..
.
.
. ..
....
.
. .
. ..
...
.
. .
69
70
1.
Determination of Saturation Field .
2.
Variation of Magnetization with
.
70
. . .. . .. . . . .. .
70
3.
Rate of Cooling from 83 0 0 C .. .. .
72
4.
Aging Following Furnace Cooling from
830*0
. . . . . . . .
. . . . .
5.
Aging Following Quenching from 8300 C
6.
Aging Following Quenching from 8300 C
andl2
0
C.
. .
.
. .
.
.
.
. . . .
72
75
75
Coefficient of Thermal Expansion Measure-
ments . . . . . . . . . . . . . . . . .*
I.
60
.
Temperature
H.
.
. . ..
. ..
..
.
Rate of Cooling from 8300 C
andl2 0
G.
57
.....
77
1.
Rate of Cooling from 8300 C . . . . .
77
2.
Aging Following Furnace Cooling . . .
80
Summary of Experimental Results . . . . .
80
1.
Aging of Quenched Invar . . . . . . .
80
2.
Furnace Cooled Invar
. .
..
..
..
82
Page
Number
Chapter
Number
83
Formation of Residual Stress
83
B.
Relief of Residual Stress
84
C.
Formation of and Disappearance of Guinier.
86
Precipitation and Re-solution of Carbide and
Graphite
.
.
.
.
. .
Summary
*
. .
*
.
.
0
.
.
D.
0
.P
88
.
.
.
.
0
.
.*
89
Conclusions
.
.
.
0
.
.
.0
90
Suggestions for Further Work
.
.
.
.
.
.
.0
91
.
0
.
.
.
.
9
.
0
.
.
0
.
0
0
92
.
Bibliography
.*
*
.
E.
Appendix A - Formulae and Data Used in
Calculating Changes in Length Due to Phase
.
. . . . . .
. .
.
.
.
1
.
.
Transformations .
Appendix B - Calculation of Length Change due
.
.
3
.
.
to Formation of Cementite in Invar
Formation of Graphite in Invar.
.
.
.
.
.
.
Appendix C - Calculation of Length Change due to
.
5
.
6
.
8
to Formation of Nickel Carbide in Invar . .
.
Appendix D - Calculation of Length Change due
Formation of Iron-Nickel Carbide in Invar
.
Appendix E - Calculation of Length Change for
Appendix F - Calculation of Length Change due
to Formation of Ferrite in Invar.
.
.
.
.
Ix
.
.
A.
Preston Zones
VIII
.
.
Discussion of Results .
.
VII
.9
-viLIST OF TABLES
Table
Number
I
II
Page
Number
Chemical Composition.... ........
. .
.........
Dimensional Changes on Aging at 700 C Following
.
..
.
.
.
.
..
.
.
.
.
.
.
.. .
.
.
.
49
...
. .
.
.
.
50
.
.
.
52
Dimensional Changes on Aging Quenched Invar
.
.
.
.
. .
.
.
.
Residual Stress at Surface of 0.375 Inch Diameter
Bars of Invar Subjected to Various Treatments
X
54
Change in Lattice Parameter of Austenite Phase
on Aging Invars of Varying Carbon Content
XI
.
.
.
66
Changes in Hardness on Aging Quenched Invars of
Varying Carbon Content...
XIII
59
Variation of Hardness of Invars of Varying Carbon
Content with Rate of Cooling from 8300 C . . . .
XII
47
Dimensional Changes on Aging Quenched Invars of
Subjected to Drop Tests,
IX
.
Dimensional Changes on Aging Quenched Invars of
Medium Carbon Content at 700 C . . ...
VIII
39
..
Dimensional Changes on Aging Furnace Cooled Invar
Varying Carbon Content. . .
VII
24
38
of Varying Carbon Content.
VI
.
..................
Both Regular and Hot Quenching.
V
19
Dimensional Changes on Reheating Aged Specimens
at 205* C
IV
.
Carbon Contents after Carubrization and
Homogenization....
III
.
.......
.
67
. .
Changes in Hardness on Aging Quenched Invars of
Medium Carbon Content at 700 C . . . . ..
. .
.
70
Page
Number
Table
Number
XIV
Variation of Both Magnetization and Curie Point
. .
of Low Carbon Invar with Treatment . . . . .
XV
73
Changes in Magnetization on Aging Furnace Cooled
Invars of Varying Carbon Content
YVII
.
.
.
..
.
.
.
.
.
.
74
.
.
76
.
.
.
.
Changes in Magnetization on Aging Quenched Invars
of Medium Carbon Content at 700 C
XIX
.
Changes in Magnetization on Aging Quenched Invars
of Varying Carbon Content
XVIII
71
Variation of Magnetization of Invars of Varying
Carbon Content with Rate of Cooling from 8300 C
XVI
-
vii
-
Variation of
. . . . . .
.
77
coefficient of Thermal Expansion of
Invars of Varying Carbon Content with Rate of
Cooling from 8309 C. . . .
XX
.
.
. .
. .
.
.
.
78
Changes in Coefficient of Thermal Expansion on
Aging Furnace Cooled Invars of Varying Carbon
Content
..............--...-
79
-
viii
-
LIST OF FIGURES
Figure
Page
Number
Number
1
Atmosphere Controlled Furnace
2
Apparatus for Measuring Coefficient of Thermal
Expansion.....
3
.
. .
...
..
.
.
.
.
.
95
.
96
97
.
.......
98
.
99
.
Dimensional Changes on Aging quenched Invar (0.07 C,
..
. .
. . . . .
.
. . .
100
Dimensional Changes on Aging Quenched Invar
C
.
101
...
Dimensional Changes oh Aging Cold Worked Invar
. . . . . . . . . .
102
Dimensional Changes on Aging Annealed'Invar
(0.07 C, 0.44 Mn, 36.8 Ni)
10
.
........
(0.07 C, 0.44 Mn, 36.8 Ni)
9
.
.
.......
(0.07 C, 0.44 Mn, 36.8 Ni) at 7 0
8
.
. .
.
Dimensional Changes on Re-Aging at 500 C
0.44 Mn, 36.8 Ni)
7
.
Dimensional Changes on Aging Invar (0.07 C,
Following Aging at 70 0 C
6
.
. .
0.44 t, 36.8 Ni) at 700
5
.
.
Dimensional Changes on Aging Quenched Invar
(0.07 C, 0.44 Mn, 36.8 Ni)
4
.
..
103
*...
Dimensional Changes on Aging Invar (0.07 C,
0.44 Mn, 36.8 Ni) Cooled at Different Rates from
Both 83 0
11
C and 315 0 C
...
..
. ..
..
.
104
Dimensional and Volume Changes on Aging Annealed
Invar (0.07 C, 0.44 Mn, 36.8 Ni) at 5400 C and
Quenching in Water to 20C. .........
105
Page
Figure
Number
12
Number
Dimensional Changes on Aging Decarburized and
Quenched Invar (0.01 C, 0.44 Mn, 36.8 Ni). . 106
13
Dimensional Changes on Aging Decarburized and
Annealed Invar (0.01 C, 0.44 Mn, 36.8 Ni). . 107
14
Dimensional Changes Remilting from Drop
Tests of Quenched and of Annealed Invar (0,7
C, 0.44
.
.
.
.
.
.108
.
Distribution of Residual Stress in Quenched
Invar (0.07 C, 0.44 Mn, 36.8 Ni)
16
.
. .
.
.
.
.
.
.
.
15
Mn, 36.8.Ni)
Plot of Lattice Parameter vs. Carbon Content
of Invar (0.44 Mn, 36.8 Ni) Heated 500 Hours
at Various Temperatures.
17
.
.
.
..
.
.
.
110
. .
.
.
111
Plot of J vs. H for Quenched Invar (0.07 C,
0.44
19
.
Solubility of Carbon in Austenitic Phase of
Invar (0.44 Mn, 36.8 Ni) . . . . . .
18
.
un,
36.8 Ni) at 130 C . .
.
.
.
.
.
.
. 112
Variation of Saturation J with Temperature
for Annealed Invar (0.07 C, 0.44 Mn, 36.8 Ni 113
20
Variation of Saturation J with Temperature
for Quenched Invars (0.10 Mn, 36.0 Ni) of
Varying Carbon Content . . . . . . . . . . . 114
21
Microstructures. . . .
.
. .
.
.
.
.
.
. .
. 115
-
ix
-
ACIaOWLEDGMENTS
The writer wishes to express his appreciation to:
Professors Morris Cohen and Benjamin L. Averbach under whose
joint direction this research was carried out, for their advice
and deep interest;
Professor John T. Norton, for use of x-ray equipment and advice
regarding x-ray technique;
John M. Fitzpatrick, Walter Fitzgerald, Leonard Sudenfield,
and Harold Ludwig for assistance in the various tests and
techniques used;
My wife, Annette Lement, for measurements of the x-ray diffraction films and for taking care of more than her share of our
domestic and social tasks.
I.
INTRODUCTION
The problem of making metal parts that will maintain constant
dimensions has become increasingly important to industrial development.
Metals changpdimensions when subjected to variations in
temperature or to aging at a constant temperature.
These dimensional
changes are due to normal thermal expansion or contraction, phase
transformations, and stress phenomena.
To minimize such dimensional
changes requires proper selection of alloy composition, method of
processing, and heat treatment.
Because of its very low coefficient of thermal expansion, the
36:64 nickel-iron alloy "Invar" has an inherent advantage over other
alloys for applications requiring constant dimensions.
However, even
with this alloy there is still the problem of achieving dimensional
stability with respect to aging in the ambient range.
Furthermore,
the coefficient of thermal expansion of Invar has been found to depend on prior thermal and mechanical treatment.
Although Invar has been used successfully in the past for such
applications as length standards, geodetic tape, clock pendulums and
wheels, and thermostatic strips, even more exacting dimensional
characteristics are being required by modern high precision equipment.
To meet this challenge, a more basi: understanding of the metallurgical phenomena responsible for the dimensional behavior of Invar
is necessary.
It is for this reason that the present thesis investi-
gation was undertaken.
-2-
II.
SUMMARY
The dimensional behavior of Invar was studied by means of pre-
cision length, specific volume, x-ray diffraction, hardness, magnetic,
and thermal expansion measurements.
SpWg;al techniques involving
decarburization, carburization, stress analysis, x-ray diffraction,
electrolytic extraction, and metallograplhic examination were
employed.
This investigation showed that there are several expansion
and contraction effects that play a significant role in the
dimensional behavior of Invar.
The metallurgical phenomena believed
responsible for these effects are as follows:
(1)
Formation of residual stress by rapid cooling, which
results in an expansion.
(2)
Relief of residual stress by aging or by mechanical "shock",
which results in a contraction.
(3)
Formation and disappearance of Guinier-Preston zones
within the austenitic solid solution, which result in an
expansion and contraction respectively at low aging
temperatures.
(/)
Precipitation and re-solution of both carbide and
graphite, which result in a contraction and expansion
respectively at moderately high aging temperatures.
-3-
III.
A.
PREVIOUS WORK
Prior Investigations
The discovery of the alloy vInvar" was made in the year 1896
by C. E. Guillaume of the International Bureau of Weights and
Measures in France.
Guillaume was looking for a cheaper alloy as a
substitute for the platinum-iridium alloy used in length standards.
He found that iron-nickel alloys in the range of about 30 to 60
percent nickel possess lower coefficients of thermal expansion at
room temperature than either pure iron or pure nickel and that the
alloy containing 36 percent nickel possesses the minimum coefficient.
This alloy was named "Invar" since it is practically invariable with
respect to temperature changes in the vicinity of room temperature.
Several theories(1) have been advanced to explain the low co-
efficient of Invar.
1.
These theories may be summarized as follows:
Existence of a compound or superlattice corresponding to
Fe2 Ni.
2.
Reversible transformation of austenite to ferrite in the
neighborhood of room temperature.
3.
Reversible transformation of a ferromagnetic austenite
phase to a paramagnetic phase coexisting below the Curie point.
4.
Effect of ferromagnetism on lattice parameter of austenite.
No experimental evidence has been presented to support the existence
of a compound or superlattice, reversible ferrite-austenite transformation, or coexistence of ferromagnetic and paramagnetic austenites as claimed in theories 1, 2 and 3.
On the contrary, evidence
(-4obtained by Owen, Yates, and Sully(2) indicates that the overall
change in length of a bar of Invar can be correlated with the change
in lattice parameter of the austenite phase.
SuLLy(
Although Owen and
have shown that below about 4700 C (8800 F) the equilibrium
structure of Invar consists of both austenite and ferrite, the
austenite to ferrite reaction does not occur at an appreciable rate
at room temperature and the amount of ferrite resulting from an
annealing treatment is probably small.
Therefore, the anomalous
expansion of Invar must be due to the effect of ferromagnetism as
claimed by theory 4.
According to the band theory (4)
, ferromagnetism may occur in
elements or alloys with unfilled d-bands provided that the magnetized state has a lower energy than the unmagnetized state.
The
relative magnitudes of the Fermi and exchange energy associated
with the d-band determines which state has the lower energy.
The
Fermi or kinetic energy depends on the height to which the band is
filled with electrons as well as on the shape of the band.
The
exchange energy which is the potential energy of electrostatic
interaction
between electrons is dependent on the relative numbers
of electrons of opposite spin.
If there are more electrons spinning
in one direction, than in the opposite direction, the exchange
energy is lower than for equal numbers of opposite spin.
In
order for ferromagnetism to occur, there must be more electrons in
the d-band spinning in one direction than in the opposite direction
so that a net magnetic moment results.
This condition can be stable
provided that the associated decrease in exchange energy is larger
than the increase in Fermi energy.
-5-
The difference between the Fermi and exchange energies is called
the energy of magnetization and is equal to the difference in
energy between the unmagnetized and magnetized states.
Knowing both
the energy of magnetization and energy corresponding to the
magnetized state as a function of atomic separation, it is possible
to determine the variation of the energy corresponding to the Unmagnetized state as a function of atomic separation.
In the case of Invar the minimum in the energy curve corresponding to the unmagnetized state is believed to have a higher
value and to occur at a smaller separation than the minimum in
energy curve corresponding to the magnetized state.
This means
that below its Curie point Invar in the ferromagnetic state is
more stable than in the paramagnetic state and has a higher lattice
parameter than would be the case if it were paramagnetic.
If the
ferromagnetism decreases, this is equivalent to a sliift of the
ferromagnetic energy curve toward that of the paramagnetic energy
curve.
Thus on heating up from absolute zero there is- found a de-
crease in lattice parameter of the austenite as loss of ferro-
magnetism occurs due to the effect of increased temperature which
causes the orientation of the magnetic domains to become more and
more random.
This contraction effect superimposed on the normal
expansion due to temperature increase results in a coefficient of
almost zero in the vicinity of room temperature.
Above room tempera-
ture, the coefficient increases with temperature until the Curie
point, about 2600 C (5000 F),is reached and the alloy is in the paramagnetic condition.
On heating above this temperature the coefficient
is fairly constant and has a value even higher than that of pure iron.
The coefficient of exoansion of Invar was found by Guillaume(5)P
Scott (6), Russell (7) and Hunter
mechanical treatments.
to be affected by thermal and
Quenching from about 8000 C (14700 F) results
in a lower coefficient than furnace cooling from this temperature.
By cold working it is possible to decrease the coefficient below the
as-quenched value and even obtain negative coefficients.
However,
low coefficients obtained by cold working are found to increase with
time.
Similarly, low temperature aging treatments increase the
value of the coefficient of either cold worked or as-quenched Invar.
On reheating cold worked Invar, Hood (9)
found that the coefficient
increases to a maximum value at about 4700 C (8750 F) and then
decreases at higher temperatures.
He suggested that the increase
in coefficient at 470 0 C (8750 F) might be due to transformation from
austenite to ferrite.
Sachs and Spretnak
(9)
disputed this theory on
the ground that the alloy is in the single phase austenite field of
the equilibrium Iron-Nickel diagram at the temperature considered.
In the same investigation Sachs and Spretnak found that cold
working is not effective in causing transformation of austenite
to ferrite in Invar.
Although Invar possesses a low coefficient of thermal expansion
at room temperature, Guillaume(10) found that this alloy is
subject
to dimensional changes which occur on aging at and above room
temperature.
Guillaume spent over thirty years of research in at-
tempting to determine the cause of this dimensional instability and
how to overcome it.
change occur:
(1)
In brief, he found that two types of dimensional
an expansion or progressive change, and (2) a
contraction or transitorr change.
Guillaume attributed these changes
to the presence of carbon in commercial heats of Invar.
He reasoned
that carbon was combined in the form of cementite (FeC) which undergoes a volume change in transforming either from the non-magnetic to
the magnetic state or vice versa.
In support of this theory, he
showed that dimensional instability increases with increasing carbon
content and decreases with increasing content of carbide forming
elements.which he assumed act to prevent the formation of cementite.
In order to stabilize Invar without use of carbide forming elements,
Guillaume recommended aging for several days at 1000 C (2120 F)
and then cooling very slowly to room temperature over a period of
several months.
B.
Possible Eplanations of Invar
On the basis of the previous work done on the dimensional be-
havior of Invar, it appears that a basic understanding of the
various phenomena observed is lacking.
In order to form a basis upon
which to plan the experimental program, possible explanations of
these ohenomena were given consideration.
The
explanations that
were considered involve several metallurgical phenomena known to
occur in steel.
A discussion of these phenomena and their possible
connection with the dimensional behavior of Invar follows.
1.
Magnetic Inversion of Cementite.
Guillaume's theory that
the low temperature expansion is associated with the magnetic inversion of cementite has been mentioned previously.
and Chevenard(
Le Chatlier
have shown that on cooling annealed steel an ex-
pansion superimposed on the normal contraction due to temoerature
0
change sets in at the cementite Curie point,210 C (4100 F).
This
effect increases with carbon content and must be associated with the
magnetic inversion of cementite since it always occurs at the Curie
point.
In order to relate this effect to Invar, it is necessary to
assume that cementite is present and that its Curie point is lowered
to at least 1000 C (2120 F) where the low temperature expansion has
been observed.
It is plausible that this might occur due to the
presence of some nickel in solid solution in the cementite.
However,
it is also necessary to assume that cementite can be retained in the
non-magnetic state below its Curie point and that a change to the
magnetic state occurs on aging.
In view of the fact that the Curie
point of other ferromagnetic materials has been found to be virtually
independent of heat treatment, it remains to be proven whether
cementite if it exists in Invar is an exception.
2.
Precipitation of Cementite, Graphite or Nickel Carbide.
cording to Hunter
Ac-
(8), the interpretation of Guillaumes' explanation
of the dimensional instability of Invar is changing solubility of
carbon in austenite during heat treatment.
In order to explain
either the expansion or contraction that occurs during the low
temperature aging of Invar it is necessary to show that the solid
solubility of carbon in 36:64 nickel-iron austenite decreases with
temperature.
The pioneering work done on the iron-nickel-carbon
system by T. Kase (12) indicates that the solid solubility of carbon
in 36:64 nickel-iron austenite may decrease with decreasing temperature.
Unfortunately, Kases' work was based on chemical analysis of
combined carbon content which could be the sum
of carbon in the
form of a carbide and carbon in solid solution.
Therefore, the
-
-9
location of the solid solubility line for carbon is still in doubt.
There' are three main possibilities as to the form of the
precipitated carbon:
cementite, graphite, and nickel carbide.
Cementite and graphite are definitely known to exist in steel
whereas the evidence for the existence of nickel carbide is sketchy.
According to Jacobson and Westgren
,
nickel carbide (Ni3 C) has
a hexagonal close-packed strouture and is stable at temperatures
below about 3000 C (5700 F).
