Mechanical Property Potential of Iron Base Infiltrated Parts
F. J. Semel
Hoeganaes Corporation, Cinnaminson, NJ 08077
ABSTRACT
The effects of composition and processing on the transverse rupture and tensile properties of iron base
infiltrated specimens are presented. It is shown that a wide variety of properties are available based on
either simple alloy modifications and/or heat treatment of a standardized base compact composition. The
observed properties compare favorably with those generally reported for the Compacted Graphite cast
irons and the plain Ductile irons in both the as-cast and heat treated conditions.
INTRODUCTION
Starting in late 2001, research efforts in this laboratory were directed to developing iron base infiltration
as a viable parts making process. The initial results of these efforts were reported in two papers that were
presented in 2004 [1, 2]. The focus in both cases was to establish a basic understanding of the process and
to define the conditions needed to implement it as a practical matter. It was shown that infiltration to near
theoretical densities was possible in several Fe-C based alloy systems. The necessary processing
conditions generally included temperatures below 1200 oC (2190 oF), times of less than ½ hour and the
use of standard hydrogen-nitrogen atmospheres with modest methane additions (typically less than 0.5
v/o) to control the carbon potential. The required infiltrant compositions were at or near the corresponding
eutectic liquidus values of the selected alloy system and the required base compact compositions were
likewise at or near the eutectic solidus values of the selected system. The possibility to combine an
infiltrant composition of one alloy system with a base compact composition of another system was also
demonstrated. In addition, it was shown that base compact densities of 6.8 g/cm3 or less were sufficient to
obtain a virtually pore free density after infiltration. Thus, a particular advantage of the technology as
compared with the traditional high pressure / high density processes [3, 4] is that the low starting densities
essentially provided the potential to press larger parts that can later be infiltrated to the same or even
higher final densities.
The as-infiltrated carbon contents of the alloy systems that were studied typically ranged from about 2 %
to about 2.35 %. In an otherwise un-alloyed Fe-C composition, carbon contents in this range normally
result in an as-infiltrated microstructure which consists of pearlite in a network of hyper-eutectoid grain
boundary carbides. Such microstructures are inherently brittle and have limited potential for structural
applications. Consequently, the possibility to use alloy additions to graphitize the hyper-eutectoid carbon
and produce cast-iron like microstructures was investigated. It was found that modest additions of either
silicon or nickel were effective in this regard. However, based on what was generally known of their
alloying effects as well as the results of a series of trials with pre-alloyed nickel compositions [2], it was
concluded that the silicon was the better choice for future development. Thus, subsequent work was
directed towards obtaining a sufficient understanding of its effects on both the infiltration and
graphitization processes to facilitate the design of silicon containing infiltrant and base compact
compositions. The ultimate objective was to normalize the development of the technology in terms of one
such alloy in each case as standard iron base infiltration compositions.
As it turned out, the defining studies indicated silicon contents that nominally averaged 0.18 % for the
infiltrant composition and 0.75 % for the base compact composition. Based on the eutectic equilibrium of
the Fe-C-Si system as indicated by the ThermoCalc program [5], the corresponding carbon contents were
4.28 % for the infiltrant and 1.91 % for the base compact. The infiltrant weight to full density in this alloy
system is about 15 % of the base compact weight. Thus, based on the infiltrant and base compact
compositions in each case, its easily shown that the respective carbon and silicon contents to be expected
after infiltration are about 2.21 % and 0.68 %. Likewise, assuming that all of the hyper-eutectoid carbon
is graphitized during the process and that the balance forms pearlite, it can also be shown that the final
density to be expected is about 7.53 g/cm3. The attendant studies of the systems reasonably confirmed
each of these expectations.
Following this, a series of trials was conducted to determine the resultant transverse rupture and tensile
properties as well as the effects on properties of minor alloy modifications of the standard base compact
composition in both the as-infiltrated and heat treated condition. The alloy modifications in the survey
included one or more of copper, nickel, manganese and molybdenum. Based on the graphite morphology
that was observed during the defining studies, the heat treatments included stress relieving, light to full
annealing, and normalizing. Regrettably, due to a lack of knowledge regarding the relevant transformation
characteristics of these alloys, the Q&T and austempering heat treatments, as generally applied to the
Ductile cast irons [6], were not attempted.
The purpose of the present report is to communicate the results of this survey as well as to discuss their
significance relative to the potential of the iron base infiltration technology versus both the cast irons and
traditional P/M.
EXPERIMENTAL PROCEDURE
The alloy modifications that were included in the survey were made exclusively to the standard base
compact composition (i.e. the infiltrant composition was not modified). In all, there were nine such
modifications. These are shown overleaf in Table 1 along with the alloy designations that are used to
identify them through the balance of the paper. The carbon contents shown in the table correspond to the
eutectic solidus values of the various alloys.
Each of the compositions was subsequently infiltrated with the standard infiltrant composition (4.28% C,
0.18% Si, bal. Fe and residual impurities). The infiltrant weight was nominally 13.5 % of the base
compact weight. Thus, allowing for the contributions of the residual impurities typical of the iron base
powders used in making the infiltrant and base compact compositions, the final infiltrated alloy contents
in each case were about 90 % of the values shown in the table.
Two iron base powders were used in making the several mixes including: Hoeganaes Corporation
Ancorsteels 1000 B and 50 HP. The Ancorsteel 50 HP was used in making the molybdenum containing
compositions. The Ancorsteel 1000 B was used in making the balance of the compositions, the infiltrant
composition and in diluting the Ancorsteel 50 HP to make the two 0.30% Mo containing compositions.