Whether precipitation of carbon
either in the form of cementite, graphite, or nickel carbide results
in an overall expansion or contraction depends on the relative
specific volume and amount of the austenite phase and carbon containing phase present after precipitation.
All three carbon-
containing phases have a higher specific volume than the austenite
phase; but this tendency for expansion is opposed by the decrease
in specific volume of the matrix solution due to depletion of
carbon.
An additional factor is the effect of nickel content on
the specific volume of the austenite phase.
Starting with pure
iron the addition of nickel increases the specific volume of nickeliron austenite to a maximUm value at 40 percent nickel after which
the specific volume decreases to the value for pure nickel.
In the event carbon precipitates in the form of cementite (Fe3 C)
there should be a decrease in specific volume of the austenite phase due
to the loss of carbon and an increase in specific volume due to
lowered iron content.
If the resultant change is an increase in
specific volume, then precipitation of cementite, which is slightly
more voluminous than the austenite phase, should result in an overall
expansion.
-
10
-
If, however, the resultant change is a decrease in the
specific volume of the austenite phase, then either an overall expansion or contraction will occur depending on whether the total
volume of the precipitated cementite phase is larger or smaller than
the decrease in volume of the original austenite phase.
In the event carbon precipitates in the form of graphite, then the
only significant change in the specific volume of the austenite phase
would be a decrease due to loss of carbon.
This change in volume of the
austenite phase is opposed by the formation of the voluminous graphite
phase which is approximately 3 1/2 times greater in specific volume.
Thus an overall expansion or contraction will be observed depending on
whether the total volume of the precipitated graphite is larger or smaller
than the decrease in volume of the original austenite phase.
If carbon comes out of solid solution in the form of nickel carbide (Ni3 C), then there should be a decrease in the specific volume of
the austenite phase due to loss of both carbon and nickel.
Since the
specific volume of nickel carbide is larger than that of the austenite
phase, an overall expansion or contraction will be observed depending
on whether the total volume of the precipitated nickel carbide is
larger or smaller than the decrease in volume of the original
austenite phase.
In order to calculate the overall length change resulting from the
precipitation of graphite or carbide, it is necessary to know the
specific volumes of the precipitated phases as well as the specific
volume of austenite as a function of both nickel and carbon.
With
the exception of the effect of carbon, sufficient data were found in
the literature from which to make these calculations.
It was there-
fore decided to determine the effect of carbon on the lattice parameter
and consequently specific volume of 36:64 nickel-iron austenite.
As
11
-
-
will be discussed later, such information enabled calculations to be
made of the overall dimensional change resulting from the precipitatiin
This
of carbide or graphite from Invar containing 0.05 percent carbon.
carbon content was selected since it is the lowest for which theaxpansion effect at low aging temperatures was observed.
Guillaume's recommendation for the elimination of the expansior
effect by a very slow cool after prior aging at 1000 C (2120 F) couldbe understood on the basis of precipitation of carbon in the form of a
carbide or graphite.
Assuming that precipitation occurs at a maxi-
mum rate at 1000 C (2120 F), then aging at this temperature would
be effective in removing most of the carbon from solid solution and
reducing the tendency for dimensional change.
However, there should
still be some carbon in solution corresponding to the solid solu-
bility at 1000 C (2120 F).
If the alloy is simply air cooled to room
temperature, the carbon may remain dissolved but eventually must
precipitate out of solution.
In order to minimize this effect, it
is necessary to precipitate all the carbon in excess of the
solubility limit at room temperature.
Theoretically all one has to
do to accomplish this is to age long enough at room temperature;
however, due to the exceedingly low rate of diffusion, such a process might take an infinite length of time.
In order to minimize the
time for complete precipitation, it would be logical to age at the
temperature of maximum rate of precipitation until the process
stops, lower the temperature so that the solid solution becomes more
supersaturated and age until the process again stops, and repeat
until room temperature is reached.
In effect, Guillaume's recom-
mended very slow rate of cooling can be considered equivalent to a
series of Wing treatments at lower and lower temperatures.
-
- 12
The opposite of precipitation, that is re-solution, could conceivably account for part of the dimensional change observed on
aging Invar.
If during slow cooling, for example, some of the carbon
is precipitated before room temperature is attained, subsequent aging
at a high enough temperature could cause re-solution as manifested
by an expansion or contraction depending on whether precipitation
results in a contraction or expansion.
If the precipitated phase
has a fine enouch particle size, it is possible for retrogression
to occur.
This phenomenon involves re-solution of particles below a
certain critical size and can orecede further precipitation on aging.
The possibility that
Transformation of Austenite to Ferrite.
3.
the low temperature expansion is due to a transformation from austenite to ferrite as suggested by Russell
is supported by the equili-
brium diagram of Owen and Sully(3) which shiows that at a temperature
of 3000
c (5700 F) a pure Invar should consist of about 55 percent
austenite
iron).
(58:42 nickel-iron)
and 45 percent ferrite (5:95 nickel-
However, in order to attain equilibrium conditions it
was
found necessar7 to severely cold work and age for long periods of
time.
After ordinary heat treatment it
is generally claimed that
Invar is entirely in the austenitic condition.
It
is conceivable
that a small amount of transformation could occur on aging at 1000
C
(2120 F) although the rate of such transformation would be very slow
at this temperature.
The effect of impurities
such as carbon, manganese,
and silicon found in commercial Invar on this transformation is not
known; however, based on the effect of these elements on ferritic
hardenability in ordinary steel,
it
is suspected that the trans-
formation would be retarded by their presence.
It is also possible that austenite transforms to a carbideferrite or graphite-ferrite aggregate.
Either an overall expansion
or contraction will occur depending on the relative amounts and
-13-
specific volumes of the constituents making up the aggregate and
how the specific volume of the untransformed austenite phase is affected.
It is obvious that there are numerous possibilities in con-
nection with this type of transformation and therefore a detailed
analysis of dimensional changes would be of great complexity due to
the many assumptions required at this point.
Assuming that transformation of austenite to ferrite results in
an overall expansion, if any ferrite were present in Invar at room
temperature, on reheating there would be a reverse transformation of
ferrite to austenite which should result in a contraction.
However,
it is doubtful whether such a contraction would occur at low aging
temperatures where an approach to equilibrium in the opposite direction
is more likely.
It is believed, therefore, that the ferrite to auste-
low aging temperatures as suggested by Russell
.
nite reaction could not account for the observed contraction at
In order to explain why the coefficient of thermal expansion
at room temperature should be lower after fast cooling than after
slow cooling, the possibility that annealing favors, whereas quench-
ing suppresses, transformation of the austenite to ferrite was considered.
If ferrite formed, the resulting two-phase alloy should
have a higher coefficient than the alloy in the single phase condition due to the much higher coefficient of ferrite containing
about 5 percent nickel as compared with austenite containing about
36 percent nickel.
4.
Transformation of Austenite to Martensite.
The possibility
of a martensite reaction occurring isothermally at a low temperature
was also considered.
Although the martensite reaction was in the
-14on
past believed to take place only/cooling, recent work by Averbach
and Cohen (14) and Kurdjumov (15) has indicated that it can also take
place isothermally.
On the basis of the austenite-martensite volume
change occurring in steel, a length increase of about 140 microinches
per inch for each percent of transformation would be expected.
If the austenite-martensite reaction were reversible, then the
contraction on aging could be accounted for.
There is also the
possibility that decomposition of martensite by rejection of carbon
on aging could at least partially account for the observed contraction.
The occurrence of an austenite to martensite transformation
could account for the fact that a lower coefficient results after
quenching than after furnace cooling in the same way as suggested
for an austenite to ferrite transformation provided that the martensite
has a higher coefficient than the austenite from which it forms.
5.
Residual Stress Formation and Stress Relief.
could account for dimensional changes on aging Invar.
Stress relief
The occurrence
of residual stress in a metal object which has been subjected to
rapid cooling from a relatively high temperature is well known.
Where
there is no phase change involved, the setting up of residual stress
must be due to the difference in cooling rate between the center and
surface portions of the metal object.
The cooling stresses usually
result in residual compression at the surface and residual tension
in the interior.
On aging relief of this residual stress occurs
giving rise to dimensional changes.
Wheth .r a contraction or ex-
pension results depends on the exact stress distribution, the
15
-
-
variation of elastic limit in both tension and compression with
temperature, and the rate of heating to and cooling from the aging
temperature.
Guillaume's recommendation for the achievement of dimensional
stability can be partially understood on the basis of stress relief.
Heating to 1000 C (2120 F) for several days might result in a sufficient degree of stress relief so that negligible dimensional
changes would occur subsequently provided that Invar is not heated
above this temperature in service.
Slow cooling from 1000 C (2120 F)
to 200 C (680 F) over a period of several months as recommended by
Guillaume is not an efficient way of achieving stress relief.
Holding longer at 1000 C (2120 F) will result in greater stress
relief than slow cooling.
Residual stress can also be introduced by cold working.
The
distribution of residual stress and the resulting dimensional changes
due to stress relief on aging will depend on how the cold working is
applied.
It is possible that the reported differences in coefficient of
expansion resulting from furnace cooling, cuenching, and cold working
Invar may be due to the relief of residual stress.
The reported
dilatometer curves which were made for the purpose of determining
coefficients of expansion usually show that heating was carried
out well above room temperature.
This could result in stress relief
as manifested by a dimensional change superimposed on the normal
heating curve and lead to a false measure of the coefficient,
particularly if an average value over a range of temperatures is
being determined.
-
-16
Without knowledge of the magnitude of irreversible
changes due to stress relief the reported values of the coefficient
are open to question.
6.
Alteration of Manetic State by Cold Work. The lowering
of the coefficient by cold work could be attribut'ed to alteration
of the ferromagnetic condition of the austenite.
It is possible that
the intensity of magnetization could be affected .by cold work in
such a way that the temperature dependence of the contraction due
to the gradual reversion from the ferromagnetic to the paramagnetic
state on heating results in a lowered overall coefficient of expansion at room temperature.
The observed effect of reheating after
cold working on the coefficient could also be explained on this
basis.
Reheating tends to remove the effects of cold working and
consequently the coefficient would be increased.
-
- 17
IV.
PLAN OF EXPERIMENTAL WOEK AND MATERIALS USED
Based on consideration of the possibilities discusged in the
previous section, a plan of the experimental work to be followed in
this investigation was decided upon.
The main outline of this plan
is as follows:
1.
Determination of the effect of quenching, annealing, and
cold working on length changes during subsequent aging of
Invars of varying carbon content.
2.
Determination of the magnitude and distribution of residual
stress in quenched Invar.
3.
Determination of the solid solubility of carbon in 36:64
nickel-iron austenite.
4.
Determination of the possible existence of cementite,
graphite, nickel carbide, ferrite, and martensite in Invar
and the conditions under which they form.
5.
Determination of the effect of heat treatment on the hardness
of Invar.
6.
Determination of the effect of thermal and mechanical
treatment on the temperature dependence of intensity of
magnetization of Invar.
7.
Determination of the coefficient of thermal expansion at
room temperature of Invar subjected to thermal and mechanical
treatments, taking into account irreversible length changes.
The materials used in this investigation were iron-nickel-carbon
alloys obtained in the form of 0.250 and 0.375 inch diameter rod
mostly in the cold drawn condition.
The chemical composition of
these materials is given in Table I.
investigated:
(1)
18
Two series of Invars were
0.44 Mn, 36.8 Ni; and (2) 0.10 Mn, 36.0 Ni.
-
-
19
-
-
TABLE I
CHEMICAL CMTPOSITION
Ni
C
Mn
Si
8
P
0.44 Mn, 36.8 Ni*
0.07
0.44
0.24
0.011
0.007
36.82
0.10 Mn, 36.0 Ni
0.02
0.09,
0.01
0.012
0.008
36.02
0.10
0.12
0.08
0.010
0.009
36.10
0.15
0.08
0.17
0.021
0.009
36.03
0.25
0.05
0.20
0.022
0.010
36.60
0.40
0.10
0.07
0.013
0.009
35.89
0.58
0.07
0.22
0.022
0.012
36.16
0.74
0.07
0.17
0.020
0.009
35.89
0.99
0.14
0.24
0.011
0.012
36.10
Invar Series
*
By decarburization and carburization techniques, Invars
varying in carbon content from 0.01 to 0.84 percent were made
from this composition.
-
- 20
V.
A.
EXPERIMENTAL EQUIPNENT AiD TECNIQUES
Heat Treatment
1.
Quenching.
Heating prior to quenching of Invar specimens
was carried out either in a lead pot furnace or a tube furnace.
The
lead pot, 10 inches in diameter by 10 inches deep inside a chromel
wound muffle, was maintained at 830 + 50 C (1525 + 100 F) for this
treatment.
It was found that distortion resulting from water quench-
ing specimens 0.250 inch in diameter by 4 inches in length could be
minimized by use of a jig consisting of two parallel rods 0.25 inch
square and 8 inehes long welded 6 inches apart across a rod 12 inches
long.
Two specimens could be tied on the jig with iron wire in a
position parallel to and on either side of the 12 inch rod.
The
loaded jig was placcd in the lead pot to completely suknerge the
specimens about 4 inches from the surface.
After 30 minutes of
heating at 8300 C (15250 F) the loaded jirg was removed from the
lead pot and quenched in a tank of water using an up and down motion.
With this procedure, distortion as measured by "bowing" of the specimens was usually found to be less than 0.005 inch.
When it was desired to quench specimens from as high as 1205* C
(22000 F) a vertical tube furnL.ce shown in Figure 1 was used because
the lead pot could not be heated above 8700 C (16000 F).
The tube
furnace contained a muffle 2 inches in diameter and 24 inches long,
wound with Kanthal A resistance wire.
A quartz tube with con-
strictions for inlet and outlet gas connections was placed inside
-
- 21
of the muffle.
The furnace provided a four inch long zone at the
center of the tube throughout which the temperature was constant
within 50 C (100 F).
Either dry nitrogen, hydrogen, or argon gas
was used as a protective atmosphere and entered the quartz tube
through the bottom of the furnace.
In practice, specimens were tied
with iron wires and lowered into the
zone.
4
inch constant temperature
Because of the lack of space for a quenching jig and the
higher temperatures employed, appreciable distortion often occurred.
This necessitated heat treating a greater number of duplicate
specimens until enough specimens with an acceptable degree of bowing,
less than about 0.01 inch, became available.
Heating of Invar specimens prior to furnace
2.. Firnace Cooling.
cooling was carried out in a small retort furnace heated by resistance
elements.
The retort contained inlet and outlet connections for a
protective atmosphere.
Hydrogen gas was found to result in the
brightest surface after furnace cooling of Invar.
The procedure
was to place the specimens in the retort, turn on the hydrogen gas,
heat to 830 + 80 C (1525
+
15*0 F) in about 3 hours, hold at tempera-
ture for 1 hour, and furnace cool to room temoerature in about 10
hours.
3.
Aging.
Aging at room temperature was carried out in a
constant temperature room maintained at 20 + 10 C (68 + 20 F).
Aging between 200 C (680 F) and 1500 C (3000 F) was carried out in
oil pots maintained within 30 C (50 F) of the desired temperature.
Aging above 1500 C (3000 F) and up to 5400 C (10000 F) was carried
out in salt pots maintained within
50
C (100 F) of the desired
-
- 22
temperature.
The following times were found necessary for the center
of specimen 0.250 inch in diameter by 4 inches in length to attain
within 50 C (100 F) of the temperature of the aging bath:
Tepperature
Medium
500 C
Oil
1 minute
1500 C
Oil
2 minutes
2050 C
Salt
1 minute
4250 C
Salt
0.5 minute
Time for Center to Reach Temperature
The time of cooling from a 3150 C (6000 F) salt bath to room temperature for specimens 0.250 inch in diameter by 4 inches in length was
found to be as follows:
Medium
Time for Center to Reach Room Temoerature
Water
1 second
Air
20 minutes
Silocel
45 minutes
4.
Carburization and Homogenization.
out in the tube furnace shown in Figure l
Carburization was carried
A gas train similar to
that used by Low and Gensamer (16) was constructed for the purpose of
carrying out carburization, homogenization, and decarburization
experiments.
Carburization was carried out by bubbling dry purified
hydrogen through liquid heptane.
The steps involved in purifying
and drying ordinary tank hydrogen consisted of passing this gas
through platinized asbestos at 4000 C (7500 F), anhydrous calcium
sulfate, anhydrous magnesium perchlorate, and a trap cooled by
liquid nitrogen.
The purpose of the platinized asbestos was to get
rid of any oxygen by catalyzing the reaction 2H2 +
02 --
+.
2H20-
The water formed by this reaction as well as any water initially
-23-
present in the tank hydrogen is removed by the two chemicals and
liquid nitrogen trap, and dry oxygen-free hydrogen gas is thus prepared.
This gas when bubbled through liquid heptane maintained at
0' C (320 F) in an ice bath acts as a carrier of hydrocarbon vapor.
A low flow rate adjusted to result in barely perceptible bubbling through
the liquid heptane was maintained during carburization.
Appropriate
temperature-time combinations were selected to result in the desired
degree of carbrization.
In order to obtain powder of a given carbon content for the
x-ray diffraction experiments, it was decided to carburize Invar
specimens 0.250 inch in diameter by 2 inches in length weighing
about 9 grams.
This size specimen also allowed enough material for
chemical analysis of carbon content at both the center and surface of
the specimen.
It was expected that after carburization a carbon
gradient would exist from surface to center and a homogenization treatment carried out in dry nitrogen would be required to equalize the
carbon content throughout the 0.250 inch diameter section.
An
initial experiment gave the following results:
Carburization Treatment
12050 C - 6 hours
12050 C - 6 hours
Homogenization
Treatment
% C at
Surface
% C at
Center
none
0.824
0.770
0.783
0.775
12050 C - 3 hours
Specimens were water quenched to room temperature after carburization
and after homogenization.
This experiment gave an idea of how much
carbon could be introduced by carburization and showed that equali-
zation throughout the 1/4 inch section to within 0.01 percent of
-
- 24
carbon was attainable.
Homogenization treatments at lower tempera-
tures were tried but the times required became excessive below
10950 C (20000 F).
A homogenization treatment consisting of 24
hours at 1095* C (20000 F) was finally selected for this work.
The carburizing treatments and resulting carbon contents of
the series of specinens used in the x-ray diffraction work are given
in Table II.
TABLE II
CARBE0
CONTENTb-FTER
Carburizing Treatment
None
9800 C
2 hrs.
-
CiJRBURIZbTION AND 'TOMOGENIZATION
Carbon Content zAfter Homogenizing 24,
Hours at 10950 C
Average
Center
Surface
0.070
0.070
0.070
0.130
0.124
0.127
10400 C
-
4 hrs.
0.270
0.266
0.268
10950 C
-
2 hrs.
0.376
0.365
0.369
1095* C
-
4 hrs.
0.421
0.411
0.416
8 hrs.
0.505
0.485
0.495
11500 C - 8 hrs.
0.616
0.638
0.627
12300 C - 8 hrs.
0.832
0.844
0.838
10950 C
Melting occurred wehen an attempt was made to carburize at 12600 C
(23000 F); therefore a carbon content of about 0.84 percent was
considered the limit attainable by this technique using an 8 hour
carburizing period.
7
5.
25
-
-
Decarburization. Decarburization was carried out by the use
of wet hydrogen.
To produce the wet hydrogen, tank hydrogen was first
passed through platinized asbestos at 400 0 C (7500 F) to remove
oxygen and then through a water saturator.
After some preliminary
experimentation, it was found that with a flow rate of 2 cubic feet
per hour and the water saturator maintained at 70 0 C (1580 F)
effective decarburization of a 0.250 inch diameter Invar specimen
could be accomplished in 24 hours at 10950 C (20000 F).