Table 1 - Base Compact Compositions Used in the Study
Carbon
Manganese
Copper
Alloy
ID
%
%
%
0.10
0.06
Si Base
1.91
0.06
Base + 0.5 Mn
1.88
0.60
0.10
Base + 1 Cu
1.87
1.06
0.10
0.06
Base + 1 Ni
1.86
0.10
Base + 2 Cu
1.83
2.06
0.10
Base + 1 Cu + 1 Ni
1.82
1.06
0.14
0.06
Base + 0.3 Mo
1.84
0.14
Base + 0.3 Mo + 1 Cu
1.81
1.06
0.18
0.06
Base + 0.5 Mo
1.79
0.18
Base + 0.5 Mo + 2 Cu
1.74
2.06
Nickel
%
0.04
0.04
0.04
1.04
0.04
1.04
0.05
0.05
0.05
0.05
Molybdenum
%
0.02
0.02
0.02
0.02
0.02
0.02
0.30
0.30
0.55
0.55
Silicon
%
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
The indicated carbon units in each case were admixed as Asbury grade 3203 HS graphite. Additional
graphite in the amount of ~ 0.06 % for the base compact mixes and ~ 0.20 % for the infiltrant mix was
added to allow for the inevitable dusting and sintering losses that occur in the process. The admix copper
additions were made in the form of Acupowder grade 8081 copper. The admix nickel additions were
added as International Nickel grade 123 nickel. The admix manganese additions were added as a
proprietary Hoeganaes Corporation ferro-manganese alloy. The admix silicon additions were likewise
added in the form of a proprietary ferro-silicon alloy. Each of the base compact mixes was lubricated with
0.45 % Lonza Acrawax C and 0.10% Baer Locher zinc stearate. The infiltrant mix was lubricated with
0.10% of the same grade zinc stearate. All of the mixes were submitted to binder treatment processing in
accordance with the Ancorbond process [7].
The base compact mixes were compacted into standard transverse rupture strength (ASTM B 528) and
dog bone tensile (ASTM E 8) specimens at a nominal density of 6.7 g/cm3. The infiltrant slugs were
compacted to the same geometries but at a constant compaction pressure of ~ 550 MPa (40 tsi). As
mentioned, the infiltrant weight was nominally 13.5 % of the base compact weight. Testing after
infiltration was in accordance with the indicated specifications. Density checks of the tensile specimens
were typically limited to one or two specimens per composition and were conducted prior to testing using
the water immersion method (ASTM B 328). TRS testing was performed on a Tinius Olsen compression
testing machine at a crosshead speed of 2.5 mm/min (0.1”/min). Tensile testing was performed on a
Zwick/Roell Z-100 tensile machine at a crosshead speed of 0.635 mm /min (0.025”/min). The machine
was equipped with a 2.5 cm (1”) extensometer and provided automated readouts of the elastic modulus,
the 0.2 % offset yield strength, the ultimate tensile strength and the percent elongation value. The reported
TRS and tensile properties in each case represent the average of tests on from three to five specimens per
condition.
The specimens were processed in an high temperature production belt furnace. Infiltration was at 1185 oC
(2165 oF) for ½ hour at temperature. The furnace atmosphere was nominally 90 % N2 and 10 % H2 by
volume and was additionally treated with 0.25 v/o of CH4 to minimize carbon losses to oxygen impurities.
To insure complete graphitization of the hyper-eutectoid carbon contents of the specimens, normal
cooling subsequent to infiltration was interrupted at temperatures just below the lower critical temperature
of the 0.75% Si Base composition at about 760 oC (1400 oF) with a slow cooling step of from 10 to 15
minutes duration.
The TRS specimens were tested in the as-infiltrated condition. The tensile specimens were tested in both
the as-infiltrated and heat treated conditions. Four different heat treatments were investigated including: a
stress relief, a sub-critical anneal, a normalizing treatment and, a partial ferritizing anneal. The stress
relief was at 200 oC (400 oF) for 1 hour in N2. The sub-critical anneal was at 755 oC (1390 oF) for ½ hour
in N2. The normalizing treatment consisted of austenitizing at 870 oC (1600 oF) for ½ hour in synthetic
DA followed by normal cooling in the cold zone of the furnace to ambient temperatures [i.e. at about
100 oC/min (3 oF/sec) in the range from 845 to 315 oC (1550 to 600 oF)].The partial ferritizing anneal
consisted of austenitizing at 925 oC (1700 oF) for ½ hour in synthetic DA, furnace cooling to 715 oC
(1320 oF) and holding for 1.5 hours in N2 followed by normal cooling to ambient temperatures. As a
matter of interest, in the case of Ductile iron a full ferritizing anneal reportedly requires a hold of about 5
hours at the sub-critical temperature plus slow cooling thereafter to 345 oC (650 oF) [8].
RESULTS AND DISCUSSION
In general, the study of the indicated compositions and conditions of heat treatment was the cumulative
outcome of a series of three smaller studies. All had the explicit objective to determine the mechanical
property potential of the iron base infiltration technology. However, each addressed different interests in
terms of the alloy additions, the heat treatments and the particular properties that were examined.
Consequently, its appropriate to present and discuss the results in terms of these smaller studies.
Tensile Properties of the Silicon Base Composition and Alloy Effects of Copper,
Nickel and Manganese
The objective of first of the three studies was to determine the tensile properties of the 0.75 % Si Base
composition and to get an indication of the effects of modest additions (~ 1 % or less) of copper, nickel
and manganese. The alloy effects of copper and nickel were of interest because they are well known P/M
additives. The manganese composition was included because there was also an interest to compare the
results with the properties of the cast irons and other than carbon and silicon, all of the several cast iron
grades that exist contain some manganese [9].
The tensile properties of the Si Base and of four alloy modifications of the Base were determined in the
as-infiltrated condition and both the stress relieved and sub-critically annealed conditions. The asinfiltrated properties of the five compositions are shown below in Table 2.
Table 2 – As-Infiltrated Properties of the Si Base and Cu, Ni and Mn Modified Mixes
Yield Strength
Ultimate Strength
Elongation
Hardness
Alloy ID
MPa
(103 psi)
% in 2.5 cm
HRA
MPa
(103 psi)
353.7
(51.3)
468.2
(70.0)
1.4
56
Si Base
422.0
(61.2)
597.1
(86.6)
2.2
60
Base + 1 Cu
376.5
(54.6)
504.0
(73.1)
1.7
55
Base + 1 Ni
430.9
(62.5)
582.6
(84.5)
1.8
60
Base + 1 Cu + 1 Ni
384.7
(55.8)
515.7
(74.8)
1.5
57
Base + 0.5 Mn
Density
g/cm3
7.47
7.43
7.46
7.25
7.34
The final carbon contents of the specimens ranged from ~ 2.0 to 2.1 %. The individual values generally
varied with the eutectic solidus values of the base compact compositions as indicated in the earlier
Table 1 and otherwise appeared to have been effected by modest dusting and sintering losses during
processing in advance of infiltration. Assuming complete graphitization of the hyper-eutectoid carbon
contents involved, the corresponding pore free densities of the specimens would be expected to range
from ~ 7.52 to 7.54 g/cm3. Thus, the densities of the first three compositions listed in Table 2 were all
upwards of 98.5 % of the pore free value. In contrast, the densities of the last two compositions listed
were obviously low in comparison with the first three. Nevertheless, both were upwards of 96 % of the
pore free value.