Chemical
analysis before and after this treatment gave the following results:
Treatment
As received
24 hours at 10950 C in wet H2
Carbon
Pyen
Nitrogen
0.070
0.0027
0.0058
0.011
0.0036
0.0077
These results show a significant decrease in carbon content along
with but a slight increase in oxygen and nitrogen contents.
B.
Precision Length Meagurements
Precision length measurements were carried out by the use of a
Sheffield Comparator of 5000X magnification.
Specimens 0.250 inch
in diameter and 4 inches long were spherically ground to a 2 inch
radius at their both ends in order to provide a reproducible high
spot for precise measurement and also to minimize the error due to
small deviations in positioning when held vertically in a jig.
The
anvil to gaging point distance of the comparator was maintained at
4.120 inches so that in making a measurement it was necessary for the
specimen to rest with its bottom end on a gage block of the proper
size which was wrung to the anvil.
By moving the jig and conse-
quently the specimen back and forth in contact with the gaging
was taken of
point a maximum reading could be taken. Next a reading
-
-26
a 4.120 inch standard block calibrated to the nearest microinch.
From the difference in readings between the specimen and standard
block and a measurement of the temperature at which the measure-
ments were taken, the length of the specimen prior to or after a
given treatment could be determined.
The sensitivity of the com-
parator used is about 5 microinches so that changes in length of a
4 inch specimen could be determined with an accuracy of within 2
microinches per inch.
The reproducibility of length change for
duplicate specimens varies with the magnitude of the change.
In
general it is believed that the precision of measurement of the
average length change of duplicate specimens is + 2 microinches per
inch or 5 percent, whichever is the larger.
In order to make a correction for the difference in coefficient
of thernial expansion between the specimen and standard block, accurate measurement of temperature is required.
Although length
measurements were made in a "constant" temperature room maintained
at 20 + 10 C (68 + 20 F), the variation of even 10 C (20 F) results
in a large error due to the great difference between the coefficient
of expansion of the tool steel standard block and the
Invar speci-
mens (12 compared with less than 2 microinches per inch per OC).
To reduce this error, a large copper block, 2 inches thick by 12
inches square was used for the purpoce of keeping both standard
block and specimens at constant temperature.
This block was machined
with grooves in order to attain intimate contact with cylindrical
specimens.
The standard block
and
a dumWy block of the same
K
-27-
material and dimensions were kept side by side on a flat surface of
the block.
A copper-constantan thermocouple was attached to the
dummy block and temperatures were measured by a potentiometer to
within + 005* C (0.100 F).
C.
Specific Volume Measurements
The weigh-in-water, weigh-in-air method was used for measurement
of specific volume.
(17)
Cohen and Kohl.
This method has been described in detail by
The accuracy of measurement is about 0.00002
cubic ems. per gram.
D.
Drop Tests
It was noted that the accidental dropping of a quenched Invar
specimen resulted in a relatively large contraction.
It was sus-
pected that this change of length might be associated with a redistribution of residual stress.
tests were carried out.
In order to check this hypothesis, drop
These tests consisted of dropping specimens
from a height of 5 feet above a concrete floor and measuring the
resultant change in length.
In order not to injure the spherical
ends of the specimen, a small electromagnet having two wound cores
When the
was used to hold the specimen in a horizontal position.
current in the electromagnet was turned off, the specimen dropped and
maintained its horizontal position on striking the floor.
The re-
bound was as high as 2 feet above the concrete floor and specimens were
usually caught on the first bounce.
E.
Heyn Analysis for Residual Stress
The Heyn analysis was used to determine the residual stress in
Invar specimens subjected to various heat treatments.
This analysis
assumes the existence of longitudinal stress but no radial or
-
- 28
circumferential stress.
In carrying out the Heyn analysis it is
necessary to determine the change of length resulting from machining
off layers from the surface.
If after reducing the area of a
cylindrical specimen to a value A, a change in length e measured
on a unit length basis results, then the force F that the machined
off surface layer exerted on the section of area A is given by the
following:
(1)
F = EAe
Where E = Young's modulus
The stress S that must have existed at the radius corresponding to
A prior to machining is given by:
(2)
S =
= -E
A
+ e
Thus the residual stress distribution can be calculated knowing the
value of E (21 X 106 p.s.i. for Invar), the rate of change of e with
A at a given A, and the value of e at a given A.
In order to deter-
mine the variation of e with A, specimens 0.250 and 0.375 inch in
diameter and 4 inches in length possessing spherical ends were
centerless ground taking off 0.001 inch from the diameter at each
pass.
area
The change of length corresponding to decreased sectional
was determined by precision length measurements.
For the most part only the residual stress at the surface was
desired and this
as determined by machining 0.020 inch in steps
of 0.001 inch off the diameter.
The value of surfaoe stress was
at the
calculated using equation (2) substituting the value of d
dA
surface and zero for e.
The accuracy of surface residual stess as
determined by the Heyn method is estimated to be about 10 percent.
-
- 29
F.
X-rar Diffraction Measurements
1.
Precision Lattice Parameter Measurements.
Precision lattice
parameter measurements of the austenite phase in Invar were made
using a symmetrical back reflection camera 10 cms. in diameter.
Specimens in the form of 325 mesh powder were used.
A cobalt
target was sel-cted in order to obtain enough high angle lines to
facilitate determination of lattice parameters by the extrapolation
method.
The accuracy of this method is about 0.01 percent, which
0
amounts to + 0.0003 A for the austenite lattice parameter.
Three lines were obtained in the back reflection diffraction
pattern resulting from the face-centered cubic austenitic phase.
These lines are as follows:
Line
hkl
1
400
Cobalt K alpha 1
0.99
Medium (diffuse)
2
400
Cobalt K alpha 2
0.99
Strong (sharp)
3
331
Cobalt K beta 1
0.97
Weak (sharp)
Radiation
sin 2e
Relative Intensity
In order to calculate the lattice parameter corresponding to each
line the following equations were used:
Where 8 = Bragg angle in degrees.
= 90 -
(1)
or 6 = 90 - 0.14129 S
S = Separation on film in mm.
D = diameter of camera (101.38 mm).
(2)
a
-
2
+
I
Where a
= lattice parameter
= wavelength of radiation in
angstroms
hkl = Miller indicies of atomic plane.
The value of the lattice parameter corresponding to each line
was plotted against sin 26.
The value obtained by extrapolating to
sin2e = 1 was taken to be the lattice parameter of the specimen.
-
- 30
The parametric method of determining the solubility of carbon in
36:64 nickel-iron -austenite was employed.
In the determination of the
phase boundary of a terminal solid solution in a two component system
by this method, there are two main steps.
First, it is necessary to
determine the relationship between lattice parameter and composition
of the solid solution phase by quenching a series of specimens of
varying composition from the temperature of maximum solubility.
Second,
specimens of an alloy exceeding the maximum solubility of the solid
solution are brought to equilibrium and quenched from temperatures up
to that corresponding to maximum solubility.
From the lattice para-
meters of the solid solutions in equilibrium with the second phase
at each temperature and the relationship between lattice parameter
and composition of the terminal solid solution, the composition-temperature or phase boundary of the terminal solid solution can be constructed.
The procedure uied for binary systems depends on the fact that
the composition of two phases in equilibrium at a given temperature
is independent of the amounts of the phases present.
In a ternary
system, the compositions of two phases in equilibrium at a given
temperature are not necessarily fixed.
Therefore, instead of
determining the relation between the lattice parameter and carbon
content at only the temperature corresponding to maximum solubility,
it was planned to determine t-nis relation at all temperatures.
It
was expected that a plot of lattice parameter vs. carbon content would
exhibit a discontinuous change in slope (but not necessarily a
horizontal break) at the carbon content corresponding to the limit
of solubility for a given temperature.
The solid solubility curve
of carbon in 36:64 nickel-iron austenite could then be determined
by plotting the carbon content at the discontinuity vs. temperature.
-
- 31
2.
Identification of Phases Present.
In order to determine
whether or not Invar contains phases other than austenite two x-ray
diffraction techniques were used.
One technique described by Averbach
and Cohen (is) consists of exposing the polished and etched surface
of a solid specimen 0.375 inch in diameter to monochromatic radiation
in a modified Debye camera.
Iron K-alpha radiation monochromatized
by a bent rock salt crystal and exposure times of 12 hours were
employed.
The other technique involves use of the regular Debye
camera and a powdered specimen made in the shape of a wire about
0.5 mi. in diameter with cellulose acetate as binder.
The specimens
were prepared by an electrolytic extraction technique used to separate
carbides from steel.
The details of this technique are given in the
following section.
G.
Electrolytic Extraction
Electrolytic extractions for possible graphite or carbides were
carried out in an acid cell designed by Blickwede (19).
In this
cell a specimen 0.250 inch in diameter by 4 inches long serves as
anoLe and is surrounded by a cylindrical copper cathode.
Both anode
and cathode are immersed in an electrolyte, 5 percent hydrochloric
acid.
A current of 0.5 amps is used which results in a current
density of 0.15 amp. per square inch.
usually about 48 hours.
The duration of a run is
Every 8 hours specimens are scraped and
washed with 2 percent hydrochloric acid to obtain the insoluble
residue.
This residue is kept at 00 C (-32* F) until vacuum filtered.
The filtered residue is washed with 200 cubic cms. of distilled water
and dried in a vacuum oven for 8 hours at 650 C (1500 F) after which
slow cooling in vacuum is carried out.
The filter and residue are
placed in a dessiccator for 4 hours and finally weighed.
From the
32
-
-
weight of the residue and the dissolved specimen, the weight percent
of the residue can be calculated.
H.
Hardness Tests
Measurements of Rockwell B hardness were made in a standard
Rockwell machine.
The values reported are accurate to + 1 Rockwell B
unit.
I.
Maanetic Measurements
Measurements of variation of intensity of magnetization with
temperature were made using the apparatus described by Zmeskal and
Cohen(20) for specimens 0.250 inch in diameter by 4 inches long.
A
rate of heating of 50 C (100 F) per minute was maintained in making
a run.
By calibration, the following relation was determined from
which to calculate intensity of magnetization (J):
J = 6.37 (D
- D ) gauss.
Ds
= deflection of galvanometer
for complete reversal of field
with specimen in secondary cell.
DA = deflection of galvanometer for
complete reversal of field without specimen in secondary coil.
A magnetic field intensity of H = 1100 oersteds which is well above
saturation was used in these measurements.
J.
Measurement of Thermal Expansion Coefficient
The determination of the coefficient of thermal expansion of
Invar at room temperature represents a more difficult problem than
for other metals because of the low magnitude involved.
In order to
measure the coefficient of Invar an apparatus shown in Figure 2 was
-
- 33
constructed.
This apparatus consists of a water bath with heater
and stirrer, two metallographic microscopes on an adjustable support,
a clamp for holding test specimens fixed with respect to the adjustable
support, and two rectangular arms 3 inches in length.
The clamp is
attached to the middle of an Invar specimen 0.250 inch in diameter
and 4 inches long by a set screw and holds the specimen in a horizontal position below the surface of the liquid bath.
The two arms
are also attached by set screws to either end of the specimen and
protrude vertically through openings about 4 inches apart in the
cover of the bath.
The specimen assembled with clamp and arms
attached is shown outside of the water bath in the front view of
Figure 2.
When this assembly is put into the water bath and a run
started only the arms can be seen as shown in the side view of Figure
2.
The arms each have a Knoop impression which serves as a marker
for measuring changes in length.
The microscopes are focussed on
these marks, and by means of a filar eyepiece on each microscope,
relative motion of the Knoop impressions and consequently change in
length of the specimen due to change in temperature of the bath can
be measured.
In making a determination of coefficient of expansion,
the temperature of the bath was varied from 200 C (680 F) to 500 C
(1200 F) as measured by a copper-constantan thermocouple attached to
the middle of the specimen.
In order to get rid
of any irreversible
length changes, measurements were repeated through the same temperature range until a constant value of the coefficient of expansion was
obtained.
The adjustable support, clamp, and arms were all made of
Invar to minimize any expansion due to temperature change.
The
accuracy of a thermal coefficient determined by this method is estimated to be about 0.1 microinches per inch per OC (0.05 per OF).
VI.
A.
-
34
-
EPERIEiTTAL RESULTS
Dimensional Changes-on Aging
1.
Invar (0.07C, 0.44. Mn, 36.8 Ni)
a.
Aging Following Quenching from 8300 C.
The effect of
aging quenched Invar on change of length is shown in Figure 3.
Specimens were water quenched to 20* C (680 F) after 30 minutes at
8300 C (15250 F) and initial measurements of length were taken.
Aging was carried out for as long as 1000 hours or more at temperatures up to 2050 C (4000 F) and specimens were air cooled to room
temperature for measurements of length after successively increasing
time increments.
As shown in Figure 3, aging at 200 C (6$0 F) results in a slight
contraction whereas aging from 50' C (1200 F) to 950 C (2000 F)
results in an initial contraction followed by an expansion.
The
greatest expansion occurs at 700 C (1580 F) th0 curve for which is
still increasing after 2000 hours.
At 950 C (200* F) the expansion
reaches a maximum in about 250 hours after which a contraction occurs.
Aging at 120* C (2500 F) to 2050 C (4900
F) results in increasing
contractions.
On the basis of these initial results it appears that on aging
at temperatures up to 2050 C (4000 F) at least two phenomena are occurring.
One of these phenomena causes a contraction which increases
with increasing aging temperature.
The other phenomenon causes an
expansion and occurs at aging temperatures up to at least 95* C (2000 F).
As will be shown later the contraction effect is due to stress relief.
For convenience, the expansion effect will be referred to as the 700 C
(1580 F) expansion since it occurs to a maximum extent at this
temperature.
-
- 35
b.
Agijng at 700 C Following Quenching From Temperatures
up to 5400 C.
The effect of water quenching to 20* C (680 F) after
1 hour at temperatures up to 5400 C (10000 F) on subsequent aging at
700 C (1580 F) is shown in Figure 4.
Specimens were first water
quenched from 830* C (1525* F) and initial measurements of length
were taken.
As shown in Figure 4, increasing initial contractions result
from quenching from temperatures up to 3150 C (6000 F).
Quenching
from 425* C (8000 F) results in less of a contraction than from
2050 C (400* F) and ouenching from 540* C (10000 F) results in an
expansion.
On subsequent aging at 700 C (1580 F), an initial contraction
followed by an expansion occurs with the exception of specimens
quenched from 4250 C (8000 F) which only show an expansion.
As judged
by the magnitu.e of the expansion which occurred after completion of
the initial contraction on aging, quenching from higher temperatures
has a greater effect than from lower temperatures.
It was noted that
quenching from 1200 C (2500 F) results in the smallest expansion.
On the basis of these results, it appears that a third phenomenon
giving rise to an expansion occurs on quenching from temperatures
above 3150 C (6000 F).
As will be shown later this expansion effect
is associated with the formation of residual stress.
c.
Re-Aging at 500 C Following Aging at 700 C.
The ef-
fect of re-aging specimens at 50' C (1200 F) after 1000 hours of
aging at 700 C (1580 F) is shown in Fig;ure 5.
These specimens were'
quenched in water from 8300 C (15250 F) to 200 C (680 F) prior to
the first aging treatment at 700 C (1580 F).
36
-
-
As shown in Figure 5, practically no change in length occurred
after 1000 hours of aging at 500 C (120* F).
This indicates that the
phenomenon responsible for the low temperature expansion effect is
0
slowed down to a negligible rate by a long aging treatment at 7 * C
It follows that the high dimensional stability exhibited
(1580 F).
on aging at 500 C (1200 F) should be duplicated on aging at 200 C
(680 F) where any expansion would be expected to occur at an even
slower rate.
d.
Aging Following Quenching From 2050 C.
The effect of
aging at temperatures from 50 0 C (1200 F) to 1500 C (3000 F) following
water quenching after 1 hour at 2050 C (4000 F) is shown in Figure 6.
Specimens were first water quenched from 8300 C (15250 F) before reheating to and quenching from 2050 C (4000 F),
The initial measure-
ment of length was taken after quenching from the latter temperature.
As shown in Figure 6, aging at 50' C (1200 F), 700 C (1580 F), and
950 C (2000 F) results in expansions which start after approximately
10, 1, and 0.5 hours respectively.
After more than 1000 hours, the
specimens aged at 500 C (1200 F) and 700 C (1580 F) are still expanding; whereas, on aging at 950 C (200* F) expansion stopped after
approximately 75 hours.
Aging at 1500 C (300* F) results in an
initial expansion during the first 0.2 hour after which a slight contraction occurs.
The effect of three long time aging periods at 70* C (1580 F)
is shown in Figure 7.
Specimens were first water quenched from
8300 C (15250 F) to 20* C (680 F) and then aged at 700 C (1580 F)
for 1000 hours.
The dimensional changes resulting from this treat-
ment are shown in Plot A of Figure 7 which is similar to the 700 C
37
-
-
(158* F) curve of Figure 3 which represents the same treatment.
The same specimens were next reheated to 2050 C (4000 F) for 1 hour and
quenched in water to 20* C (680 F).
A contraction of 112 microinches
per inch resulted from this treatment.
Since the specimens had al-
ready undergone a net expansion of 32 microinches per inch during
the 1000 hour age at 70' C (158* F), they were 80 microinches per
inch shorter than their initial
length.
On subseq7uent aging at
700 C (158* F) as shom in Plot B of Figure 7, an expansion again
occurs although this time the small initial contraction is absent.
The magnitude of the expansion after 1000 hours is 30 microinches
per inch, which is sLightly smaller than the expansion which occurred during the first aging treatment at 70 0 C (1580 F).
On re-
heating to 2050 C (4000 F) for 1 hour and water quenching to 200 C
(680 F) a second time, a contraction of 44 microinches per inch
results.
On re-aging at 700 C (158* F) as shown in Plot C of Figure 7,
an expansion similar to Plot B occurs.
The results of Figure 7 indicate that the 700 C (1580 F)
expansion is part of a reversible reaction that can be repeated over
and over again by reheating to 2050 C (4000 F) and re-aging at a
lower temperature.
If this is the case, one would expect that on re-
heating to 2050 C (4000 F) after the first
aging period at 700 C
(1580 F) there should be a contraction +4hich equals rather than exceeds the expansion which occurred on aging at the lower temperature.
However, heating of quenched Invar to 2050 U (4000 F) results in a
contraction due to stress relief even if the 700 C (1580 F) aging
treatment is omitted.
The magnitude of this contraction was found
to be 70 microinches per inch, which accounts for the difference.
-
- 38
On reheating to 205* C (400* F) for the second time, a contraction
results which is virtually the same as the expansion which occurred
during the second aging period at 70 0 C (1580 F).
This shows more
clearly that the expansion and contraction effects are related.
The
expansion on aging at 700 C (1580 F) for the third time illustrates
that this effect can be repeated over and over again.
In order to determine whether the same relation between expansion and contraction holds for aging temperatures other than 700 C
(1580 F), the specimens aged as shown in'Figure 4 were reheated to
2050 C (4000 F) for 1 hour and quenched in water to 200 C (680 F).
The resulting contractions are compared with the previous expansions
in Table III.
TABLE III
DIMENSIONAL CHANGES ON REHEATING AGED SPECIMENS AT 2050 C
Initial
Aging Treatment
Microinch Per
Inch Change
Quenched in Water
After
Microinch Per
Inch Change
1250 hours at 50* C
+30
1 hour at 2050 C
-36
2700 hours at 700 C
+37
1 hour at 2050 C
-45
1250 hours at 950 C
+28
1 hour at 2050 C
-37
1250 hours at 1500 C
+13
1 hour at 2050 C
-19
For each aging treatment, the contraction on subsequent reheating
to 2050 C (4000 F) is only about 6 to 9 microinches per inch larger
in magnitude than the previous expansion.
Thus more evidence points
to the existence of a contraction effect that is the reversible
counterpart of the 700 C (1580 F) expansion and which differs from
the stress relief effect found on aging directly following quenching.
For convenience this new contraction effect will be referred to as
the 2050 C (4000 F) contraction.
39-
e,
Hot Quenching from 2050 C to 700 C.
In order to de-
termine whether the initial quench from 2050 C (4000 F) to 200 C
(6* F) was necessary in order for the expansion to occur on subsequent aging at 700 C the following experiment was tried.
A series
of specimens initially water quenched from 830* C (15250. F) to 200 C
(680 F) were all reheated to 2050 C (4 0
Next one
F) for 1 hour.
was quenched in water to 200 C (680 F)
pair of these specimens
whereas the remaining specimens vere hot quenched in
oil at 700 C
(1580F) and aged for increasing increments of time before cooling to
200 C (680 F).
Allowing for the change in length due to the one
hour treatment at 2050 C (4000 F) as determined by measuring the
first pair of specimens, the change in length with time which occurred at 70 0 C (158* F) after hot quenching could be calculated.
In
Table IV, the results of this calculation are compared with the re-
sults of water quenching from 205* C (4000 F) to 200 C (680 F) and
then aging at 700 C (1580 F).
TABLE IV
DIMFNSIONAL CHANGES ON AGING AT 700 C FOLLOWING BOTH
REGULAR AND HOT QUENCHING
Microinch Per Inch Length Change
On Aging at 700 C
200 hrs.
Quenching Treatment
20 hrs.
Quenched in water from 205* C to
200 C
+15
+28
0
+20
Quenched in oil from 205* C to 700 C
The results of Table IV reveal that both quenching procedures
result in expansions.
However, aging at 700 C (1580 F) after regular
quenching results in an expansion which starts sooner and has a
larger magnitude after 200 hours than the expansion that occurs after
hot quenching directly to and aging at 700 C (1580 F).
40
-
-
On the basis
of this experiment it is doubtful whether the 700 C (1580 F) expansion
can be attributed to a retrogression effect.
Any particles that
precipitated out at 2050 C (4000 F) or on cooling down to 70 0 C
larger than the critical
(158* F) during hot quenching, would be
size necessary for re-solution to occur at 70 0 C (158* F).
.Aging
Following Cold Working.
drawn Invar is shown in Figure 8.
The effect of aging cold
This material was received in the
cold drawn condition, having been reduced 25 percent in area to a
diameter of 0.250 inch following water quenching from approximately
8000 C (14700 F) to 200 C (680 F).
Aging was carried out at tempera-
tures from 50* C (1200 F) to 2050 C (400* F) and the specimens were
air cooled to room temperature for ea.ch measurement of length.
As shown in Figure 8, the dimensional behavior on aging is
similar to that shown in Figure 3 for non-cold drawn Invar.
However, with cold drawn Invar the expansion effect at 500 C (1200 F)
and 700 C (1580 F) is smaller and the contraction effect at 150* C
(30r0 F) and 2050 C (4000 F) is larger than with quenched Invar.
It is believed that aging of cold drawn Invar results in a contraction effect which is also due to stress relief.
g.
Aging Following Furnace Cooling from 830' C.
of a furnace cooling treatment is shown in Figure 9.
slowly heated to 8300 C (15250 f), held
to 200 C (680 F).
The effect
Specimens were
60) minutes, and furnace cooled
After an initial measurement of length, specimens
were aged at temperatures up to 2050 C (4000 F) and air cooled to
200 C (680 F) for succeeding measurements of length.
-41-
As shown in Figure 9 aging of furnace cooled Invar results in
expansion and contraction effects of smaller magnitude as compared
with aging of quenched Invar.
Whereas the 700 C (1580 F) expansion
is believed due to the same cause in both cases, it
is doubtful
whether the contraction that occurs on aging furnace cooled Invar can be
attributed to stress relief.
h-.
Rate of Cooling from 8300 C and, 3150 C.
The effect of
rate of cooling from 8300 C (15250 F) and from an aging temperature
of 3150 C (6000 F) is shown in Figure 10.
Three sets of specimens
were furnace cooled, air cooled, and water quenched from 83 0 0 C
(15250 F) to goo C (680 F) and initial measurements of length were
taken.
All specimens were aged at 315* C (6000 F).
Specimens of
each set were cooled in silocel, air, and water from the aging
temperature for successive measurements of length.
As shown in Figure 10, the effects of furnace and air cooling
from 8300 C (15250 F) are similar.
Also, the rate of cooling from
the aging temperature does not have an appreciable effect on dimensional
change of these two sets of specimens.
However, water quenching
from 8300 C (15250 F) results in more than five times as large a contraction as does furnace or air cooling from this same temperature.
There is also an appreciable difference between cooling in water as
compared to cooling in silocel or air from the 3150 C (6000 F)
aging temperature after an initial water quench from 8300 C (15250 F).
Cooling in water from the aging temperature results in the same amount
of contraction as does cooling in silocel or air after an aging time
of 0.2 hour; however, specimens cooled in water continue to contract
with increasing aging time, whereas the more slowly cooled specimens
remain fairly constant in length.
-
- 42
i.
Quenching in Water from
54Q0
more about the nature of the.grpansion,
C.
In order to learn
resulting from water
quenching from an aging temperature of 5400 C (10000 F), simultaneous
measurements of specific volume and change in length were carried
out starting with furnace cooled material.
The purpose of this ex-
periment was to determine (a) whether the expansions in length are
accompanied by corresponding changes in volume; and (b) whether the
expansions in length depend on the time of aging or number of water
quenches from the aging temperature.
The results are shown in
Figure 11 in the form of two plots of length change and a plot of
specific volume.
Plot A shows the effect of increasing increments of
aging time up to 100 hours on dimensional change, whereas Plot B
shows the results of 10 one-hour aging treatments.
volume measurements in Plot C
Plot B.
The specific
were made on the same specimens as for
The dimensional and specific volume results reveal that
there is virtually no volume change even after an expansion of 1700
microinches per inch Mrhich should correspond to an increase in
volume of 0.51 percent or approximately 62 X 10-5 cubic cms. per
It also shows that 10 one-hour aging and quenching treatments
gram.
iesult in a greater expansion than a total of 100 hours at 5400 C
(10000 F) which involved quenching to 200 C (680 F) only 6 times.
Thus it appears that the expansion is dependent to a larger extent on
the number of quenching treatments than on the cumulative time of
aging.
Based on the specific volume and dimensional evidence, it appears that the expansion effect resulting from rapid cooling from
relatively high aging temperatures must be due to plastic deformation
associated with the formation of residual stress.
During cooling the
43
-
-
surface layer is elongated in tension giving rise to an increase in
length without a corresponding increase in specific volume.
2.
Decarburized invar (0.01 C, 0.44 Mn, 36.8 Ni)
a.
Aging Following Quenching from 830
C.
In order to
obtain an idea of the effect of carbon content on dimensional changes
which occur during aging, Invar specimens oontaining 0.07 percent
carbon were reduced to 0.01 percent carbon by a wet hydrogen treat-
ment consisting of 24 hours at 10950 C (20000 F).
After this
treatment, the specimens were heated for 30 minutes at 8300 C (15250 F)
and water ouenched.
Aging was carried out at tepoeratures from 200 C
(680 F) to 2050 C (4000 F) and specimens were air cooled from the
aging temperature for succeeding measurements.
The results of the above treatment is shown in Figure 12.
The 70 0 C (158* F) expansion appears to have been eliminated by the
initial decarburization treatment and therefore must be a carbon
dependent reaction.
A sizeable contr.ction occurs on aging at
700 C (1580 F) which is the temperature of maximum expansion on
aging quenched Invar containing 0.07 percent carbon.
This behavior
indicates that on aging quenched 0.07 percent carbon Invar at 700 C
(1580 F) a contraction and expansion are occurring simultaneously
and the actual magnitudes of these effects are not revealed by
Figure 3.
It should be noted that magnitudes of the contraction
found after aging both 0.07 percent carbon and 0.01 percent carbon
Invar for 1000 hours at 205* C (4000 F) are approximately equal.
It is believed that the contraction which takes place in both
compositions is due to the same cause, relief of residual stress.
I
b.
Aging Following Furnace Cooling from 830* C.
The effect
of furnace cooling decarburized Invar is shown in Figure 13.
Speci-
mens were heated to 8300 C (15250 F), held kD minutes, and furnace
cooled to room temoerature.
Aging was carried out at temperatures
from 200 C (680 F) to 205* C (4000 F) and specimens were air cooled
from the aging temperature for succeeding measurements of length.
The result
of the furnace cooling treatment is to virtually
eliminate the contraction effect.
Such would be the case if relief
of residual stress results in the contraction observed on aging decarburized and quenched specimens.
Furnace cooling is much more
effective in eliminating the contraction of 0.01 percent carbon than
of 0.07 percent carbon Invar.
This behavior indicates that the con-
traction effect following furnace cooling is carbon dependent to a
greater extent than the contraction following water quenching from
8300 C (15250 F).
The contraction effect following furnace cooling
Invar containing 0.07 percent carbon seems to be the same as that
which occurs on reheating this material after aging below 2050 C
(4000 F).
Since the 700 C (1580 F) expansion is carbon dependent,
it follows that its reversible counterpart, the 2050 C (4000 F)
contraction, should also be carbon dependent.
It has already been
shown that the 700 C (1580 F) expansion can occur isothermally
after .hot quenching from 2050 C (400* F) to 70* C (1580 F).
This
indicates that the 700 C (158* F) expansion may occur to some
extent during furnace cooling from 8300 C (15250 F) once a temperature below about 2050 C (400* F) is reached.
Subsequent aging would
allow the reverse reaction to occur and result in a contraction at
temperatures above about 1500 C (3000 F) as was 'found to be the
case.
-
- 45
c.
Summay.
The results ofaging 0.07 and 0.01 percent
carbon Invars (0.44 Mn, 36.8 Ni) after water quenching, cold
drawing, and furnace cooling indicate that the following effects
occur:
(1)
A contraction due to stress relief of water cuenched and
cold drawn specimens.
The magnitude of this effect in-
creases with temperature.
(2)
An expansion due to the formation of residual stress in
specimens that are quenched in water from temperatures
above about 315* C (6000 F).
The magnitude of this
expansion effect increases with temperature.
(3)
An expansion which occurs on aging below about 2050 C
(400* F).
This expansion occurs to a maximum extent on
aging at 70 0 C (1580 F).
It is due to a reversible carbon
dependent reaction.
(4)
A contraction which is the reversible counterpart of
the 700 C (1580 F) expansion.
aging above 950 C (2000 F).
This contraction occurs on
It is essentially completed
if aging is carried out at 2050 C (4000 F).
3.
Invars of VaryinF Carbon Content (0.02 to 0.58 C. 0.10 Mn.
36.0 Ni)
a.
Aging Following Furnace Cooling from 8300 C.
The effect
of aging furnace cooled Invars of varying carbon content is shown in
Table V.
Following furnace cooling from 8300 C (15250 F) to 200 C
(680 F), specimens were reheated for 1 hour at 2050 C (4000 F) and
water quenched to 200 C (680 F),
This latter treatment results: in
contractions which increase with carbon content.
Beyond about 0.40
-
- 46
percent carbon the magnitude of the contraction is approximately
constant.
After the water quench from 2050 C (4000 F)) specimens wer'e aged
for 500 hours at temperatures varying from 20* C (680 F) to 1500 C
As shown in Table V, aging has little effect on Invar con-
(3000 F).
taining 0.02 percent carbon.
carbon contents.
However, expansions result with higher
For a given aging temperature the magnitude of the
expansion increases with carbon content up to about 0.40 percent
carbon.
For a given carbon content, the aging temperature resulting
in the maximum expansion is either 700 C (1580 F) or 950 C (2000 F).
The behavior of this series of Invars again illustrates the
carbon dependence and the "reversibility" of the phenomenon res-
ponsible for the 70 0 C (1580 F) expansion.
Slow cooling from 8300 C
(15250 F) evidently allows sufficient time for this expansion effect
to occur once the temperature falls below about 2050 C (4000 F).
On reheating to 2050 C (4000 F) the reaction goes in the opposite
direction and a contraction occurs.
By water quenching to 20* C
(680 F) re-occurrence of the expansion effect on cooling is avoided.
However, on subsequent aging below 205* C (4000 F) the expansion
takes place.
The 0.02 and 0.10 percent carbon Invars (0.10 Mn, 36.0' Ni) are
respectively similar to the 0.01 and 0.07 percent carbon Invars
(0.44 MA, 36.8 Ni) as far as the 700 C (1580 F) expansion effect
is concerned.
This indicates that the manganese content of Invar
is not significant at least up to 0.44 percent.
However, it is be-
lieved that higher manganese contents affect the 700 C (1580 F)
expansion.
It was found that this phenomenon does not occur in a
free machining grade of Invar containing 0.07 percent carbon, 0.88
percent manganese, and 0.16 percent selenium.
Assuming the selenium
-
- 47
TABLE V
DIMENSIONAL CHANGES ON AGING FURNACE COOLED* INVARS
OF VARYING CARBON CONTENT
Carbon
Content
Percent
0.02
Microinch Per Inch Length Change
1 hour**
500 hro. 500 hrs. 500 hrs. 500 hrs. 500 hrs.
at 205 0C
at 500C at 7000 at 950C at11QC
at 200
0
-5
-7
-
2
- 10
- 2
0.10
-32
-5
+29
+ 60
+ 33
+19
0.15
-55
+18
+38
+ 90
+ 40
+25
0.25
-80
+28
+80
+ 95
+105
+41
0.40
-105
+33
+95
+115
+118
+50
0.58
-110
+38
+79
+130
+100
+60
-* 8300 C, furnace cooled to 200 C.
*3*
All specimens were reheated for 1 hour at 2050 C, water quenched
to 200 C, and measured prior to aging for 500 hours at 20 to
1500 C.
48-
is all combined with manganese in the form of MnSe in this grade of
Invar, there would be 0.76 percent "uncombined" manganese, presumably
in solid solution in the austenitic phase.
It appears, therefore,
that a minimum manganese content which is between 0.44 and 0.76
percent is necessary to suppress the 700 C (1580 F) expansion.
b.
Aging Following Quenching in Water from 8300 C.
The
effect of aging quenched Invars of varying carbon content for 350
hours at aging temparatures up to
VI.
5400
C (10000 F) is shown in Table
As was found to be the case for Invar containing 0.07 percent
carbon, 0.44 percent manganese, and 36.8 percent nickel, aging of
this series of varying carbon content at low aging temperatures
results in dimensional changes which are due to the combined action
of stress relief and the 700 C (1580 F) expansion effect; whereas
at high aging temperatures, both stress relief and residual stress
formation on subsequent cooling occur.
However, on aging above
3150 C (6000 F) extremely large contractions occur which cannot be
explained on the basis of stress relief alone.
For a given carbon
content, the magnitude of the contraction is a maximum at
425* C
These large contractions are definitely carbon dependent
(8000 F).
and could be due to the precipitation of carbide and/or graphite
from the austenitic solid solution.
c.
Aging Following iuenching from 8300 C and 12050 C.
The effect of water quenching from 8300 C (15250 F) and 12050 C
(22000 F) on dimensional change
of Invars of medium carbon content
aged at 700 C (1580 F) is shown in Table VII.
49
-
-
TABLE VI
DIMENSIONAL CHiANGES ON AGING QUENCHED* INVARS
OF VARYING CARBON CONTENT
Carbon
Content
Percent
Microinch Per Inch Length Change after 350 Hours at
700 C
95* C
0.02
-14
-
9
-71
-71
-
0.10
+ 55
+ 53
-40
-53
- 152
- 35
0.15
+ 83
+ 96
-32
-51
- 320
-
0.25
+143
+126
-25
-54
- 549
- 357
0.40
+135
+110
-24
-69
-1548
-1420
0.58
+138
+121
-21
-53
-3202
-1823
Aging
205* C
Temperature
3150 C
4250 C
Quenchrin rater from 8300 C to 200 C.
*
*
Air cooled from aging temperature to 200 C.
40
5400 C
+ 45
58
-
- 50
TABLE VII
DIMENSIONAL CHANGES ON ACTING QUENCHED* INVARS OF
MEDIUM CARBON CONTENT AT 700 C
Carbon
Quenched
Content
Percent
From
0C
0.25
830
178
0.25
1205
152
0.40
830
140
0.40
1205
180
0.58
830
143
0.58
1205
173
*
Microinch Per Inch Length Change
After Aging 500 Hours at 700 C
Quenched in water to 200 C.
These results reveal that the 700 C (1580 F) expansion is
greater after water quenching from 1205* C (22000 F) than from
8300 C (1525* F).
It also appears that for a 12050 C (22000 F)
quench, the magnitude of the 700 C (1580 F) expansion increases with
carbon content; whereas for a 8300 C (15250 F) quench, the expansion
is approximately constant beyond about 0.25 percent carbon.
The
explanation of these results could be that the solubility of carbon in the austenite phase increases with temperature and th.t the
magnitude of the 700 C (1580 F) expansion is largely dependent on
the amount of carbon in solid solution.
d.
Su~may.
The results of aging 0.02 to 0.58 percent
carbon Invars (0.10 Mn, 36.0 Ni) after water quenching and furnace
cooling indicate that stress relief, formation of residual stress,
the 700 C (1580 F) expansion, and the 2050 C (4000 F) contraction
occur.
The carbon dependence of the 700 C (1580 F) expansion
and the 205* C (4000 F) contraction extends beyond the 0..07 percent
-
- 51
carbon content dealt with in the previous section.
Large con-
tractions occur on aging above about 3150 C (6000 F).
This effect
increases with carbon content and is believed due to the precipitation of carbide and/or graphite.
B.
Drop Tests
The results of drop tests carried out on both quenched and furnace
tooled specimens of Invar (0.070, 0.44 Mn, 36.8 Ni) 0.250 inch in
diameter are shown in Figure 14.
These results show that increasing
contractions result from repeated dropping of quenched specimens
whereas practically no dimensional change results from dropping of furnace
Oooled-
specimens.
After ten drops, an average contraction of 375
microinches per inch occurred in the quenched specimens.
This is
much larger than the extent of the contractions observed on aging
quenched specimens.
Specimens of quenched Invar which had been subjected to 10
drops were subsequently aged at temperatures up to 315? C (6000 F).
A comparison of the dimensional changes resulting from aging specimens
which were "dropped" and specimens which were not "dropped" is given
in Table VIII.
As shown in Table VIII, the effect of dropping specimens is to
substantially reduce the contraction or stress relief effect that
occurs on subsequent aging of quenched Invar.
specimens actually results
Aging of "dropped"
in immediate expansions on aging at
both 700 C (1580 F) and 150* C (3000 F).
At 3150 C (6000 F),
"dropped" specimens undergo less than one-half the contraction as
compared with specimens that are not "dropped".
-
- 52
TABLE VIII
DIMENSIONAL CHANGES ON AGING QUENCHED*
INVAR SUBJECTED TO DROP TESTS
Aging Temperature*
Condition
Microinch Per Inch Length Change
0.2 hrs. 1 hr. 10 hrs, 100 hrs.
Dropped
10 times
700 C
+ 2
+9
+ 9
+ 22
Not Dropped
700 C
- 4
- 5
+3
+ 15
Dropped
10 times
1500 C
+8
+ 8
+12
+ 10
Not Dropped
1500 C
-32
-32
-35
- 42
Dropped
10 times
3150 C
-25
-25
-23-
- 23
Not Dropped
3150 C
-67
-67
-62
-107
*
Water quenched from 830' C to 200 C.
*
Specimens were air cooled from aging temperature to 200 C.
C.
Residual Stress
1.
Complete Stress Distribution.