In the case of the Base + 1Cu + 1Ni specimens, the low density is thought to indicate an unfavorable
effect of the combination of the copper and nickel on one or more of the densification mechanisms
involved in the process. Other than infiltration which, of course, is the principle mechanism, the process
that was used in the study was specifically designed to effect about 20 to 25 % of the densification by
liquid phase sintering. Thus, the low density, in this case, was very probably the result of a poor liquid
phase sintering response due possibly to an adverse effect of the alloy additives on the dihedral angle of
the system. Liquid phase sintering may have also contributed to the low density of the Base + 0.5 Mn
specimens as well. However, in this case, there were clear indications that the effect was due chiefly to
gas formation, probably the result of on-going MnO reduction, subsequent to infiltration.
As expected, the tensile and hardness results in the table showed that the Si Base composition had the
lowest overall properties. In comparison, each of the alloy compositions exhibited substantially higher
strength values and in most cases, modestly higher ductility and hardness values as well. The greatest
increases in all four properties were in the specimens of the Base + 1Cu and the Base + 1Cu + 1Ni
compositions. In view of the apparently poor liquid phase sintering response of the latter composition, the
general implication of the findings was that copper was the single most effective alloy addition.
The stress relief anneal generally led to very modest improvements in the ultimate strength and ductility
values but to little or no change in the yield strength, hardness or density values. In contrast, the subcritical anneal led to fairly substantial ductility increases but to equally significant decreases in strength
and hardness. The density, in most cases, was unaffected by this treatment. The results of the sub-critical
anneal are shown below in Table 3.
Table 3 – Sub-Critically Annealed Properties of the Si Base and Cu, Ni and Mn Modified Mixes
Yield Strength
Ultimate Strength
Elongation Hardness Density
Alloy ID
MPa
(103 psi)
% in 2.5 cm
HRA
g/cm3
MPa
(103 psi)
315.8
(45.8)
475.8
(69.0)
2.1
55
Si Base
7.47
363.4
(52.7)
561.9
(81.5)
2.7
57
Base + 1 Cu
7.39
326.8
(47.4)
490.2
(71.1)
2.5
55
Base + 1 Ni
7.46
334.4
(48.5)
506.8
(73.4)
3.4
53
Base + 1 Cu + 1 Ni
7.25
324.1
(47.0)
475.8
(69.0)
2.1
54
Base + 0.5 Mn
7.34
The presence of free graphite in these compositions makes them similar in many respects to the cast irons.
Apart from alloy content and the microstructure of the iron base matrix, it is the morphology of the
graphite precipitates that largely determines the properties of the cast irons. In general, there are four
different morphologies which comprise the predominant types in each of the four principal cast iron
grades. In order of decreasing symmetry and correspondingly, of decreasing potential in terms of
mechanical properties, these include: 1) the nodular or spheroidal type of the Ductile cast irons; 2) the
temper carbon type of the Malleable cast irons; 3) the vermicular or compacted type of the Compacted
Graphite cast irons; and, 4) the flake type of the Grey cast irons [10]. The predominant graphite
morphology of the present compositions including those that have yet to be discussed was the vermicular
or compacted type. Thus, it is of interest to compare the properties of the present compositions with those
of the Compacted Graphite (or CG) cast irons.
For the record, a micrograph showing the vermicular or compacted graphite morphology that typified the
compositions of the study is shown overleaf in Figure 1.
Figure 1 – Typical Graphite Morphology of Iron Base Infiltrated Specimens
For purposes of the indicated comparison, the mechanical properties of two grades of the CG cast iron as
reported in the open literature are presented in Table 4 [11]. These data include the as-cast condition as
well as two conditions of heat treatment: the fully ferritized; and, the normalized conditions. Based on the
similarities of the microstructures of the iron based matrices involved, the as-cast and normalized
conditions are reasonably comparable to the present as-infiltrated condition. The ferritizing condition,
however, is specifically designed to increase ductility and is not comparable with either the as-infiltrated
condition or the sub-critically annealed condition. These results were included here primarily as general
information and to provide a point of comparison with findings that will be introduced later.
A brief review of the data in Table 4 will show that the highest strength and hardness properties are
associated with the normalized condition and the highest ductilities are, of course, those of the ferritized
condition. Predictably, the strength and hardness values of the nickel containing grade were generally
better than those of the un-alloyed grade.
Table 4 - Typical Tensile Properties Of Compacted Graphite Cast Irons
Yield
Tensile
Strength
Strength
Elongation
Iron
Matrix (a)
Condition
MPa
(ksi)
MPa
(ksi)
%
60% F
263
(38.1) 325
(47.1)
2.8
As-Cast
Ferritized (b)
100% F
231
(33.5) 294
(42.6)
5.5
Normalized (c)
90% P
307
(44.5) 423
(61.3)
2.5
328
(46.7) 427
(61.9)
2.3
As-Cast
…
Ferritized (b)
100% F
287
(41.6) 333
(48.3)
6.0
Normalized (c)
90% P
375
(54.4) 503
(73.0)
2.0
Hardness
HRA (e)
48
47
52
53
49
56
Nickel
%
1.5
1.5
1.5
(a) F, ferrite; P, pearlite. (b) Annealed, 2 hr. at 900 oC (1650 oF), furnace cooled to 690 oC (1275 oF), held 12 hr.,
cooled in air. (c) Austenitized 2 hr. at 900 oC (1650 oF), cooled in air. (e) Converted from Brinell values.
Comparison of the data in this table with those in the earlier Table 2 will show that the strength and
hardness of all of the infiltrated compositions were significantly better than those of both the CG irons in
the as cast condition and of the plain CG iron in the normalized condition. Otherwise, the strength and
hardness of the Si Base composition closely approached those of the nickel containing grade in the
normalized condition while those of the remaining four compositions were either equivalent or
substantially better than those of the nickel grade in the normalized condition.