The distribution of residual
stress in quenched Invar (0.07 C, 0.44 Mn, 36.8 Ni) as determined
by the Heyn analysis is shown in Figure 15 which also shows the
magnitude of the successive contractions which resulted from reduction of the cross section area by machining.
As calculated from
the slope of the change in length curve, water quenching Invar
from 8300 C (15250 F) to 20* C (680 F) results in residual compression
at the surface and residual tension at the center of a 0.250 inch
-
- 53
diameter specimen.
The aagnitudes of the residual stress at the
surface and center were both found equal to about 22,500 psi.
The
value of the residual stress at the center was determined by extrapolation because of difficulty in reducing the cross section area
below 0.012 square inch.
Extrapolation of the stress curve for
smaller cross sections than 0.012 square inch was carried out by
making the area under the curve for residual compressive stress approximately equal to that for residual tensional stress. These areas
should be equal since the condition of static equilibrium is that
the exterior compressive force equals the interior tensional force.
Since the Heyn analysis for residual stress is only approximAte,
it may-be that the residual stress curve is actually a straight line
instead of varying slope as shown.
This is suggested by the fact
that the surface and center stresses are approximately equal and
that the change from compression to tension occurs at approxima.tely
the mid-area point.
2.
6urface Stress Values.
The effect of various treatments
on the residual stress at the surface of 0.375 inch diameter Invar
specimens containing 0.02 and 0.40 percent carbon (0.10 Mn, 36.0 Ni)
is shown in Table IX.
(680 F) results
Water quenching from 8300 C (15250 F) to 200 C
in approximaitely the same magnitude of residual com-
pressive stress as was found at the surface of 0.250 inch diameter
Invar (0.07 C, 0.44 Mn, 36.8 Ni) given the same treatment.
iIging
one hour at 3150 C (6000 F) results in considerable stress relief,
which accounts for the observed contraction.
times also results in stress relief.
Dropping specimens 10
The residual stress after
dropping is approximately the same as after a 3150 C (6o0o
F)
54
-
-.
TABLE IX
RESIDUAL STRESS AT SURFACE OF 0.375 INCH DIAMETER
BARS OF INVAR SUBJECTED TO VARIOUS TREATMFNTS
Change in Length
Due to Last Step
Carbon
Content
Treatment
of Treatment
Microinches per Inch
Residual Compressive
Stress at Surface
Lbs. oer So. In.
0.02
8300 C, water quench
24,000
0.40
8300 C, water quench
21,000
0.02
a) 8300 C, water quench
b) 3150 C, 1 hour, air cool
0.40
-325
6,000
-368
13,000
+415
13,000
+296
13,000
a) 83 0 0 C, furnace cool
b) 540* C, 1 hour, water cuLench
0.40
10,000
a) 8300 C, water quench
b) Dropped 10 times
0.02
-126
a) 8300 C, water quench
b) Dropped 10 times
0.40
7,000
a) 8300 C, water quench
b) 3150 C, 1 hour, air cool
0.02
-114
a) 83 0 0 C, furnace cool
b) 5400 C, 1 hour, water qu ench
-
55,
aging treatment; however, dropping results in approximately three
times as large a contraction as does aging at 3150 C (6000 F).
It
should be noted that greater stress relief occurs in the 0.02 than
in the 0.40 percent carbon Invar as a result of either aging or
dropping.
This behavior may be due to the lower elastic limit of
the lower carbon material.
Attempts to determine the surface residual stress in furnace
cooled Invar gave erratic results.
This could be due to the low
resistance to plastic deformation of furnace cooled Invar, which
may allow residual stress to be introduced by the machining operation.
Reheating furnace cooled specimens to 5400 C (10000 F) and water
quenching to 200 C (680 F) results in a large expansion as has been
noted previously.
That this expansion is associated with the intro-
duction of residual stress is shown in Table IX .
A surface residual
stress of 13,000 psi. in compression results from this treatment.
This residual stress is about one-half that resulting from water
quenching from 8300 C (15250 F) to 200 C
(680 F), which corresponds
to a more drastic cooling treatment.
D.
X-rM
1.
Measurements
Determination of Solid Solubility of Carbon.
A plot of
lattice parameter vs. carbon content of Invar (0.44 Mn, 36.8 Ni)
obtained by heating 500 hours at 8300 C (15250 F), 7050 C (13000 F),
and 5400 C (10000 F) is shown in Figure 16.
features of this plot:
There are two distinctive
a) an increase in lattice parameter with
carbon content up to the solubility limit at a given temperature;
and b) constancy of lattice parameter for carbon contents exceeding
the solubility limit at a given temperature.
From the intersections
-56-
of the straight portions of the plot with the sloped portion for
low carbon contents, solubility limits at the three temperatures studied
were determined.
Plotting solubility limit vs. temperature gives the
solubility curve shown in Figure 15.
This curve was extrapolated to
lower temperatures on the basis that for an ideal solution the
logarithm of the mole fraction of solute is proportional to the
reciprocal of the absolute temperature.
The result of the ex-
trapolation shows that carbon is practically insoluble at 200 C
(680 F).
The fact that the austenitic lattice parameter is constant for
carbon contents exceeding the solubility limit indicates that the
second phase at temperatures at least above 5400 C (10000 F) must
either be graphite or a carbide that has approximately the same iron-
nickel ratio as does the solid solution.
An iron-nickel carbide
corresponding to (Fe2 NiQ . fulfills this description; however, the
existence of such a carbide has not been established.
The variation in lattice parameter of Invar austenite with
carbon content as determined from Figure 16 amounts to 0.00053 angstrom
per 0.01 percent of carbon.
This information enables one to cal-
culate the overall change in length resulting from precipitation
of a carbon-containing phase from solid solution.
Such calculations
are given in Appendix B, C, D, and E assuming the formation of
cementite (Fe3 C), graphite, nickel carbide (Ni3 C), and a hypothetical
iron-nickel carbide (Fe-,NiC) respectively.
These calculations are
based on the formulae and data given in Appendix A.
In each case it
is assumed that an Invar containing 0.05 percent carbon, 36.0 percent nickel, and 63.95 percent iron initially entirely austenitic
precipitates all of its carbon in the form of the second phase.
The
-
- 57
calculated overall changes in length are as follows:
Precipitated Phase
Microinch per Inch Length Change Resulting
from Precipitation of 0.05 percent Carbon
Cementite
-410
Graphite
-170
Nickel Carbide
-600
Iron-Nickel Carbide
-440
The results of these calculations'show thAt a contraction should
occur if any of the above phases precipitate.
The contraction is
smallest for the precipitation of graphite because it has the largest
specific volume.
These results indicate that precipitation of either
graphite or carbide cannot account for the 700 C (1580 F) expansion.
The relatively small contraction effect associated with the
precipitation of graphite makes it improbable that this is the sole
explanation of the large contraction effects shown in Table VI for
aging at temperatures above 3150 C (6000 F).
For example, on aging an
Invar containing 0.58 percent carbon at .4250 C (8000 F), even if
all the carbon in excess of the solubility limit came out in the
form of graphite, a contraction of about 500 microinches per inch
should result.
This is much smaller than the observed contraction
of about 3,200 microinches per inch actually observed.
It is more
reasonable to assume that the contraction is due to precipitation of
carbide out of solid solution.
2.
202 C.
Changes in Lattice Parameter due to AginZ at 700 C to
The effect of aging Invars (0.44 Mn, 36.8 Ni) of varying
-
- 58
carbon content for 500 hours at 700 C (1580 F), 950 C (2000 F), and
205* C (4000 F) on the lattice parameter of the austenitic phase is
shown in Table X.
The initial treatment was to quench in water from
8300 C (15250 F) to 200 C (680 F).
The results shown in Table X
indicate that there is no significant change in lattice parameter
for carbon contents up to about 0.13 percent.
With higher carbon
contents, aging at 70* C (1580 F) results in significant increases
in lattice parameter as compared to the as-quenched condition.
Smaller increases were found on aging at 950 C (2000 F); whereas,
aging at 2050 C (4000 F) produced little or no change.
From a comparison with Table VI for aging temperatures up to
2050 C (4000 F), it appears that the changes in austenite parameter
referred to the as-quenched condition vary in the same manner as
the overall length changes.
In order to compare the magnitudes of the
lattice parameter and length change for aging at 700 C (1580 F),
average values were determined for carbon contents ranging from 0.25
to 0.63 percent.
The average lattice parameter change as calculated
from Table X is an expansion of 0.0008 angstrom , which is equivalent
to a length change of +220 microinches per inch.
This is in good
agreement with the average overall length change of about +160
microinches determined from Table VII allowing for a stress relief
effect of -16 microinches per inch which is the magnitude of the
contraction resulting from aging the 0.02 percent carbon Invar at
700 C (1580 F).
On the basis of these results it appears that the
overall increase in length resulting from the 700 C (1580 F) expansion effect is due primarily to the change in parameter of the
austenite.
This indicates that the 700 C (1580 F) expansion is due
-
- 59
TABLE X
CHANGE IN LATTICE PARAMETER OF AUJTENITIC PHASE
ON AGING INVARS OF VAKING CARBON
CONTENT
Carbon
Content
Percent
Lattice Parameter in Angstroms
500 hours
500 hours
500 hours
As Quenched* at 700 C
at 950 C
at 2050 C
0.01
.3.5937
-3.5939
3.5941
3.5941
0 .07
3.5963
3.5964
3.5962
3.5962
0 .13
3.5991
3.5994
3.5995
3.5994
0 .27
3.6025
3.6029
3.6030
3.6025
0.42
3.6023
3.6032
3.6028
3.6024
0 .50
3.6020
3.6031
3.6025
3.6024
C .63
3.6025
3.6032
3.6028
3.6023
o .84
3.6014
3.6020
3.6019
3.6010
*
Quenched in water from 830* C to 20* C.
- 6o-
to what is happening to the austenitic solid solution rather than
being associated with a phase transformation.
As will be discussed
later, this reversible carbon dependent reaction is believed due to
clustering of solute atoms within solid solution as a result of a preprecipitation process.
Such clustering is known as Guinier-Preston
zones.
Debye Patterns of Electrolytic Extractions.
3.
In oiu er to
check whether the 700 C (1580 F) expansion is associated with the
formation of carbide or grqwhite, x-ray diffraction patterns were
taken of the residue obtained by electrolytic extraction of an Invar
containing 0.15 percent carbon.
This carbon content was telected
in order to maximize the 700 C (1580 F) expansion without exceeding the
solubility limit at 830" C (15250 F) from which quenching was carried
Electrolytic extractions were made after the following heat
out.
treatments:
(a)
quenching in water from 8300 C (15250 F) to 200 C
(680 F); and (b) aging for 350 hours at 700 C (158* F) after treatment (a).
Diffraction lines were found after each treatment.
These lines
could not be identified as either cementite, graphite, or nickel
carbide.
From prior experience with difficulties involved in
electrolytic extractions, it is believed that the patterns may be
due to oxide or hydrate phases which form during the extraction.
4.
Existence of Martensite or Ferrite.
In order to determine
whether transformation of austenite to either ferrite or martensite is
responsible for the 700 C (1580 F) expansion, long time exposures were
made of specimens of Invar containing 0.40 percent carbon using a
modified Debye camera and monochromatized iron radiation.
Specimens
a) quenched in water from 830" C (15250 F) to 20" C (68" F), b) quenched
-
- 61
and aged 500 hours at 700 C (1580 F), and c) furnace coolpd from.
8300 C (1525* F) to 200 C (680 F) were x-rayed.
In the case of
each treatment, only lines resulting from the austenitic phase appeared.
The absence of martensite or ferrite lines does not necessarily
prove that these phases are absent since the sensitivity of the
method used is about 0.5 percent.
Furthermore, if either phase
were present in the form of extremely fine particles in amounts
even above 0.5 percent, the resultingc diffraction lines might be so
diffuse as to be undetectable.
Therefore, all thi t can be said is
that there is no direct evidence for the existence of either martensite or ferrite in 0.15 percent carbon, Invar.
The possibility that an isothermal transformiation of austenite
to
martensite results in the 700 C (1584 F) expansion cannot be
reconciled with the carbon dependence of this phenomenon.
Increasing
amounts of carbon in solid solution should stabilize the austenite
to an increasing extent and thereby decrease the extent of transformation, whereas the 700 C (1580 F) expansion actually increases
with increasing carbon content.
Furthermore, on the basis of a
martensite reaction it is difficult to understand why no such expansion occurs in Invar of very low carbon content.
The carbon dependence of the 700 C (1580 F) expansion is also
in conflict with the possibility of a transformation of austenite to
ferrite.
It would be expected that increased carbon content would
make the austenite more stable with respect -to transformation to
to ferrite.
Furthermore, there is some question as to whether such
-
- 62
a transformation would result in an overall expansion or contraction.
According to the calculations made in Appendix F, the length changes
associated with the formation 1, 10, and
45 percent ferrite con-
taining 5 percent nickel are as follows:
Amount of Ferrite
Overall Length Change
Present
Microinches per Inch
11
+160
10
+930
45
-3,000
These calculations neglect the effect of carbon content and
assume that in the austenite that remains untransformed sufficient
diffusion occurs to make both the nickel and iron contents uniform
throughout the solid solution.
On this basis small amounts of
transformwtion to ferrite result in expansions whereas large amounts
result in contractions.
In view of the low aging temperatures at
which the 700 C (1580 F) expansion occurs it is doubtful whether the
assumption of complete homogenization of the untransformed austenite
by diffusion is a cgood one.
It is more likely that due to
limited diffusion only the chemical composition of the austenite in
the immediate vicinity of the ferrite formed would be affected
by the transformation.
Assuming that both the ferrite formed and
the affected austenite are at the ecquilibrium compositions (5 and
58 percent nickel respectively), the overall change would be a
contraction.
The magnitude of the contraction would depend on how
much ferrite formed.
If equilibrium conditions could be attained
throughout, 45 percent ferrite would form according to the ironnickel equilibrium diagram of Owen and Sully
and an overall contraction
of 3,000 microinches per inch would :esult.
-
63
-
On this basis, a
contraction of about 67 microinches per inch should occur for each
one percent of ferrite formed.
On the basis of the role carbon plays in iron-carbon austenite
it seems more logical to assume that the iron-nickel-carbon austenite
of Invar would transform to a ferrite-carbon phase aggregate rather
than to ferrite alone.
This type of reaction would be in accord with
the carbon dependence of the 7Q0 C (1580 F) expansion since the
transformation of iron-carbon austenite to pearlite, a ferritecementite aggregate, is speeded up by increasing carbon content.
However, as has been calculated, the formation of either graphite
or carbide alone results in a contraction.
Since forming ferrite
is also likely to result in a contraction, it is doubtful whether
the 700 C (1580 F) expansion can be explained on the basis of the
formation of either a ferrite-carbide or ferrite-graphite aggregate.
E.
Metallographic Examination
Metallographic examination was carried out on specimens con-
taining 0.15 and 0.58 percent carbon.
All specimens were water
quenched from 8300 C (15250 F) to 200 C (680 F).
Aging was carried
out for 500 hours at temperatures up to 6500 C (12000 F).
In the as-quenched condition, the structure of the 0.15 percent
carbon Invar consists of twinned grains of austenite whereas the 0.58
percent carbon Invar consists of twinned grains of austenite plus
streaks of graphite elongated in the longitudinal direction (Figures
23A and 23B).
Chemical analysis of the 0.58 percent carbon Invar
-
- 64
showed the presence of 0.39 percent graphite, which is approximately
equal to the difference between the total carbon content and the
solubility limit.
No graphite was found in the 0.15 percent carbon
Invar by chemical analysis.
Examination failed to show any significaat difference in microstructure between specimens in the as-quenched and quenched-and-aged
at 70 0 C (1580 F) conditions regardless of carbon content.
Thus
there is no metallographic evidence that would associate the 70 0 C
(158* F) expansion effect with a phase transformation.
Aging at higher temperatures results in no significant change in
microstructure up to 4250 C (8000 F).
At this temperature precipi-
tation of a carbide phase occurs at the grain boundaries (Figure 23C).
This effect occurs at the temperature of maximum contraction as
shown in Table VI.
Although a greater contraction occurs in the
0.58 percent carbon Invar as compared with the 0.15 percent carbon
Invar, less grain boundary carbide precipitation was found in the
higher carbon material.
It is therefore believed that precipitation
of carbide occurs to a larqe extent within the grains of the 0.58
percent carbon Invar.
Aging above 4250 C (8000 F) also results in grain boundary
carbide precipitation; however, the extent of this precipitation
decreases with temperature.
This is in line with the results of
Table VI, which show a decrease in the magnitude of the contraction
above 4250 C ,(8000 F).
After aging at 6500 C (12000 F) practically
no grain boundary precipitate is visible in either composition.
On aging quenched Invar having excess graphite present at
4250 C (8000 F), there is opportunity for precipitation of carbide
both at the grain boundaries and within the grains.
The presence
65
-
-
of graphiGt. is
believed to provide lIterfaces whicn lower the surface
energy required for precipitation within the grains.
This could ex-
plain why less grain boundary carbide precipitation occurs in the 0.58
perc.ent carbon Invar even though there is a greater total amount of
precipitation as compared with 0.15 percent carbon Invar.
F.
Hardness Tests
1.
Rate of Cooling from 8300 C.
The effect of rate of cooling
from 8300 -C (1525O F) on the Rockwell B hardness of Invars (0.10 Mn,
36.0 Ni) of varying carbon content is shown in Table XI.
For each
carbon content, the hardness in the as-received or cold worked condition is higher than in the heat treated condition.
The chief effect
of a 8300 C (15250 F) treatment is to result in recrystallization
and substantial softening.
The hardness of Invar containing 0.02 per-
cent carbon is not sigrificantly affected by the rate of cooling from
8300 C (1525 -F).
However, rate of cooling does affect the hardness
of higher carbon Invar.
For a given carbon content there is practi-
cally no difference between water quenching and air cooling; whereas,
furnace cooling results in decreased hardness to a significant extent
in the range of about 0.10 to 0.25 percent carbon.
For both water
ouenching and air coolin: the variation of hardness with carbon content shows a maximum at about 0.25 percent carbon.
For furnace
cooling the maximum is displaced to about 0.+0 percent carbon.
2.
Teperature and Time ofAging.
The effect of aging tempera-
ture and time of' aging on the hardness of Invars (0.10 Mn, 36.0 Ni)
of varying carbon content initially water quenched from 8300 C
(15250 F) to 200 C(680 F) is show~n in Table XIT.
These results
show that the hardness of quenched Invar is not significantly affected
by aging as long as 500 hours at temperatures up to 3150 C (6000 F).
In particular, there is no hardness change associated with the 700 C
-
- 66
TABLE XI
VARIATION OF HARDNESS OF INVARS OF VARYING
CARBON CONTENT WITH RATE OF COOLING FROM 8300 C
Carbon
Content
Percent
*
Ro ckwell B Hardness
Water
Air
Cold Drawn* Quenched
Cooled
Furnace
Cooled
0.02
89
69
70
69
0.10
96
77.5
78.5
74
0.15
99,5
82
82
76
0.25
102.5
86.5
87
81.5
0.40
101.5
84
84
83
0.58
101.5
83
82.5
82
0.74
81
82
79
0.99
82
80.5
79
As received condition:
8000 C, quenched in rater to 200 C,
cold drawn (27 percent reduction in
area).
-
- 67
TABLE XII
CHANGES IN HARDNESS ON AGING QUENCHED* INVARS
OF VARYING C.ARBON CONTENT
Carbon
Content
Percent
Aging**
Time
Hours
200 C
0.02
1
69
Rockwell B Hardness
10
100
500
1
0.10
70
C
69
77.5
10
100
77.5
500
1
0.15
82
1
0.40
**
82
82
81.5
82.5
77.5
78.5
82
82.5
84
82.5
83.5
84
78
78
78.5
79
78.5
83.5
84.0
84.5
85.5
84.5
84.5
84.5
85.5
84.5
85.5
84.5
82.5
81
78.5
77
-77.5
82.5
83.0
83.5
84.5
83
83.5
83.5
81
82
82
82
81
84
83
80.5
1
10
100
*All
82
82
78
79
100
500
500
82
78.5
78.5
78.5
79.5
87
100
0.99
80.5
78.5
79
80
81
71
86
80.5
78
79.5
500
10
80
71
71
72
87
10
100
0.74
78
70.5
72
72
72
87
87.5
87.5
88
500
1
78
79
79
70
71.5
71.5
70
70,
87.5
87.5
87.5
88
10
100
0.58
69.5
70
70.5
86.5
87
87.5
87.5
10
100
500
69.5
70
70
71
81.5
1
0.25
31*2C 425_ C 540 C 6500 C
82
83
84
10
100
500
2050 C
81-5
specimens were initially
83
83
83.5
82
80
79
78.5
81
81
81.5
82
78
81
76
81.5
81
81
81.5
81.5
82
80
79.5
81
79
77
75
87
77.5
.77
78
80.5
79
83
78.5
75.5
77
79
77
76
76.5
81.5
76.5
74
75
76
73
73.5
81
77.5
71.5
71.5
78.5
81
77
70.5
70
quenched in water from 8300 C to 200 C.
All specimens were quenched in water from aging temperature to 200 C.
-
- 68
(1580
F) expansion.
At higher aging temperatures than 3150 C (6000 F) decreases ia
hardness occur.
The higher carbon Invars soften to a greater extent
For a given carbon content, the time
than the lower carbon Invars.
rate of hardness change increases with aging temperature.
The most
rapid changes occur at 6500 C (12000 F) which was the highest aging
temperature investigated.
In order to explain these hardness results, the following
Precipitation of carbide at the grain
considerations are involved.
boundaries at 425* C (8000 F) should result in an increase in
hardness but this is opposed by a hardness decrease due to lowering
of the carbon content.
The net result of these effects is a
retardation of softening in the lower carbon Invars and actual
softening in the higher carbon Invars in which most of the precipitation occurs within the grains.
The reverse situation occurs on
aging above 4250 C (8000 F) where re-solution of the grain
boundary carbide was observed.
In this case the hardness decrease
due to removal of the grain boundary precipitation is opposed by
the hardness increase due to increased carbon content of the
austenite.
Since the overall effect is. a decrease in hardness
when precipitation occurs
in the higher carbon Invars, an overall
increase in hardness would be expected on re-solution.
However,
what actually occurs is a decrease in hardness on aging the higher
carbon Invars above 425* C (8000 F).
This could be explained if
simultaneous precipitation of graphite, the more stable phase,
occurs
during the re-solution of the carbide phase.
Precipi-
tation of graphite would lower the hardness by decreasing the
carbon content of the austenite and due to the fact that graphite
-
- 69
is a phase of extremely low hardness.
This explanation is in
accord with what occurs during the graphitization of white cast iron.
On this basis, the decrease in the contraction which occurs on
aging above 4250 C-,(8000 F) can also be explained.
Because of the
ligher specific volume of graphite as compared to carbide, the resultant conversion of carbide to graphite should result in an expansion ;r what amounts to the same thing, a decreased contraction
starting from the as-quenched condition.
The austenite solid
solution could be continually decreasing in carbon content while
the process of carbide re-solution and graphite precipitation was
occurring.
At a high enough aging temperature re-solution of
graphite should occur due to the increase in solubility of carbon
with temperature.
The hardness results indicate why furnace cooling results in
lower hardness than water quenching from 8300 C (15250 F).
EvidenQLy
slow cooling allows Invar to spend sufficient time at temperatures
in the vicinity of 6500 C (12000 F), which allows softening to
occur. presumably due to both for.iation of graphite and lowering
of the carbon content of the austenite.
3.
Aging Followin=
Quenching from 830* C and 1205* C.
The
effect of aging at 700 C (158* F) following water quenching from
8300 C (15250 F) and from 12050 C (22000 F) on the hardness of
Invars of medium carbon content is shown in Table XIII.
The rasults shown in Table XIII indicate that the hardness of
Invars whose carbon contents exceed the solubility limit at 8300*C
(15250 F) can be increased by quenching from 12050 C (22000 F) to
200 C (680 F).
Heating to the higher temperature allows more carbon
-
- 70
to go into solid solution and the hardness consequently increases.
Aging to result in the 70' C (1580 F) expansion effect does not
change the hardness to a significant extent.
TABLE XIII
CHANGES IN HARDNESS ON AGING QUENCHED INVARS OF MEDIUM
CARBON CONTENT AT 70 0 C
Carbon
Content
Rockwell B Hardness
Quenched in Water from 8300 C
Quenched in Water from 12050C
Aged at 700 C
Aged at 70 0 C
Percent.
AsQuenched
for 500 hours
As Qienched
for 500 hours
9.25
86
86
85
85.5
0.40
84
84
87.5
89-
0.58
83
83
90.5
91.5
G.
Magnetic Tests
1.
Determination of Saturation Field.
A plot of intensity of
magnetization (J) vs. magnetic field intensity (H) for quenched
Invar (0.07 C, 0.44 Mn, 36.8 Ni) at a temperature of 130 C (550 F)
is shown in Figure 18.
of
This plot shows that a saturation intensity
magnetization (J) of approxinmately 1000 gauss is obtained with
an applied magnetic field intensity (H) of 500 oersteds or over.
In order to insure saturation, a magnetic field intensity (H) of
1100 oersteds was used in all the magnetic tests subsequently des-
cribed.
2.
Variation of Mapnetization with Termperature.
of intensity of
The variation
magnetization with temperature of furnace cooled
Invar (0.07 C, 0.44 Mn, 36.8 Ni) is shown in Figure 17.
Similar
results were found for water ciuenched and for cold dramn Invar.
A
comparison of saturation J values at 200 C (680 F) and Curie points
-
- 71
for a) cold drawn, b) water quenched, and c) furnace cooled Invar is
given in Table XIV.
Curie points were determined by extrapolation
as indicated in Figure 19.
TABLE XIV
VARIATION OF BOTH MAGNETIZATION AND CURIE POINT OF LdW
CARBON INVAR WITH TREATMENT
Treatment
Saturation Intensity of
Magnetization (J) at 200 C,
Gauss
Curie Point OC
Cold drawn
(25 percent reduction
1000
272
8300 C, water quench
1010
268
8300 C, furnace cool
1020
266
in area)
As shown in Table XIV, the magnetic properties of low carbon
Invar are only slightly altered by heat treatment or cold working.
It is doubtful whether cold working affects the magnetic condition
sufficiently to result in an appreciable change in coefficient of
expansion.
The effect of carbon content on variation of saturation J with
temperature is shown in Figure 20.
Specimens of Invar (0.10 Mn,
36.0 Ni) were water quenched from 8300 C (1525* F) to 200 C (680 F)
and tested in this condition.
Both magnetization at a given tempera-
ture and the Curie point increases with carbon content.
The Curie
points for 0.02, 0.10, and 0.40 percent carbon are 2480 C (4800 F),
2550 C (4920 F), and 2700 C (518* F) respectively.
This amounts to
an average increase ofabout 0.60 C (1.00 F) in the Curie point per
0.01 percent carbon.
This variation in the Curie point may be
associated with the reported increase in coefficient of thermal ex-
pansion with carbon content.
-
, 72
3.
from 8300 C.
Rate of Coolin
The effect of rate of cooling
from 830* C (1525* F) on intensity of magnetization at 200 C (68* F)
of Invars (0.10 Mn, 36.0 Ni) of varying carbon content is show.n in
Table XV.
Up to 0.25 percent carbon, the magnetization in the as-
received (cold drawn) condition is lower than after a 8300 C
(15250 F) heat treatment.
Furnace and air cooling from 8300 C
(15250 F) are equivalent up to about 0.25 percent carbon.
With higher
carbon contents, furnace cooling results in higher magnetization
than does air cooling.
In all cases, water quenching from 8300 C
(15250 F) results in lower magnetization than do furnace or air
cooling.
For a given treatment, the magnetization increases with
carbon content up to 0.40 percent carbon.
4.
Aging Following Furnace Cooling from 8300 C.
The effect of
reheating furnace cooled Invar (0.10 Mn, 36.0 Ni) for 1 hour at 2050 C
(400* F), water auenching to 200 C (680 F), and then aging for 500
hours at 70 0 C (1580 F) on intensity of magnetization at 200 C
(680 F) is shown in Table XVI,
For carbon contents above 0.02
percent, the 205* C (4000 F) treatment results in lowered magnetization.
Aging at 700 C (158* F) for 500 hours restores the magne-
tization to about the furnace cooled value in all cases.
It
therefore appears that an increase in magnetization is associated
with the 70* C (1580 F) expansion.
Since this phenomenon also
occurs during furnace cooling from 8300 C (15250 F) to 200 C (680 F)
the higher magnetization resulting from furnace cooling as compared
with water quenching can be at least partially accounted for.
Since reheating furnace cooled Invar to 2050 C (4000 F) results in a
contraction which is the reversible counterpart of the 700 C
-
- 73
TABLE XV
VARIATION OF MAGNETIZATION OF INVARS OF VAIrING
CARBON CONTENT WITH RATE OF COOLING FROM 8300G
Carbon
Content
Percent
Saturation Intensity of Magnetization (J) at 200 C in
Gauss
Air Cooled
Cold Drawn* Water Quenched
Furnace Cool d
0.02
970
995
1005
1005
0.10
1015
1030
1040
1040
0.15
1025
1030
1040-
1040
0.25
1050
1050
1070
1060
0.40
1065
1060
1070
1100
0.58
1035
1020
1030
1060
*
As received condition:
8000 C, water quenched to 200 C, cold
drawn(27 percent reduction in area).
74
-
-
TABLE XVI
CHANGES IN MAGNETIZATION ON AGING FURNACE COOLED*
INVARS OF VARYING CARBON CONTENT
Carbon
Content
Percent
Saturation Intensity of Magnetization (J) at 200C in Gauss
2050 C. Water Quench 500 hrs. at 70 0 C
Furnace Cooled*
0.02
1005
1010
1005
0.10
1040
1035
1040
0.15
1040
1030
1040
0.25
1060
1050
1070
0.40
1100
1085
1105
0.58
1060
1045
1070
*
Furnace cooled from 8300 C to 200 C.
75
-
-
(1580 F) expansion, the reduction in magnetization from the annealed value can be understood.
5.
Aging Following Quenching from 8300 C.
The effect of
aging following water quenching from 8300 C (1525* F) to 200 C
(680 F) on the
iatensity of magnetization of Invars of varying
carbon content is shown in Table XVII.
As was expected, due to tae
700 C (1580 F) expansion effect aging below 2050 C (400* F) results
in an increase in the magnetization in Invars of all carbon contents with the exception of the lowest.
Following aging at
2050 C (4000 F) the magnetization is only slightly changed from
the as-quenched value.
6.
Aging Following Quenching from 8300 C and 12050 C.
The
effect of water quenching from 8300 C (15250 F) and from 12050 C
(22000 F) to 200 C (68* F) and then aging at 700 C (1580 F) on
intensity of magnetization of Invars of medium carbon content is
shown in Table XVIII.
-
- 76
TABLE XVII
CHANGES IN MAGNETIZATION' ON AGING QUENCHED
INVARS OF VARYING CARBON CONTENT
Saturation Intensity of Magnetization (J)
at 200 C in Gauss
Carbon
Content
Percent
Aged 150 Hours
As
Quenched*
2050 C
700 C
0002
995
985
1000
1000
0.10
1030
1040
1035
1030
0.15
1035
1050
1050
1040
0.25
1050
1080
1080
1060
0.40
1060
1090
1085
1070
0.58
1025
1055
1050
1035
*
Quenched in water from 8300 C to 20* C.
-
- 77
TABLE XVIII
CHANGES IN MAGNETIZATION ON AGING QUENCHED INVARS OF
MEDIUM CARBON CONTENT AT 700 C
Carbon
Saturation Intensit- of Magnetization (J) at 200 C in Gauss
Quenched in water from 8300C Quenched in Water from 12050C
Content
Percent
As Quenched
Aged at 700 C
for 500 hours
0.25
1050
1080
1055
1085
0.40
1060
1090
1090
1130
0.58
1025
1055
1085
1125
As quenched
Aged at 700 C
for 500 hours
As shown in Table XVIII, water quenching from 12050 C (22000 F)
results in a higher magnetization than from 8300 C (1525* F) to
200 C (680 F).
Also on subsequent aging at 700 C (1580 F), a greater
iicrease in magnetization occurs in specimens quenched from the
higher temperature.
These results are in line with the fact that
the solubility of carbon increases with temperature and that the
magnitude of the 700 C (1580 F) expansion is largely dependent on
the amount of carbon in solid solution.
H.
Coefficient of Thermal
1.
Expansion Measurements
Rate of Cooling from 830
C.
The effect of rate of cooling
from 8300 C (15250 F) on the coefficient of thermal expansion at
200 C (680 F) of Invars (0.10 Mn, 36.0 Ni) of varying carbon content
is shown in Table XIX.
With the exception of the Invars containing
0.25 and 0.58 percent carbon, the coefficient of thermal expansion
in the as-received (cold worked) condition was found to be the
same as in the quenched condition.
Furnace cooling from 8300 C
(1525* F) results in a substantially increased coefficient of thermal
expansion as compared with water quenching from the same temperature.
This holds for all the carbon contents investigated.
-
- 78
TABLE XIX
VARIATIOP OF COEFFICIENT OF THEiRHAL EXPAISION OF
INVARS OF VARYING CARBON CONTENT E ITH RATE
OF COOLING FROM 8300 C
Carbon
Content
Percent
Coefficient of Ther
Exhansion
Qual
at 20 C*
Cold Drawn*
Water Quenched
Furnace Cooled
0.02
0.0
0.0
0.5
0.10
0.0
0.0
1.2
0.15
0.1
0.2
1.6
0.25
0.2
0.7
1.9
0.40
1.0
0.9
2.3
0.58
1.1
0.6
2.2
*
Microinches per inch per *C.
*
As received condition:
8000 C, quenched in water to 200 C,
cold drawn (27 percent reduction in
area).
-
- 79
TABLE XX
CHANGES IN COEFFICIENT OF THE1fALT EXPANSION ON AGING
FURNACE COOLED INVARS OF VARYING CARBON CONTENT
Carbon
Content
Percent
Coefficient of Thermal Exroansion at 200 C*
Reheated 1 hour
Furnace Cooled
at 2050 C and
Aged 500 hours
from 8300 C
Water Quenched
at 700 C
0.02
0.5
0.4
0.4
0.10
1.2
1.0
1.1
0.15
1.6
1.1
2.0
0.25
1.9
0.40
2.3
1.9
0.58
2.2
1.4
*
Microinches per inch per
2.2.
0
C.
2.0
2.
A ing1Followng
Furnace Cooling.
80
-
-
The effect of reheating
furnace cooled Invar (0.10 Mn, 36.0 Ni) for 1 hour at 2050 0
(4000 F), water quenching to .200 C (680 F), and aging for 500
hours at 70 0 C (1580 F) is shown in Table XX.
With the exception
of the 0.25 percent carbon Invar, the 2050 C (4000 F) treatment
results in a decrease in the thermal expansion coefficient.
After
aging for 500 hours at 70 0 C (158* F) the coefficient tends to
increase to the furnace cooled value for carbon contents up to
0.15 percent; whereas for higher carbon contents, the furnace
cooled value is surpassed.
These results indicete that an in-
crease in coefficient is associated with the 70 0 C (1580 F)
expansion.
Since on furnace cooling from 8300 C (15250 F) this
phenomenon will occur, the higher coefficient of furnace cooled
as compared with water quenched Invar can be at least partially
accounted for.
I.
Summary of Experimental Results
1.
Aging of Quenched Invar.
The experimental results can be
summarized by considering the phenomena that are believed to occur
during the aging of quenched Invar in the following temperature
ranges:
a.
200 C (680 F) to 950 C (2000 F)
Aging in this temperature range results in a contraction
due to stress relief and an expansion due to formation of GuinierPreston zones.
Formation of Guinier-Preston zones results in an
increase in magnetization and coefficient of expansion.
There is
no significant change in hardness or microstructure in this range.
b.
81
-
-
950 C (2000 F) to 3150 cO (6000 F)
Aging in this temperature range results in continued
stress relief as manifested by further contraction.
In addition,
the Guinier-Preston zones disappear, giving rise to a contraction
and loss of magnetism.
There is no significant change in hardness
or microstructure in this range.
c. 3150C6000
F) to 5400 C(10000 F)
Aging in this temperature range results in continued
stress relief as manifested by further contraction.
The rate of'
cooling from the aging temperature to room temperature now
becomes a factor in dimensional change.
Water quenching results
in an expansion due to formation of residual stress.
Precipitation
of carbides occurs both at grain boundaries and within grains and
results in larger contractions.
The higher the carbon content, the
greater is the extent of carbide precipitation; however, less
precipitation occurs at grain boundaries in higher carbon Invars.
Precipitation of carbides at grain boundaries opposes the hardness
decrease due to loss of carbon from solid solution.
Thus, there
is a greater decrease in hardness in the higher as compared with
lower carbon Invars.
d.
540* C (1000 F) to 60
C (12000 F)
Aging in this temperature range results in continued
contraction due to stress relief and increased tendency for expansion
due to residual stress formation on cooling from the aging temperature
to room temperature.
Re-solution of the carbide phase and precipi-
tation of graphite occurs.
82
-
-
The combined re-solution and precipi-
tation process results in an expansion due to the higher specific
volume of the graphite compared to the carbide phase.
The formation
of graphite and continual decrease in carbon content of the austenite
result in lowered hardness.
e .650
C (12000 F) to 830. C _1525_ FI
Aging in this temperature range results in continued
contraction due to stress relief and increased tendency for expansion due to residual stress formation on cooling from the aging
temperature to room temperature.
Solution of graphite occurs, which
results in an exoansion as well as an increase in hardness.
2.
Furnace Cooled Invar
Furnace cooling from 8300 C (15250 F) to 200 C (680 F) can
result in precipitation of some graphite and/or carbide above about
3150 C (6000 F), as well as formation of Guinier-Preston zones below
about 950 C (2000 F).
Lowering of carbon content of the austenite
due to precipitation of graphite or carbide should result in decreased magnetization and coefficient of thermal expansion; but
this is opposed by the increase in these properties due to the formation of Guinier-Preston zones.
The latter effect is evidently
greater than loss of carbon since the magnetization and coefficient
of expansion of furnace cooled Invar is greater than that of quenched
Invar.
The lower hardness resulting from furnace cooling as com-
pared with water quenching is probably due to precipitation of
graphite.
vilr.
83
-
-
DISCUSSION OF RESULTS
The results of this investigation indicate that there are
several expansion and contraction effects that are of importaze in,
considering the dimensional behavior of Invar.
The metallurgical
phenomena that are bClicved resporsiblc for tLe oLerved
changep are as follows:
liyenvional
1) formation of residual stress 2) relief
of residual stress, 3) formation and disappearance of GuinierPreston zones, and 4) precipitation and re-solution of carbide and
graphite.
A.
A discussion of each phenomenon follows:
Formation of Residual Stress
It was found that an expansion occurs as a result of the for-
mation of residual stress during rapid cooling.
Formation of residual
stress is believed to involve plastic deformation of the surface
layer of an Invar bar according to the following mechanism.
On rapid
cooling from a high enough temperature, the surface of the bar will
attain room temperature before the interior has cooled appreciably.