In contrast, the ductilities of the infiltrated compositions were generally not as good as the cast irons.
However, the differences in the as-cast and normalized conditions were not great and may have simply
been a natural consequence of the greater strengths and hardnesses of the infiltrated compositions. In
addition, the geometries of the test specimens that were used in each case were significantly different and
may have also contributed to the ductility differences. For example, the properties of the cast irons were
reportedly based on 25 mm (1”) diameter rounds whereas the infiltrated compositions were, as previously
indicated, based on the standard P/M dog bone geometry.
The fact that the strengths and hardnesses of the infiltrated compositions were generally better than those
of the CG cast irons is thought to be attributable to the inherently lower densities of the cast irons. For
example, compared with the present compositions, the cast irons generally have both higher carbon and
higher silicon contents and each lead to significantly lower pore free densities. In the case of the CG
irons, the carbon reportedly ranges from 2.5 to 4.0 % and the silicon from 1.0 to 3.0 % [9]. At a midrange carbon of 3.25 % and a mid-range silicon of 2.0 %, quantitative estimates indicate that their pore
free density ranges from about 7.25 g/cm3 in the normalized condition (~ 90 % pearlite) to about
7.17 g/cm3 in the fully ferritized condition (100 % ferrite). In comparison, at a mid-range carbon of
2.05 % and a silicon of 0.68 %, the same method of estimation indicates that the pore free density of the
present Si Base composition ranges from 7.53 g/cm3 in the normalized condition (~ 95 % pearlite) to
about 7.45 g/cm3 in the fully ferritized condition.
It is also of interest to compare the properties of the present compositions with those of the standard P/M
grades. Since the microstructures of the infiltrated compositions are predominantly pearlitic, the most
realistic comparison is with the predominantly pearlitic P/M grades or, in effect, with the 0.6 to 0.9 %
carbon containing grades in the as-sintered condition. MPIF Standard 35 lists three such grades which are
otherwise plausibly comparable in terms of their total ‘second’ alloy contents. These include the
following: F-0008, FC-0208 and FN-0208. The highest tensile properties that the Standard lists for each
are shown below in Table 5.
Table 5 – Tensile Properties of Comparable Standard P/M Grades
Yield Strength
Ultimate Strength
Elongation
MPIF
Grade Designation
MPa
(103 psi)
MPa
(103 psi)
% in 2.5 cm
275.8
(40.0)
393.0
(57.0)
1.0
F-0008-35
448.2
(65.0)
517.1
(75.0)
<1.0
FC-0208-60
379.2
(55.0)
620.6
(90.0)
3.0
FN-0208-50
Hardness
HRA *
44
52
54
Density
g/cm3
7.0
7.2
7.4
* Converted from HRB values.
Limiting the considerations to the Si Base and Base + 1Cu compositions, comparison of the data in this
table with those in the earlier Table 2 will show that the properties of the Si Base were far superior to
those of the F-0008 grade and otherwise approached those of the FC-0208 grade. Similarly, the properties
of the Base + 1Cu composition were generally superior to those of the FC-0208 grade and rivaled those of
the FN-0208 grade. Here again, the higher density of the infiltrated compositions is almost certainly the
major underlying cause of the indicated differences.
Alloy Effects of Molybdenum and Molybdenum Plus Copper
The second in the series of the three smaller studies mentioned had the objective to determine the alloy
effects of modest additions of molybdenum and molybdenum and copper to the Si Base composition. It
consisted essentially of infiltrating the four molybdenum containing compositions that are indicated in the
earlier Table 1 and of determining their tensile properties in each of three conditions including: the asinfiltrated, the stress relieved, and the sub-critically annealed conditions. As is generally well known,
molybdenum is more frequently combined with nickel in P/M applications than with copper. Thus, it may
be of interest to note that the preference for copper in this instance was largely based on its relatively
better performance than nickel in the earlier study.
As in the earlier study, the properties in the stress relieved condition were in most cases very similar to
those of the as-infiltrated condition. However, in the case of the Base + 0.5 Mo + 2 Cu composition, the
stress relief effected significant increases in both the yield and ultimate strength values of the order of
70 Mpa (10,000 psi) each. Thus, in this instance, it is appropriate to present and discuss the stress relieved
properties rather than the as-infiltrated ones. These are shown below in Table 6.
Table 6 -Tensile Properties of the Molybdenum Containing Alloys in the Stress Relieved Condition
Elastic
Yield
Ultimate
Elongation Hard. Den.
Modulus
Strength
Strength
Alloy ID
6
3
3
GPa (10 psi) MPa (10 psi) MPa (10 psi) % in 2.5 cm RHA g/cm3
162.0 (23.5) 450.2 (65.3) 648.1 (94.0)
2.0
56
Base + 0.3 Mo
7.49
162.7 (23.6) 570.2 (82.7) 685.4 (99.4)
1.3
58
Base + 0.5 Mo
7.53
1.7
61
Base + 0.3 Mo + 1 Cu 171.0 (24.8) 588.1 (85.3) 748.1 (108.5)
7.50
1.3
69
Base + 0.5 Mo + 2 Cu 160.7 (23.3) 557.8 (80.9) 753.6 (109.3)
7.53
Comparison with the data in Table 2 will show that the infiltrated densities of these compositions were a
little higher than those of the earlier study. The two highest values listed (7.53 g/cm3), in fact, equaled the
best available estimates of the corresponding pore free densities. As a matter of interest, assuming no
graphitization of the hyper-eutectoid carbon, estimates place the limiting pore free density of the iron base
infiltration process at about 7.64 g/cm3. Thus, higher infiltrated densities than the present values are
possible and are sometimes observed. However, their occurrence indicates incomplete graphitization of
the hyper-eutectoid carbon and the likely presence of coarse grain boundary carbides which are almost
certain to have adverse effects on ductility and ductility related properties (e.g., ultimate strength,
toughness and machinability).
In addition to the strength and elongation values, the table also lists the elastic modulus values that were
observed in the tests. These data were included because this property is chiefly affected by density and the
high densities of the present compositions were expected to manifest as increased modulus values. In fact,
the values shown in the table are generally higher than those quoted in MPIF Standard 35 for the majority
of P/M grades which, of course, typically involve lower densities. However, there was also a lot of
scatter in the present data and there is reason to suspect that somewhat higher values may have been
observed if the method of testing had been more reliable. For example, in a study of the effects of carbon
content on the properties of Ductile iron, the observed modulus values for carbon contents in the present
range (2.0 to 2.1 %) averaged about 10 % higher than the above values at ~182 GPa (26.5 x 106 psi), [12].
A review of the remaining data in Table 6 will show that strength and hardness generally increased and
ductility decreased with increasing alloy content. Thus, the Base + 0.3 Mo composition had the lowest
ultimate strength and hardness and the highest elongation values whereas the Base + 0.5 Mo + 2 Cu
composition had the highest strength and hardness and the lowest elongation values. The correlation with
alloy content, however, was not perfect. It broke down somewhat in the case of the yield strength. For
example, the yield strength increased with increasing alloy content up to the Base + 0.3 Mo + 1 Cu
composition but decreased again in the case of the Base + 0.5 Mo + 2 Cu specimens. This and the fact
that the ultimate strength and elongation differences between these compositions were not large (i.e.
almost certainly not statistically different) suggested that the leaner of the two may actually be closer to
the optimum in terms of alloy content. As will be seen, the properties in the sub-critically annealed
condition tended to support this idea but also showed an interesting aspect of the more highly alloyed
composition as well. These findings are shown below in Table 7.
Table 7 - Sub-Critically Annealed Properties of the Molybdenum Containing Compositions
Elastic
Yield
Ultimate
Elong.
Hard. Den.
Modulus
Strength
Strength
Alloy ID
MPa (103
% in 2.5
RHA g/cm3
GPa (106 psi) MPa (103 psi)
psi)
cm
Base + 0.3 Mo
172.4 (25.0) 344.1 (49.9) 568.1 (82.4)
3.3
51
7.49
Base + 0.5 Mo
171.0 (24.8) 383.4 (55.6) 588.8 (85.4)
3.7
55
7.53
Base + 0.3 Mo + 1 Cu 186.2 (27.0) 373.0 (54.1) 588.1 (85.3)
5.1
52
7.50
Base + 0.5 Mo + 2 Cu 162.7 (23.6) 490.9 (71.2) 608.8 (88.3)
2.1
60
7.53
Comparison of these results with those in Table 6 will show that the anneal in this case had the same
general effects as seen in the previous study (Table 2 vs. Table 3). The strength and hardness values
decreased and the elongation values increased. However, in contrast with the previous study, the
elongation improvements in this instance were substantially larger than the earlier ones both on a
percentage basis and, in most cases, in terms of the final absolute values. Undoubtedly, the best example
of the latter is the elongation value of the Base + 0.3 Mo + 1 Cu composition which at 5.1% rivaled those
of the CG irons in the fully ferritized condition (Table 4). The reasons underlying the greater elongations
in this instance, however, were uncertain. Since the processing in the two studies was nominally the same,
its reasonable to speculate that they were some how related to the compositional differences (e.g.,
possibly to an effect of the molybdenum on carbide spheroidization kinetics). However, whatever the
reason, it will be clear that its investigation was generally beyond the scope of the study.
The results in Tables 6 and 7 also show that both the density and the elastic modulus properties were
essentially unaffected by the anneal. The constancy of these values and especially of the density is an
indication that carbide spheroidization rather than graphitization was the primary metallurgical effect of
the anneal. If significant graphitization had occurred during the process, then the density and, at least in
theory, the modulus values would have decreased. As a matter of interest, metallographic examinations
confirmed the existence of significant carbide spheroidization in connection with the anneal.
Returning to the general question of alloy effects, comparisons showed that the strength and hardness
properties of the molybdenum containing compositions were substantially higher than those of the
compositions of the earlier study in both the as-infiltrated and stress relieved conditions. This is indicated
by the data in Tables 2 and 6 but, of course, the comparison in this instance in not direct because these
tables refer to different conditions (as-infiltrated vs. stress relieved). In the case of the data in Tables 3
and 7, however, the comparison is direct and clearly show the indicated superiority of the molybdenum
containing compositions in the sub-critically annealed condition.
Copper and molybdenum are commonly added to the Compacted Graphite cast irons and here again,
where reasonably direct comparisons were possible, the indications were that the properties of the present
iron base infiltrated compositions were generally superior in strengths and hardness and reasonably
comparable in ductility [13, 14]. For example, based on a correlation of the effects of molybdenum on the
tensile properties of the CG irons, the ultimate strength to be expected in the predominantly pearlitic iron
is reportedly ~ 475 MPa (68,900 psi) at a molybdenum content of 0.3 % and ~ 522 MPa (75,600 psi) at a
content of 0.5 % [15]. As will be evident, both values are significantly inferior to the ultimate strength
values shown in each of the foregoing tables for the Base + 0.3 Mo and Base + 0.5 Mo compositions.
More interesting, perhaps, is a comparison of the properties of the present compositions with those of the
well known P/M grades that contain molybdenum. The properties of three such grades as selected from
the Low Alloy Steel and Diffusion Alloyed Steel categories of MPIF Standard 35 are shown below in
Table 8. In each case, the data correspond to steels in the as-sintered condition with carbon contents in the
eutectoid range from 0.4 to 0.9 %. Significantly, the strength, hardness and modulus values that are
shown in the table are essentially the highest values listed in the Standard for steels in the as-sintered
condition.
Table 8 – Properties of Selected Molybdenum Containing P/M Steels
Elastic
Yield
Ultimate
Elong.
MPIF
Modulus
Strength
Strength
Grade Designation
GPa (106 psi) MPa (103 psi) MPa (103 psi) % in 2.5 cm
FLN2-4405-60
162.0 (23.5) 482.7 (70.0) 689.5 (100.0)
2.0
FD-0208-65
158.6 (23.0) 503.3 (73.0) 710.2 (103.0)
1.0
FD-0408-65
168.9 (24.5) 489.5 (71.0) 861.9 (125.0)
2.0
Hard.
Den.