However, the shrinking of the surface is resisted by the interior
which is put in hydrostatic compression and cannot undergo plastic
deformation.
Accordingly, the surface layer is put in tension and
plastic flow occurs because of the large magnitude of the stress
involved.
When the interior cools it tends to contract further but
this contraction is resisted by the cold surface layer.
The net
result is that when both the .'urface and interior reach room temperature the bar as a whole has elongated and there is an equilibrium
stress distribution with the surface in compression and the interior
in tension.
Since the resultant expansion is due solely to plastic
deformation there is no appreciable change in specific volume.
B.
84
-
-
Relief of Residual Stress
On aging, relief of residual stress occurs.
If, as in-quenched
Invar, the compressive stress at the surface is equal to the tensional
stress at the interior, stress relief could occur in the following
manner,
On heating to the aging temperature, the temperature of the
surface of a bar increases faster than does that of the interior.
Since the elastic limit decreases with temperature, the compressive
stress at the surface exceeds the elastic limit sooner than does the
tensional stress at the interior.
Plastic flow in compression
therefore occurs until the surface compressive stress is decreased
to the elastic limit corresponding to the aging temperature.
As
plastic flow in compression occurs at the surface, the bar will contract and some of the tensional stress at the interior will be relieved.
By heating at a high enough aging temperature complete
stress relief can be achieved providing no residual stress sets in
as a result of a phase change.
However, very slow cooling from the
aging temperature must be carried out to avoid re-formation of
residual stress.
In the aging experiments that were carried out in
this investigation cooling from the aging temperature was mainly
done in air.
This rate of cooling was found to avoid the formation
of resiaual stress provided that the aging temperature does not exceed about 3150 C (6000 F).
Because of the extremely low coefficient
of expansion of Invar up to its Curie point, there is less danger
of introducing residual stress as compared with materials of higher
expansion coefficients.
It was found that stress relief can be achieved by subjecting
quenched Invar to mechanical shock.
Dropping on a concrete floor is
believed to achieve stress relief in the following way.
On striking
the floor transverse elastic waves are set up in the specimen.
These waves are equivalent to an applied stress which varies from
tension to compression with time at any point along the barand the
maximum magnitude of which occurs at the surface.
The addition of
an compressive elastic stress component to the residual compressive
stress at the surface could exceed the elastic limit.
Thus at the
surface plastic flow could occur in compression and stress relief
would be achieved in the same manner as described for aging.
This
would result in a contraction as observed; however, why a larger
contraction occurs by "shock" stress relief-than by "thermal" stress
relief even though thesurface stress is reduced the same amount is
not understood.
It may be that "shock" stress relief results in a
different triaxial stress distribution than "thermal" stress
relief even when the longitudinal stress at the surface is reduced
by the same amount.
"Shock" stress relief could account for the fact that repeated
water quenching from an aging temperature of 3150 G (6000 F) results
in larger contractions than repeated cooling in air or silocel.
Due
to localized pressure areas set up by steam formation during water
quenching it is possible that transverse elastic waves are produced.
Stress relief of cold drawn Invar rod was found to result in
a contraction.
The actual distribution of residual stress in cold
drawn Invar prior to aging was not determined.
Generally, cold
drawing results in tension at the surface and compression at the
interior.
With such a stress distribution, a contraction would occur
on aging if the maximum compressive stress exceeds the maximum
tensional stress.
-
- 86
In addition to the macroscopic distribution of residual stress
in cold drawn Invar there is also the possibility of residual stress
on a microscopic scale.
Due to unequal plastic deformation resulting
from differences in grain orientatiori, the residual stress may vary
in sign from one grain of the metal to the next.
relief called creep recovery should occur.
On aging stress
If plastic flow during
cold drawing has occurred primarily in tension, the creep recovery
effect would be expected to result in a contraction.
C.
Formation andDiappearance of Guinier-Preston Zones
The 700 C (1580 F) expansion and its reversible counterpart,
the 2050 C (4000 F) contraction, are believed to be associated with
a pre-precipitation brocess occurring within the austenitic solid
solution.
This hypothesis is based on the correlation between
lattice parameter and overall length changes associated with these
phenomena as well as the failure to find evidence of a phase transformation resulting in the formation of either. ferrite, martensite,
carbide, or graphite at the low aging temperatures involved.
It is possible that prior to actual precipitation of a carbon
containing phase, clustering may occur within the .olid solution due
to diffusion of carbon,and perhaps nickel, to particular lattice
sites in order to result in a lowering of free energy as compared to
the random condition.
Such clustering could result in an overall
increase in lattice parameter and account for the observed expansion,
On the basis of the Band Theory of Invar referred to in Chapter IV,
it would be expected that there should be an increase in both
magnetization and thermal expansion coefficient associated with the
increase in austenite parameter.
Due to low solute concentration
build-up, there might be little or no hardness change associated with
this process.
-
- 87
Even though such clustering within solid solution occurs at a
low aging temperature it
might still
take an extremely long aging
time for the carbide phase to actually precipitate.
Raising the
temperature should speed up the process; however, a temperature may
be reached where thermal fluctuations would tend to wipe out the
clusters and decrease the austenitic lattice parameter, thus resulting in a contraction.
The reversible relation between the low
aging temperature expansion and contraction effects can be explained
on this mechanism.
The carbon dependence of the 700 C (1580 F) expansion and the
2050 C (4000 F) contraction is in accord with a pre-precipitation
process.
The more carbon in solid solution the greater the tendency
for clustering to occur.
It is believed that any carbon out of
solid solution would ha.ve only a slight influence on these phenomena.
At the present time there are two main theories with reference
to the initial stage of the precipitation process.
According to
Guinier and Preston, the initial stage is the occurrence of platelike clusters of solute atoms within solid solution; whereas ac-
cording to Barrett and Geisler, the initial stage is the immediate
precipitation of extremely thin lamellae of a transitional phase.
The proposed mechanism for the reversible carbon dependent changes
in Invar is in conformity with the theory of Guinier and Preston.
However, direct evidence of the occurrence of Guinier-Preston zones
on aging Invar has not been obtained by x-ray methods.
An investi-
gation to confirm the existence of these zones is a logical sequence
but would require studies with single crystals.
-88-
D.
Precipitation and Re-solution of Carbide and Graphite
Evidence was found that precipitation of a carbide phase occurs
on aging quenched Invar above 3150 C (6000 F) and results in a comparatively largecontraction.
This precipitation is believed to
occur both at the grain boundaries and within the grains of the
austenitic solid solution.
The magnitude of precipitation is a
maximum at about 4250 C (8000 F).
At higher aging temperatures, it
is believed that the carbide redissolves and carbon is re-precipitated in the form of graphite within the grains of the austenite.
This conversion of carbide to graphite is essentially completed at a
temperature of 6500 C (12000 F).
Above this temperature re-solution
of graphite should occur because of the increase in solubility of
carbon with temperature.
The precipitation of carbon from golid solution is in accord
with the solubility curve determined for the austenitic phase.
Depending on the carbon content of Invar, water quenching from 8300 C
(15250 F) retainr
up to. 0.17 percent carbon in solid solution.
Since
the solubility of carbon decreases with temperature and is approximately nil at room temperature, as-quenched Invar consists of a supersaturated austenitic solid solution plus any excess graphite insoluble
at 8300 C (15250 F).
On aging the tendency is for carbon to come
out of solution; however, moderately high aging temperatures are
.required for precipitation to occur at an appreciable rate.
Whether carbon comes out in the form of carbide or graphite depends
on the energy barriers to each process.
It appears that conditions
are right for initial formation of a metastable carbide phase in
preference to the more stable graphite phase.
89
-
-
However, at higher
aging temperatures the energy barriers ehange and the formation of
graphite is favored.
This appears to occur by re-solution of the
carbide phase and precipitation of graphite rather than by decomposition of the carbide phase directly to graphite.
aging temperatures,
At still higher
solution of graphite must occur in accordance
with the solubility curve determined for Invar austenite.
E.
Summary
The dimensional behavior of Invar is believed affected by
several phenomena which overlap to some extent.
The phenomena re-
sulting in expansions are formation of residual stress, formation
of Guinier-Preston zones, re-solution .of carbide, and re-solution
of graphite.
The phenomena resulting in contractions are stress
relief, disappearance of Guinier-Preston zones, precipitation of
carbide, and precipitation of graphite.
Stress relief increases with aging temperature.
However, there
is a tendency for residual stress formation on subsequent cooling
which increases with aging temperature, but this effect can be
prevented if cooling from the aging temperature is slowly carried
out.
The formation of Guinier-Preston zones is restricted to aging
temperatures below about 950 C (158* F).
in the disappearance of these zones.
on aging above 3150 C (6000 F).
Higher temperatures result
Precipitation of carbide occurs
Re-solution of carbide and simultaneous
precipitation of graphite occurs on aging above 4250 C (8000 F).
Re-solution of graphite occurs on aging above 6500 C (12000 F).
VIII.
1.
-
-90
CONCLUSIONS
The metallurgical phenomena believed responsible for dimensional
changes in Invar are residual stress formation, stress relief,
formation and disappearance of Guinier-Preston zones, and
precipi-
tation and re-solution of both carbide and graphite.
2.
Residual stress formation occurs on rapid cooling from aging
temperatures aboye approximately 3150 C (6000 F) and results in
an expansion.
3.
Stress relief incurred by either aging or mechanical shock results in a contraction.
4.
Guinier-Preston zones form on aging at temperatures up to about
950 C (2000 F) and result in an expansion; whereas on aging above
this temperature these zones disappear and a contraction results.
5.
Precipitation of carbide and graphite occurs on aging above 3150 C
(6000 F) and results in contractions; re-solution of these
phases results in expansions.
6.
The coefficient of thermal expansion at 200 C (680 F) was found
to increase as a result of formation of Guinier-Preston zones.
-91-
I.
SUGGESTIONS FOR FURTHER WORK
The t'ollowing investigations would be valuable in further clarifying the dimensional behavior of Invar:
1.
Confirmation of the formation of Guinier-Preston zones by
determination of the existence of characteristic streaks
in Laeue patterns of single crystals of austenitic Invar
when aged to result in the 700 C (1580 F) expansion.
2.
Determination of the composition and crystal structure of
the carbide phase which precipitates on aging above about
3150 C (6000 F).
3.
Determination of the effect of both carbide and graphite
precipitation on the coefficient of thermal expansion at
200 C (680 F) of Invars of varying carbon content.
92
-
-
BIBLIOGRAPHY
1.
J. S. Marsh, "The Alloys of Iron and Nickel, Vol. 1
-
Special
Purpose Alloys," McGraw-Jill Book Company, Inc. New York and
London 1938, Chap. VI, P. 135.
2.
E. A. Owen, E. L. Yates, and A. H. Sully, "An X-ray Investigation of Pure Iron-Nickel Alloys.
Part 4.
The Variation
of Lattice Parameter with Composition," Proc. Phys. Soc.,Vol.
49, 1937, P. 315.
3.
E. A. Owen and A. H. Sully, "On the Migration- of Atoms in
Iron-Nickel Alloys," Phil. Mag. Vol. 31, 1941, P. 340.
4.
W. Shockley, "The Quantum Physics of Solids -
Monograph
B - 1184, Bell Telephone Laboratories.
5.
C. E. Guillaume, "Changements Passagers and Permanents des
Aciers au Nickel (Transitory and Permanent Changes of Nickel
Steels)," C. R. Acad. Sci., Paris, Vol. 136, 1903, P. 356.
6.
H. Scott, "Expansion Characteristics of Low-Expansion Nickel
Steels," Trans. Am. Soc. Steel Treat., Vol. 13, 1928, P. 829.
7.
T. F. Russell, "Low Expansion Nickel Steel," Engineering (London),
Vol. 128, 1929, P. 400.
8.
M. A. Hunter, "Low-Expansion Alloys," Metals Handbook, American
Society for Metals, Cleveland, 1948, P. 601.
9.
S. R. Hood, Discussion on Paper by G. Sachsand J. W. Spretnak,
"The Structure and Properties of Some Iron-Nickel Alloys,"
Trans. A.I.M.E. Vol. 145, 1941, P. 356.
10.
93
-
-
C. E. Guillaume, "Recherches Metrologiques Sur Les Aciers Au
Nickel (Metrologic
Researches on Nickel Steels)," Travaux et
Memoires du Bureau International des Poids et Mesures, Vol. 17,
1927, P. 113.
11.
P. Chevenard and A. Portevin, "Dilatometric Analysis of Some.
Alloys," Revue de i4etallurgic Memoires, Vol. 22, 1925, P. 357.
12.
T. Kase, "On the Equilibrium Diagram of the Iron-CarbonNickel System," Science Report, Tohoku Imperial University,
Ser. 1, Vol. 14, 1925, P. 173.
13.
B. Jacobson and S. Westgren, "Nickel Carbide and its Relation
to Other Carbides of the Element Series Scandium-Nickel",
Z. Physik. Chemie, ser. B, Vol. 20, 1933, P. 361.
14.
B. L. Averbach and M. Cohen, "The Isothermal Decomposition of
Martensite and Retained Austenite," Trans. A.S.M., Vol.
41,
1949, P. 1024.
15.
G. V. Kurdjumov and 0. P. Maksimova, "Kinetics of the Transfromation of Austenite to Martensite at Low Temperatures," Doklady
Akad. Nauk SSSR 61, No. 1, 83(1948).
16.
J. R. Low and M. Gensamer, "Aging and the Yield Point in
Steel," Trans. A.I.M.E., Vol. 158, 1944, P. 207.
17.
M. Cohen and P. K. Koh, "The Tempering of High Speed Steel,"
Trans. A.S.M., Vol. 27, 1939, P. 1015.
18.
B. L. Averbach and M. Cohen, "X-ray Measurement of Retained
Austenite by Integrated Intensities," T.P. 2342, Metals
Technology Feb. 1948.
19.
D. J. Blickwede, "Effect of Vanadium and Carbon on the
Constitution of High Speed Steel," M.I.T. Thesis, 1948.
20.
0. Zmeskal and M. Cohen, "Siultaneous Measurement of
Magnetic and Dilatometric Changes", Review of Scientific
Instruments, Vol. 13, 1942, P. 346.
94
-
-
-4
FIGURE 1 - ATMOSPHERE CONTROLLED
FURNACE
FIG. 1
FRONT
SIDE
VIEW
-
-q6-
VIEW
FIGURE 2 - APPARATUS FOR MEASURING
COEFFICIENT OF THERMAL EXPANSION
FIG. 2
FIGURE
3
DIMENSIONAL CHANGES ON AGING QUENCHED INVAR (0.07C,0.44Mn S36.8 Ni)
+
8300C, QUENCHED IN WATER TO 200C, AGED AS INDICATED,
50
-
-
AIR COOLED TO 200C.
700C
(.
5000
UI)
0
wA
~
950C
7~
-
.~
zu
4
C
'
m:
0
z
U
120C
'.7...
0
X
-50
150 OC
T
1
4
-lO0
205~.~
I.~
)CV
~
----------------------------------------------------------------------------------------------------------------------------4--
-t
I
I'D
I
0.1
I
I
I
0.5
I
I)
I
I
5
I
I 11
10
50
AGING TIME - HOURS
I
IL
100
500
1000
FIG. 3
+200
t
(
FIGURE 4
FIGURE 4
54000, WATER QUENCH
+100'[
N) AT 70OC
DIMENSIONAL CHANGES ON AGING INVA R (0.07c O 44 Mn
0
QUENCHED IN WATER TO 20 C AFTER I HOUR AT TEMPERATURE INDICATED,
AGED AT 70*0, AIR COOLED TO 200C
1368
-t50
ZERO CORRE SPONDS TO INITIAL LENGTH
AFTER QUEN CHING IN WATER FROM
8300C TO 2 )0c
-150*C, WATER QUENCH
z
0
200C
120 0 C WATER QUENCH
Ln
4250C. WATER QUENCH
41m,
0
cr
50[
2050C, WATER QUENCH
-100
315*C, WATER QUENCH
-150
1 ! 1
0.1
0.5
I
50
10
5
100
AGING TIME - HOURS AT 700C
500
1000
FIG. 4
FIGURE
DIMENSIONAL CHANGES
ON RE-AGING
5
AT 50*C
FOLLOWING AGING AT 700C.
8300 C,QUENCHED IN WATER
PLOT A-AGED AT 700C
TO 200C
PLOT B-RE-AGED
AFTER 1000 HOURS AT 700C
AT 500C
A -AGED
A-GDAT70
+251
0
X
0
z
AT 70*C
"f
f
cr
w
w
0
25
B-AGED AT 50*CB-AGED AT 5000
-
a
+
z
--
0
-25
--
-
---
---
-
-------------
I
U.'
--.
------
____________________
-
---
__---__-
--t-------
~~ii~J
10
50
100
AGING TIME-HOURS
-------------.------- t--
____________________
5
500
1000
FIG 5
FIGURE 6
DIMENSIONAL CHANGES ON AGING QUENCHED INVAR (0.070, 0.44 Mn, 36.8 Ni).
8300C, QUENCHED IN WATER TO 200C, REHEATED FOR I HOUR AT 2050,
QUENCHED IN WATER TO 20*C, AGED AS INDICATED, AIR COOLED TO 200C
t251
1500C
l-e
OF
z
950C
25
0
Cr
700C
ii
2 50
500C
t
0
0.1
i
I
I
I
I
I
I
0.5
I
Ii'
5
10
AGING TIME -HOURS
I
i
50
II
I
100
ill
I
500
1000
FIG.6
~~1
FICURE 7
DIMENSIONAL CHANGES ON AGING QUENCHED INVAR (007 C 0.44 Mn, 36.8 Ni) AT 700C
PLOT A - 830 0 CQUENCHED IN WATER.TO 20 0 C) AGED AT 70 0 C
PLOT B - REHEATED TO 205 0 C, HELD I HOUR, QUENCHED IN WATER TO 20*C, AGED AT 700C
PLOT C -- SAME TREATMENT AS B
A- FIRST AGING
ZERO CORRE SPONDS TO INITIAL LENGTH
AFTER QUEN CHING IN WATER FROM
8300C TO 2 0 0 C
z
w
a-
OK.
PERIOD AT 700C
-
50
t
ft
()
w
0 -50
0
7
CORRESPON DS TO LENGTH AFTER
*Cl
QUENCHING IN WATER FROM 205C
TO 200C
B- SECOND AGING PERIOD AT 70 0 C
C-TIHIRD AGING PERIOD AT 70-C
-100
CORRESPONDS TO LENGTH
AFTER QUENCHING IN WATER FROM
2050C TO 200C FOR SECOND TIME
H
0. 1
i
0.5
I
5
10
AGING T IME A T 70*
50
100
- HOUR S
500
1000
FIG. 7
FIGURE
DIMENSIONAL CHANGES ON AGING COLD WORKED INVAR
(0.07C, O 44Mn, 36,8 NI)
REDUCED 25 PEF CENT IN AREA BY COLD DRAWING FOLLOWING
ABOUT 8000C TO 20*C
WATER QUENCHING FROM
+501
70*C
0
5oc
0
uj
k'
9'
0
cr
k
_bO F
9'
50*C
2050C
-100L
0
0.1
0.5
I
5
10
AGING TIME
--
50
HOURS
100
I
if-
500
1000
FIG 8
FIGURE 9
DIMENSIONAL CHANGES ON AGING ANNEALED INVAR (0.07 C, 0.44 Mn, 36.8 Ni)
8300C, FURNACE
I
-. 4.
COOLED TO 2000, AGED AS INDICATED, AIR COOLED TO 200C
z
a:
LAJ
i
U)
0
700C
20*C
1500C
2050C
"m lo
6w
0r
-50
H
U.'