RHA
56
56
58
g/cm3
7.30
7.25
7.40
As those familiar with the MPIF system of grade designation will appreciate, the alloy contents of the
indicated grades are generally both different and in particular, higher than those of the present infiltrated
compositions. All three of the P/M grades contain at least 0.5 % molybdenum and 1.75 % nickel. In
addition, each of the diffusion alloyed (FD pre-fixed) grades nominally contain 1.5 % copper. However,
despite these differences, comparison of the data in this table with those in the earlier Table 6 will show
that the strength, hardness and modulus values of each of the two copper containing variants of the
infiltrated compositions were superior to the those of the FLN2-4405 and FD-0208 grades and were either
superior to or closely approached those of the FD-0408 grade. Otherwise, the ductility values of the two
data sets were reasonably comparable.
Alloy Effects of Copper in the Si Base and Two Additional Heat Treatments
The objective of the third and last of the three smaller studies mentioned was to examine the effects on
properties of copper modifications of up to 2 % of the Si Base composition in five conditions as follows:
as-infiltrated, stress relieved, sub-critically annealed, normalized and partially ferritized. The study also
included determinations of the TRS properties in the as-infiltrated condition. These results are shown
below in Table 9.
Table 9 - TRS Properties of the Si Base and Copper Modified Base in the As-infiltrated Condition
Transverse
Dimensional
Rupture Strength
Change
Hardness
Density
Alloy ID
3
%
RHA
g/cm3
MPa
(10 psi)
1051.5
(152.5)
0.59
58
Si Base
7.44
1100.4
(159.6)
0.77
60
Base + 1 Cu
7.40
1148.7
(166.6)
0.63
62
Base + 2 Cu
7.43
The TRS properties were chiefly of interest because target applications include Grey cast iron
components and rupture strength is a commonly cited property of the Grey irons. However, the latter is
typically quoted in terms of the breaking load of a standard test specimen and is not directly comparable
with the usual P/M values. For comparison, standard P/M specimens were prepared from a Grey iron
component of interest. They exhibited an average TRS value of 465 MPa (67,500 psi), a hardness of
53 RHA, and a density of 7.23 g/cm3.
The tensile properties of the subject compositions in each of the three process conditions that have so far
been cited in the paper are shown in Table 10.
Table 10 - Tensile Properties of the Si Base and Two Copper Containing Compositions in the AsInfiltrated, Stress Relieved and Sub-Critically Annealed Conditions
Yield Strength
Ultimate Strength
Elongation Hardness Density
Alloy ID
MPa
(103 psi)
% in 2.5 cm
HRA
g/cm3
MPa
(103 psi)
As-Infiltrated
366.7
(53.1)
502.6
(72.9)
1.9
59
Si Base
7.46
410.3
(59.5)
613.7
(89.0)
2.4
60
Base + 1 Cu
7.39
481.3
(69.8)
604.7
(87.7)
1.5
60
Base + 2 Cu
7.44
Stress Relieved
354.4
(51.4)
528.8
(76.7)
2.3
59
Si Base
7.46
413.0
(59.5)
606.1
(87.9)
2.2
59
Base + 1 Cu
7.39
484.0
(70.2)
580.6
(84.2)
1.1
61
Base + 2 Cu
7.42
Sub-critically Annealed
298.6
(43.3)
524.7
(76.1)
3.4
56
Si Base
7.46
322.0
(46.7)
535.1
(77.6)
4.0
54
Base + 1 Cu
7.42
382.0
(55.4)
535.7
(77.7)
3.1
56
Base + 2 Cu
7.43
A review of these data will show that the densities of the Si Base and Base + 1 Cu compositions were a
little lower than earlier (Tables 2 & 3) and that the density of the Base + 2 Cu composition was essentially
intermediate of these two. All of the values were upwards of 98 % of the pore free value (~7.53 g/cm3).
Comparison of the tensile properties of the Si Base and Base + 1 Cu compositions with those of the
earlier study will show that the two data sets were generally similar. In many cases, the ultimate strength
and elongation values in the present data were marginally higher while the yield strength and hardness
values were either the same or marginally lower. The general trends in the data with respect to the effects
of the heat treatments were also similar. Relative to the as-infiltrated condition, the stress relief either
slightly increased the ultimate strength and elongation values or had no effect while the sub-critical
anneal generally decreased the strength and hardness values and increased the elongations.
In the case of the Base + 2 Cu composition, the major effects of the additional copper appeared to be to
increase the yield strength and hardness and decrease the elongation values relative to both the Si Base
and Base + 1Cu compositions. The ultimate strength values of the higher copper composition were either
the same or marginally lower than those of the Base + 1 Cu composition. The indicated increases in the
yield strength were in all cases fairly substantial (≥ 70 MPa ≅ 10,000 psi) whereas the hardness increases
were generally marginal. There was also a similar indication in the case of the yield strength of the high
Cu variant of the earlier molybdenum containing compositions but only in the sub-critically annealed
condition (Table 7).
The general indication of the study as a whole with respect to copper was that its most consistent effects
were in increasing the yield strength and, to a lesser degree, the hardness properties. For example, at 1 %
copper, the yield strength and hardness increases were attended by similar or greater increases in the
ultimate strength and by increased ductilities. However, at 2 % copper, whereas the yield strength
continued to increase, the effects on the ultimate strength and hardness were both small and mixed and
were otherwise accompanied in all cases by decreased ductilities.
The properties of the subject compositions in the normalized condition are shown below in Table 11.
Table 11 – Properties of the Si Base and Copper Modified Bases in the Normalized Condition
Yield Strength
Ultimate Strength
Elongation Hardness Density
Alloy ID
MPa
(103 psi)
% in 2.5 cm
HRA
g/cm3
MPa
(103 psi)
584.0
(84.7)
781.2
(113.3)
2.2
65
Si Base
7.44
638.5
(92.6)
832.9
(120.8)
2.1
65
Base + 1 Cu
7.40
732.9
(106.3)
835.7
(121.2)
1.1
67
Base + 2 Cu
7.41
A review of these data will show that the heat treatment had virtually no effect on density but very
substantial effects on each of the other properties listed. For example, a general comparison of the tensile
and hardness values in the table with those in the balance of the paper will show that in most cases, they
surpassed the best of the latter in strength and hardness and were otherwise similar in ductility in the asinfiltrated and stress relieved conditions.
Both these properties and the as-infiltrated and stress relieved properties in the earlier Table 10 are
generally much higher than those of any of the known CG irons. However, they are comparable with the
properties of the Ductile irons in the plain or essentially un-alloyed condition (i.e. containing from 3 to
4 % C, 0.1 to 1.0 % Mn, and 1.8 to 2.8 % Si) [9]. Remarkably, in fact, although marginal in ductility, the
strength and hardness values indicated by the present findings significantly exceed those of the latter in
most instances. For example, the minimum requirements of the three highest strength grades of plain
Ductile iron in accordance with ASTM A 536 are shown below in Table 12.
Table 12 – Minimum Tensile Property Requirements of Ductile Iron According to ASTM A 536
Yield Strength Ultimate Strength
Elongation Typically Recommended
ASTM
3
Process Condition
Grade Designation MPa (103 psi)
MPa
(10 psi)
% in 2.5 cm
413.7
(60.0)
551.6
(80.0)
3.0
As Cast
80-60-03
482.7
(70.0)
689.5
(100.0)
3.0
Normalized
100-70-03
620.6
(90.0)
827.4
(120.0)
2.0
Oil Quenched & Tempered
120-90-02
A cursory comparison of the present findings with these data will show the following: 1) the requirements
of the 80-60-03 grade were closely approached by the properties of the Base + 1 Cu composition in the
as-infiltrated and stress relieved conditions; 2) the strength and hardness requirements of the 100-70-03
grade were significantly exceeded by the properties of the Si Base composition in the normalized
condition; and, 3) the requirements of the oil quenched and tempered 120-90-02 grade were met by the
properties of the Base + 1 Cu composition in the normalized condition.
To be fair, what is being called normalizing here may be more akin to sinter hardening. As is generally
well known, the normalizing heat treatment derives its name from the fact that it consists of austenitizing,
usually at a low temperature above the upper critical, followed by normal cooling in still air. As applied
to cast irons, the general aims are to eliminate hyper-eutectoid carbides if they exist, refine the grain size
and produce an iron base matrix that has a predominantly pearlitic microstructure. The cooling rate of the
process naturally depends on the mass of the casting and may be less than ~10 oC/min (0.3 oF/sec).
In comparison, the findings in the present case derive from specimens that were ‘normally’ cooled in the
cooling zone of a P/M furnace. As mentioned in the procedure section, the average cooling rate was
estimated to be ~100 oC/min (3 oF/sec) in the temperature range from 845 to 315 oC (1550 to 600 oF).
This, of course, is essentially an intermediate cooling rate in the sinter hardening range and, at the very
least, would be expected to produce a finer grain size and a smaller interlamillar spacing than would be
typical of a normally cooled large casting.
In fact, metallographic examinations showed the microstructures of the resulting iron base matrices of all
three compositions to be 100% pearlitic. In each case, the grain size was refined relative to the asinfiltrated condition and the interlamillar spacing of the pearlite was exceedingly fine; being completely
irresolvable at 1000X. Thus, in view of the fact that the pore free density of the Ductile irons is about the
same as that of the CG irons, the excellent properties of the present compositions in comparison with the
Ductile iron requirements were thought to be largely attributable to the combined effects of their higher
densities and the relatively higher cooling rates that typify normal P/M processing.
The properties resulting from the partial ferritizing anneal of the subject compositions are shown below in
Table 13.
Table 13 – Response of the Si Base and Copper Modified Bases to the Partial Ferritizing Anneal
Yield Strength
Ultimate Strength
Elongation Hardness Density
Alloy ID
MPa
(103 psi)
% in 2.5 cm
HRA
g/cm3
MPa
(103 psi)
239.3
(34.7)
442.7
(64.2)
3.9
51
Si Base
7.42
315.1
(45.7)
497.1
(72.1)
3.5
55
Base + 1 Cu
7.34
437.8
(63.5)
551.6
(80.0)
1.8
58
Base + 2 Cu
7.38
The aim in attempting to partially ferritize the subject compositions was to get some idea of what the iron
base infiltration technology has to offer in terms of ductility. Unfortunately, however, the particular
treatment that was used here was not sufficiently ferritizing to do this. Accordingly, a comparison of the
above results with those in the earlier Table 10 will show that while the treatment did result in increased
ductility values relative to the as-infiltrated and stress relieved conditions, it was generally not as effective
in this regard as the simpler sub-critical anneal.
In cast irons, ferritizing is normally accomplished by a treatment that consists of first austenitizing at a
temperature that is sufficiently above the critical to dissolve any hyper-eutectoid carbides that may exist
followed by furnace cooling to a lower temperature that is just below the critical. The actual ferritizing of
the structure occurs at the lower temperature and typically requires minimal holding times of several
hours (cf., the ferritizing treatment that was noted in the case of the CG irons in Table 4). The specific
details of the process differ according to the type of cast iron being treated [8].
In the present case, limited dilatometric studies of the transformation characteristics of the Si Base
composition had suggested the particular treatment that was used here. As it turned out, analysis of the
findings combined with the results of metallographic examinations of the broken tensile specimens in
each case indicated that the treatment fell short of producing the desired effects for two reasons as
follows. First and foremost, the hold time (1.5 hrs) at the low temperature (715 oC) was simply too short;
and second, the low temperature itself may have been marginally high with respect to the Base + 1 Cu
composition and was almost certainly too high for the Base + 2 Cu composition. Thus, for now, the
ductility potential of the iron base infiltration technology remains for future studies to demonstrate.
Summary and Conclusions
The mechanical property potential of iron base infiltration as a novel P/M technology was presented in
terms of the tensile and transverse rupture properties and property improvements that are obtainable by
simple alloy modifications and/or heat treatment of a standardized silicon containing base composition.
Nine different alloy modifications of the Si Base composition involving modest additions of one or two of
copper, nickel, molybdenum and manganese (Table 1) as well as limited investigations of four different
heat treatments versus the as-infiltrated condition were included in the study. The latter included a low
temperature stress relief, a sub-critical anneal, normalizing and a partial ferritizing anneal.