0.5
I
5
1)
100
50
AGING TIME - HOURS
500
1000
FIG 9
FIGURE
10
DIMENSIONAL CHANGES ON AGING INVAR (0.07 C,0.44 Mn, 36.8 Ni)
COOLED AT DIFFERENT RATES FROM BOTH 8300C AND 3150C
-
-
-
-
Ol-
A
0
ft.-W
-25
----
AL
AIR COOLED
I
x
v
0
0
-25
x
8300C TO 200C
*
z
w
x
FURNACE COOLED FROM
0
I
0
n
r7l
FROM 8300C
TO 20OC
0~
0
z
-
-
-
MEDIUM USED FOR
COOLING FROM 3150C
I
0
* WATER
X AIR
0-
__
501
SILOCEL
QUENCHED IN WATER FRO A 8300C TO 200C
-100
x0
0
-150
I-
0.1
0.5
I
5
10
50
100
AGING TIME-HOURS AT 3150C
i I II
500
1000
FIG. 10
I
FIGURE
x
t1600
II
DIMENSIONAL AND VOLUME CHANGES ON
AGING ANNEALED INVAR (0.07 C, 0.44 Mn)
36.8 NI) AT 5400C AND QUENCHING IN
cr +0
WATER TO 200C.
83O0 'C, FURNACE CO )LED TO 2 0 *C, AGED AT
5400C AS INDICATE D
00
x- +1200
(nf
PLOT B-10 HOURS TOTAL
AT 5400C
QUENCHED 10 TIMES
So
x
IN WATER TO 2000C
LENGTH CHANGES
+400[-+
/00
/
ul
PLOT A -100 HOURS TOT AL AT 5400C
QUENCHED 7 T IMES IN WATER TO 2OOC
x
/00.0
~
0
5
0
+20
SPECIFIC
LA-i
a-
VOLUME
CHANGES
_-eEL-___
O
P LOT C - 10 HOURS TOTAL AT 54 0*C
QUENCHED 10 TIMES IN WATER TO 200C
Q)
Q
0
,Z)
Q0
-20
C
I ___________________
0.1
0.5
I
I
I I oil
5
10
50
100
AGING TIME -HOURS
AT 5400C
500
1000
FIG. I
FIGURE
I
12
+100K
DIMENSIONAL CHANGES ON AGING DECARBURIZED AND QUENCHED INVAR
(o0
C, 0.44 Mn, 36.8 Ni)
DECARBURIZED IN WET HYDROGEN FOR 20 HOURS AT 10950C
8300 C, QUENCHED IN WATER TO 200C, AGED AS INDICATED, AIR COOLED TO 2C *c
+50
Ir
0i
w
0
20*0
4
a~
(I)
-50
\
1500C
2050C
-100[
0
QN
0.1
0.5
I
0.5~
AN10
AGING
I
I I
5I1
50
TIME -HOURS.
100
500
1000
FIG. 12
FIGURE 13
DIMENSIONAL CHANGES ON AGING DECARBURIZED
AND ANNEALED INVAR
(0.01C, 0.44 Mn, 36.8 Ni)
DECARBURIZED IN WET HYDROGEN FOR 20 HOURS AT 1095 0 C
8300c) FURNACE COOLED TO 20*C, AGED AS INDICATED, AIt COOLED TO 20*C
f 10
200C
-10
70 C
0
1O
-4-
---
t
ui
i sC:"o
0
F-,
-
-10
1o
U
205" C.
-ICy
H
0
-ci
i
DI
I
II
0.5
!
I
I I I
I
5
I
II
50
10
AGING TIME -HOUR
I
I I I I
100
I
500
1000
FIG. 13
-~
iRThILUI 1IL~HiilJJ JiL
FIGURE 14
DIMENSIONAL CHANGES RESULTING FROM DROP TESTS OF QUENCHED
AND OF ANNEALED INVAR (0.06C,0.44 Mn,36.7 Ni).
z
-100
e 830 0 C, QUE NCHED IN WATER TO 200C
X 830*C, FUR NACE COOLED TO 200C
Lu-I
cr
-200
z
0
0r
-300
-400
0
I
I
I
2
6
4
3
5
NUMBER OF DROPS
7
8
9
10
FIG. 14
-
-lo9
FIGURE 15
DISTRIBUTION OF RESIDUAL STRESS
IN QUENCHED INVAR
-100-
(.07 C, 0.44 Mn, 36.7 Ni)
830 0 CQUENCHED IN WATER
TO 20 0 C,CENTERLESS GROUND.
430)000
RESIDUAL STRESS CALCULATED
FROM CHANGES IN LENGTH BY
USE OF HEYN ANALYSIS.
-200-
z
w
-300-
+20,000
w
+10,000
-400-
0
z
0
az
-500-
0
0
w
a:_
D
-10,000
SURFACE
CENTER
w
a:.-20, 000
-600-
-700RESIDUAL
STRESS
-30,000
-800-
CHANGE
11N LENGTHi
/
-900-
I,
FRACTION OF DIAMETER
I
I
I1il
I . 0.7
0.0 030.4.0.5I
0.6
.08
0.9
0.100
0.01
0.02
0.03
0.04
CROSS SECTION AREA-SQUARE IN.
CENTER
-V
I
I. 0
0.1049
SURFACE
-1000
F IG.
15
FIGURE 16
PLOT
OF LATTICE
INVAR (0.44 Mn
TEMPERATURES.
PARAMETER
VS. CARBON
36.8 Ni) HEATED
CONTENT
500 HOURS
OF
AT VARIOUS
80
0
60
40
cr
/
20
8050 C
/
w
/
w
/
6000
/
cr
705
-
80
60
mu~~
5i4 0 0C
u
91
C
C
0
40
H
0
2013.5900
U.u
0.1
0.2
0.3
0.4
0.5
CARBON CONTENT -PER
0.6
CENT
0.7
0,8
F IG. 16
-III-
FIGURE
17
SOLUBILITY OF CARBON IN AUSTENITIC
PHASE OF INVAR (0.44 Mn, 36.8 Ni)
90C
80C
70C
60C
50C
400
300
100
_
I
(.
0i
0 .0
_________
0 DETERMINED
EXPERIMENTALLY
'' CALCULATED
ASSUMING IDEAL
SOLUTION
I
_________
1
_________
1
.1
_________
0.4
0.3
0.2
0.1
CARBON CONTENT- PER CENT
,
200 !I
1
_______________________
0.5
FIG.
17
PRO ii
FIGURE
PLOT OF J VS. H FOR QUENCHED
18
INVAR (0.07C, 044 Mn,36.8 Ni) AT 130C
0
830 0C, QUENCHED IN WATER TO 20 C
1200
_ _
_
_
_
-.
I
_
I
11000
_
_
-
I
II
I
I
I
z
SATURATION
\
I
z
Z 3600
600
I
4
.?7.
uz
0
>-
400
z
w
z
200
0
0
N
200
400
IUU
1000
600
800
MAGNETIC FIELD INTENSITY (H) IN OERSTEDS
l'+uu
IOuu
'ou'
r-000
FIG.
18
I-Mimi
-'/13-
flj
FIGURE 19
100
VARIATI ON OF SATURATION J WITH
TEMPE RATURE FOR ANNEALED INVAR
(0.07C, 0.44 Mn, 36.8 Ni).
90(
830*CF URNACE COOLED TO 200 C.
MAGNET IC FIELD INTENSITY (H) = 1100 OERSTEDS
80C
i
X
70C
U,
60
-x
i
z
-.
0
--
50C
i
HEATING CURVE
x COOLING CURVE
x
z
o400
i-
x
z
x
w
z 300
200
100
01
0
-CURIE
POINT
=266*C
4-i
i
50
100
150
200
TEMPERATURE*C
250
L
300
350
FIG.
19
FIGURE
1100-
20
1
VARIATION OF SATURATION J WITH
TEMPERATURE FOR QUENCHED INVARS
(0.10 Mn, 36.ONi) OF VARYING CARBON CONTENT.
1000-
8300c QUENCHED IN WATER TO 20 0C
MAGNETIC FIELD INTENSITY
900
1100 OERSTEDS.
800-
0700-
z
o600
N
z
__
500
_
_
_
_
0
>400
--
-
x
200-
100
E
0 0
L.400
50
I50
100
TEMPERATURE
200
00
0.02c
30
3C
0
0.1OC
FIG.
20
FIGURE 21
-
CuSO4 + HCO Etch
FIG. 21A
-
-
500x
Longitudinal Section of 0.58 Percent Carbon Invar in AsQuenched Condition. Twinned Austenitic Grains and Graphite
Streaks.
Unetched
FIG. 21B
MICROSTRUCTURES
1500X
Longitudinal Section of 0.58 Percent Carbon Invar In AsQuenched Condition. Graphite Streaks.
CuS04 + HC1 Etch
150OX
FIG. 21C- Transverse Section of 0.15 Percent Carbon Invar Water
Quenched and Aged 500 Hours at 425* C. Carbide Precipitation at Grain Boundaries.
FIGURE 21
--
APPENDIX A
FORMULAE AND DATA USED IN CALCULATING CHANGES IN LENGTH
DUE TO PHASE TRANSFORMATIONS
(a)
Change in length in terms of change in volume:
aL =1 AV
where
L = change in length
Lo = initial length
AV = change in volume
Vo = initial volume
(b)
Specific volume from x-ray data:
NO
An
where
V = specific volume
N
= Avogadro's number
a
= lattice parameter in A
0
n = number of atoms in
unit cell
A = mean atomic weight
(c)
Change in specific volume due to change in lattice parameter
and atomic weight:
AV =3Aa
(d)
~A
Lattice parameters of iron-nickel austenites at 150 C.
according to Owen and Sully(3)
V
Nickel Content
Lattice Parameter
Nickel Content
Lattice Parameter
in Atomic Percent
in Angstroms
in Atomic Percent
in Angstroms
100
3.5171
3.5027
3.5241
3.5278
3.5378
3.5494
3.5626
3.5691
3.5779
3.5828
3.5858
3.5881
96.58
93.65
90.63
82.13
73.03
62.66
57.19
50.91
46.74
44.26
41.50
(e)
39.46
37.24
36.01
33.97
33.34
33.07
32.48
32.29
31.13
30.60
29-25
27.12
3.5887
3.5886
3.5883
3.5859
3.5847
3.5842
3.5833
3.5829
3.5808
3.5798
3.5769
3.5743
Lattice parameters of iron-nickel ferrites at 150 C according
to Owen and Sully
:
Nickel Content
in Atomic Percent
Lattice Parameter
in Angstroms
2.96
2.8624
5.73
8.56
2.8633
2.8634
-
-3
APPENDIX B
CALCULATION OF LENGTH CHANGE DUE TO FORMATION OF
CaMENTITE IN INVAR
(1)
Assume complete precipitation of 0.05 percent carbon as Fe 3 C
according to the following equation:
I gram A -- +- 0.9925 gram A' + 0.0075 gram Fe3 C
where A = austenite containing:
At= austenite' containing:
(2)
0
0.00
30
63 95
36.3
63.7
Specific volume of Invar containing 36 percent nickel and 64
percent iron calculated from formula (a) Appendix A using 3.587
angstroms as the lattice parameter of 36:64 nickel-iron austenite
from (d) Appendix A:
(6.023 X 10) (3)
(3)
,= 0.12234 cubic ems. per gram.
Increase of volume of 36:64 nickel-iron austenite due to solution
of 0.05 percent carbon assuming an increase of 0.0023 angstroms
in lattice parameter as determined from Figure 16.
AV
(4)
3.587
+ 0.00023 cubic cms.
Volume of 1 gram of A-phase:
VA
(5)
3(0.0023) (0.12234)
36 + A V = 0.12257 cubic ems.
Change in specific volume of 36:64 nickel-iron austenite due to
increase of 0.30 percent nickel calculated from formula (b)
Appendix A knowing from (d) Appendix A that an increase in
-
-4
lattice parameter of 0.0Q4 angstrom occurs:
i(.584
AVNi
(6)
.9
5
Lper
% (0.12234)= + 0.00002 cubic cms.
gram
Volume of 0.9925 grams of A'-phase:
VA, = (0.9925) (0.12236) = 0.12145 cubic ems.
(7)
Volume of 0.0075 grams of Fe 3 C having a specific volume of
0.13000 cubic cms. per gram:
VFe3C = (0.0075) (0-13000) = 0.00097 cubic cms.
(8)
Change in length for reaction (1) calculated using equation (a)
Appendix A:
AL
L
-
1
3
(0.12145 + 0.00097 - 0.12257
0.12257
AL
=-410
microinches per inch
= 0 .00041
I I
r
-5.APPENDIX C
CALCULATION OF CHANGE OF LENGTH DUE TO FORMATION OF GRAPHITE IN
INVAR
(1)
Assume complete precipitation of 0.05 percent carbon as graphite
according to the following equation:
1 gram A -- +-0.9995 gram A' + O.0U05 gram G
(2)
where A = austenite containing:
5
A'= austenite containing:
0.00
Ni
Fe
36
63.95
36
64
From equation (4) Appendix B:
VA = 0.12257 cubic ems.
(3)
Volume of 0.9995 grams of A'-phase of specific volume equal to
0.12234:
VAt = (0.9995)
(4)
(0.12234) = 0.12228 cubic cms.
Volume of 0.0005 gram of graphite of specific volume equal to
0.44500:
VG = (0.0005) (0.45500) = 0.00023 cubic cms.
(5)
_4 L _i (0.12228 + 0.00023 - 0.12257)
L
3
0.12257
AL = -170 microinches per inch
L
-0.00017
6-
APPENDIX D
CALCULATION OF LENGTH CHANGE DUE TO FORMATION OF NICKEL CARBIDE
IN INVAR
(1)
Assume complete precipitation of 0.05 percent carbon as Ni3 C
according to the following equation:
1 gram A -0O.9922
gram A' + 0.0078 gram Ni 3 C
where A = austenite containing:
A'= austenite containing:
(2)
C
Ni
3
6395
0.00
3564
64.46
-
Fe
From Equation (4) Appendix B:
VA = 0.12257 cubic ems.
(3)
Change in specific volume of 36:64 nickel-iron austenite due
to decrease of 0.46 percent nickel calculated from formula (b)
0
Appendix A knowing from (d) Appendix A that a decrease of 0.0005 A
in lattice parameter occurs:
Ni=L
(4)
+
61:
(.12234)= -0.00002 cubic cms. per gram
Volume of 0.9922 grams of At phase of specific volume equal to
0.12232 cubic cms. per gram.
VAt = (0.9922) (0.12232) = 0.12137 cubic ems.
(5)
Specific volume of Ni 3 C according to reference and formula (b)
Appendix A:
(6.023
Ni 3 C
X 1023) (0.8660)
(188.08) (0.6667)
(2.646)2(4.329) x 10-24 12608
cuiic ems.
per gram.
-7-
(6)
Volume of 0.0078 grams of Ni 3 C:
VN13C
AL
1 (0.12137) + 0.00098 - 0.12257) = -0.0006
0.12257
3
L
or
cms.
= (0.0078) (0.12608),= 0.00098 cubic
L
= -600 microinches per inch
APPEDIX E
CALCULATION OF LENGTH CHANGE FOR FORMATION OF IRON-NICKEL
CARBIDE IN INVAR
(1)
Assume complete precipitations of 0.05 percent carbon as
Fe 2NiCaccording to the following equation:
1
1 gram A
(2)
0.9924 gram At + 0.0076 gram Fe 2NiC
where A =austenite containing:
Ni
C
0.05 36.0
Fe
63.95
At= austenite containing:
0.00 36.0
64.0
From equation (4) Appendix B:
V = 0.12257 cubic ems.
(3)
Volume of 0.9924 gram of A'-phase of specific volume equal
to 0.12234:
cms.
VA, = (0.9924) (0.12234) = 0.12142 cubic
(4)
Volume of 0.0076 grams of Fe2NiC which is assumed to have the
same specific volume as cementite:
0.0076 (0.13000) = 0.00099 cubic cms.
VC
(5)
Change in length for reaction (1) calculated using equation
(a) Appendix A:
SL_
1
3
L
or
L
=
(0.12142 + 0.00099 - 0.12257)
0.12257
-44+0 microinches per inch
=-0.000
-
- 9
APPENDIX F
CALCULATION OF LENGTH CHANGE DUE TO FORMATION OF FERRITE IN INVAR
A.
Formation of 1 percent Ferrite
(1)
Assume transformation of austenite to ferrite occurs according to following equation:
1 gram A-.-0.99 gram A' + 0.01 gram F
= austenite containing
Ni
36.0
Fe
64.0
A'= austenite containing
36.3
63.7
= ferrite containing
5.0
95.0
where A
F
(2)
Volume of 1 gram of A-phase from equation (2) Appendix B:
VA = 0.12234 cubic ems.
(3)
Difference in specific volume between At and A phase according to equation (5)
Appendix B.
LWNi = +0.00002 cubic cms. per gram
(4)
Volume of 0.99 gram of A' phase:
VA, = 0.99 (0.12234 + 0.00002) = 0.12114 cubic cms.
(5)
Specific volume of F-phase calculated according to
formula (b) of Appendix A, knowing from (e) Appendix A
that the lattice parameter of the ferrite phase = 2.8629
angstroms
F
(6)
(6.023 X 1023) (2.8629)3 (102 2 4
(56.04)
2
Volume of 0.01 gram of F-phase:
VF = (0.01) (0.12610) = 0.00126 cubic cms.
0.12610
(7)
Change in length is given by formula (a) Appendix A:
A 10.12114 + 0.00126 - 0.12234
0.12234
L
or
B.
10
-
-
W A
=
orL
=
0.000160
0006
+160 microinches per inch
Formation of 10 Percent Ferrite
(1)
1 gram A--0.90
where A
grams At + 0.10 grams F
N).
Fe
36.0
64.0
austenite containing: 39.5
60.5
austenite containing:
A'
F = ferrite containing:
5.0
95.0
(2)
VA = 0.12234 cubic cms.
(3)
For increase of 3.5 percent nickel, corresponding increase
in lattice parameter = 0.0017 angstrom according to (d)
Appendix A.
Change in specific Volume due to increased
nickel content is given by formula (c) of Appendix A:
(0-001.)
'Ni =
(4)
3.488
-
56.85
)
(0.12234)= -0.00004 cubic cms.
-prgem
gram
per
Volume of 0.90 grams of A'-phase:
VA, =(0.90) 9 [0.12234 - 0.00004]
(5)
0.11007 cubic CMs.
Volume of 0.10 grams of F-phase:
VF = 0.10 (0.12610) = 0.01261 cubic ems.
(6)
AL
L
or
L
-
0.11007 + 0.01261 0,12234
3 (0.12234)
= + 930 microinches per inch
+000093
- Il-
C.
Formation of 45 percent Ferrite
(1)
1 gram A-*.0.55 gram At + 0.45 gram F
Ni
where A = austenite containing: 36.0
A'= austenite containing:
F = ferrite containing:
Fe
64.0
58.0
42.0
5.0
95.0
(2)
VA = 0.12234 cubic cms.
(3)
For nickel content of 58.0 percent, lattice parameter
of A' phase equals 3.569 angstroms according to (d)
A.
Specific volume of A' phase is given by formula (b)
of Appendix A.
(6.023 X 1023) (3.569 x
(57.50) (4)
VA' (4)
8)3
10
= 0.11905 cubic cms.
per gram
Volume of 0.55 grams of A? phase:
VA' = 0.55 (0.11905) = 0.06548 cubic ems.
(5)
Volume of 0.45 grams of F-phase of specific volume equal
to 0.12610:
0.05675 cubic ems.
VF = 0.45 (0.12610)
(6)
1 (0.12223
3
L
or.
L
0.12234)
-0.0030
0.12234
L~-
-3,000
iicroinches per inch