Initially, the as-infiltrated tensile properties of the Si Base and four alloy modifications of the base were
presented (Table 2) and discussed. The alloy modifications involved additions of copper, nickel, copper
plus nickel and manganese. The as-infiltrated densities varied from a low of 7.25 g/cm3 to a high of
7.47 g/cm3 versus a pore free value of ~ 7.53 g/cm3. The properties of all four of the alloy modifications
were superior to those of the Si Base composition. The Base + 1 Cu composition exhibited the best
overall properties. Based on this and the infiltrated densities of the various compositions, nickel and
manganese were eliminated from further consideration in the study.
The five compositions were also tested in the stress relieved and sub-critically annealed conditions. In
most cases, the stress relief led to slight increases in ultimate strength and elongation but had little to no
effect on the yield strength, hardness or density. The sub-critical anneal generally decreased both strength
and hardness and increased elongation (Table 3). There was no significant effect of the anneal on density.
It was explained that most of the carbon in the silicon containing compositions of the study is in the form
of graphite precipitates of the vermicular or compacted variety which are also common to the so-called
Compacted Graphite or CG cast irons. Based on this similarity, the as-infiltrated properties of the Si Base
and Base + 1 Cu compositions were compared with those of a plain and an alloyed CG iron in both the ascast and normalized conditions (Table 4). The comparison generally showed that the properties of the Si
Base composition were superior to those of the plain CG iron and similar to those of the alloyed one and
that the properties of the Base + 1Cu composition were superior to both. The as-infiltrated properties of
the Si Base and Base + 1 Cu compositions were also compared with those of various well known P/M
grades with somewhat similar results (Table 5). In general, the alloy contents of both the P/M grades and
the CG irons in these comparisons were higher than those of the infiltrated compositions but their
densities were appreciably lower. Thus, the generally better properties of the infiltrated compositions in
the two cases were attributed to their higher densities.
The results of a similar study of the alloy effects of molybdenum and molybdenum plus copper on the Si
Base composition were next presented (Table 6) and discussed. The data in this case included the elastic
modulus values that were observed as well as the usual tensile properties. It was noted that the modulus
values were generally higher than those of most of the standard P/M steels and that this was expected in
view of the higher densities of the infiltrated compositions. Otherwise, the data showed that strength and
hardness increased and elongation decreased with increasing alloy content. Relative to the earlier study,
the strengths and hardnesses of the these compositions were all substantially higher than those of the Si
Base composition and with the exception of the leanest alloyed one of the present series, higher than those
of the Base + 1 Cu composition as well.
The properties of the molybdenum containing compositions in the sub-critically annealed condition were
also presented (Table 7) and discussed. As previously, the general effect of the anneal was to decrease
strength and hardness and increase elongation. Interestingly, however, in spite of the fact that the
strengths of these compositions were generally higher than those of the earlier study, their elongation
values were also generally higher as well. Metallographic examinations indicated that the primary
microstructural result of the anneal was carbide spheroidization and it was speculated that the better
ductilities in this case may be an effect of the molybdenum additions on spheroidization kinetics.
Limited comparisons with cast irons indicated that the as-infiltrated properties of the molybdenum
containing compositions were generally superior to those of the molybdenum containing CG irons.
Comparisons were also made with selected P/M steels in the standard MPIF alloy and diffusion alloyed
categories. The selected grades included the three that were reported as having the highest properties in
the as-sintered condition (Table 8). Owing to significant compositional differences that basically favored
the P/M steels, the comparisons were not direct. Nevertheless, they showed that the strength, hardness and
modulus values of each of the two copper containing variants of the subject infiltrated compositions were
better than those of two of the P/M steels and closely approached those of the third. Otherwise, the
ductility values of the two data sets were similar.
The results of a final series of studies which involved the Si Base composition and two copper containing
variants at 1 and 2% Cu were now presented and discussed. The studies included determinations of the
TRS and tensile properties of the three compositions in the as-infiltrated condition and the tensile
properties in the stress relieved, sub-critically annealed, normalized and partially ferritized conditions.
The TRS properties were presented (Table 9) and discussed first. It was noted that they were primarily of
interest because target applications included Grey cast iron components and the transverse rupture
strength is a commonly cited property of the Grey irons. Grey iron data were subsequently introduced that
clearly suggested that its properties in this respect are no match for those of the infiltrated compositions.
The properties of the three compositions in the as-infiltrated, stress relieved and sub-critically annealed
conditions were presented (Table 10) and discussed next. The properties of the Si Base and Base + 1 Cu
compositions were in most cases marginally higher than those of the earlier study in which they appeared
but the trends in the data with regard to the effects of the two heat treatments were about the same as
earlier. The major effects of the additional copper in the Base + 2 Cu composition were to increase the
yield strength and decrease the elongation relative to the Base + 1 Cu composition.
Possibly the most interesting results in terms of the mechanical property potential of the iron base
infiltration technology were the properties that were produced with these three compositions in the
normalized condition. In terms of strength and hardness, they basically represented the best properties of
the study as a whole and in particular, far exceeded any properties known to be possible with the CG cast
irons. Consequently, they were presented (Table 11) and discussed primarily with reference to the
minimum property requirements of the three highest strength grades of the Ductile cast irons in
accordance with ASTM A 536 (Table 12). The data showed that the properties of the Si Base composition
generally exceeded the requirements of the low and intermediate level grades of the three and that the
properties of the Base + 1 Cu composition easily met those of the highest strength grade.
The final study with these compositions involved a partial ferritizing anneal with the objective to
demonstrate the ductility potential of the technology. Regrettably, although the resulting elongation
values were improved relative to the as-infiltrated and stress relieved conditions (Table 13), they were
generally no better than those in the sub-critical annealed condition. Subsequent metallography,
nevertheless, confirmed the existence of the ferritization in all three of the compositions. The general
indications with regard to the failure to achieve the desired ductility effects were that the time at the
ferritizing temperature had been too short and the temperature had been a little too high.
ACKNOWLEDGMENTS
Special thanks are due to the Ben Franklin Technology Partners of Pennsylvania for funding a part of this
research and to Messrs. W. B. Bentcliff, G. Golin and T. Murphy of the Hoeganaes Laboratory for their
help in obtaining the data and figures used in preparing the manuscript.
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