Processing of Ferro-Phosphorus-Containing Mixes in Low Hydrogen Atmospheres
K.S. Narasimhan, D.J. Kasputis & G. Fillari
Hoeganaes Corporation
&
J. C. Lynn
DaimlerChrysler Corporation
Presented at PM2TEC2002
World Congress on Powder Metallurgy & Particulate Materials
June 16-21, 2002, Orlando, Florida
Abstract
Ferrophosphorus-containing premixes with iron are used extensively in magnetic applications. Recent
trends toward reducing cost necessitate reduction of hydrogen content in the sintering atmosphere.
This reduction of hydrogen leads to occasional brittle fracture in parts made from ferrophosphorus
mixes. The objective of this investigation is to develop a robust powder that could be used in low
hydrogen-containing sintering atmosphere. A design of experiments was developed to include residual
elements and oxygen level in the ferrophosphorus powder. The results suggest controlling the oxygen
in the ferrophosphorus is critical in achieving high impact energies while sintering in low hydrogen
atmospheres. Addition of a minute amount of graphite can improve the impact strength of the
otherwise low impact material.
Background
Phosphorus is an effective alloying element in iron enhancing the magnetic properties, increasing yield
strength and improving the ductility of P/M steels. Phosphorus is added as ferrophosphorus, mostly as
Fe3P, although Fe2P is also used. There are a number of articles that describe the benefit of using
phosphorus1-5. Fe3P acts as a liquid phase sintering aid and the mechanism of sintering is referred to
as transient liquid phase sintering. Phosphorus has the unique ability to be a solid solution hardener
and accelerate the sintering process. Phosphorus-containing P/M parts exhibit high impact strength
nearly 1.5 times that of sintered iron at the same density.
The amount of phosphorus that is generally added to iron powder is about 0.45 w/o. Higher levels of
phosphorus can lead to improved magnetic properties such as saturation induction. However, use of
phosphorus is limited to 0.8 w/o. At higher levels of phosphorus, processing conditions have to be
extremely well controlled to avoid phosphorus segregation to the grain boundaries, which can lead to
brittle fracture.
Pure iron powders such as Ancorsteel 1000B and Ancorsteel 1000C are ideally suited for magnetic
applications as they can be pressed to high densities and can achieve high values of saturation
induction (Bmax). Low coercive force (Hc) and high permeability (µmax) P/M parts made out of pure iron
powders are used in flux return path for direct current motors, antilock braking system wheel sensors,
solenoid plungers and bodies, exhaust gas recirculation (EGR), valve bodies, rotors for permanent
magnet motors, etc (See Table 1). In most of these applications, shape capability of P/M allows for
replacement of low carbon steels and lamination steels.
Table I: Property Comparison of P/M with Low Carbon Steels
AISI 1008
Ancorsteel
1000B @ 7.3
gm/cm³
Ancorsteel 45P @
7.35 gm/cm³
1900
2700
3500
Oe(kG)
14.4
15
15.1
Hc (Oe)
3
2.1
1.9
Yield Strength
42,000
21,000
42,000
(psi/MPa)
(285)
(145)
(285)
56,000
32,800
59,400
(psi/MPa)
(385)
(225)
(405)
Elongation (%)
37
13.7
12
Property
Max. Perm
Induction @ 15
Induction Level (kG)
Tensile Strength
20
18
16
Saturation
Induction
Induction @ 15 Oe
Ancorsteel 45P
14
12
Induction @ 15 Oe
Ancorsteel 1000B
10
8
6.8
6.9
7.0
7.1
Part Density (g/cm³)
Figure 1: Induction of P/M Materials, Function of Density & Alloying
7.2
7.3
Maximum Permeability
4000
3500
1260 °C (2300°F)
2
3000
1120 °C (2050 °F)
2500
2000
1500
6.70
6.80
6.90
7.00
7.10
7.20
7.30
7.40
Sintered Density (g/cm³)
Figure 2: Maximum DC Permeability as a Function of Sintering Temperature
Maximum Permeability
4000
3500
Ancorsteel 45P
3000
2500
Ancorsteel 1000B
2000
1500
6.8
6.9
7.0
7.1
7.2
7.3
Part Density (g/cm³)
Figure 3: Permeability as a Function of Part Density
Density plays a critical role on the magnetic properties, especially on saturation induction and
permeability. Figures 1 and 2 show the effect of density on the permeability and induction. Generally,
parts are sintered at 1120 °C (2050 °F) although sintering at 1260 °C (2300 ° F) can further enhance
the magnetic properties (Figure 3). The sintering atmosphere is generally dissociated ammonia (75v/o
H2).
This manuscript reviews a specific problem encountered in the processing of ferrophosphoruscontaining premixes for magnetic applications that require impact strength. Parts made from
ferrophosphorus premixes occasionally develop low impact energies. This phenomenon is generally
observed when the sintering atmosphere is low in hydrogen, e.g. 10 v/o H2, or 6v/o H2 compared to
75v/o H2 atmosphere which is generally used. In order to better understand the phenomenon, the test
matrix outlined in this report was formulated.
Test Procedure:
Mechanical Testing
Tensile tests were performed using dogbone samples tested according to ASTM Standard E8 and
MPIF Standard 10, unnotched Charpy impact tests were conducted according to MPIF Standard 40.
Tensile testing was performed on a 267,000 N (60,000 lbs.) Tinius-Olsen universal testing machine with
a crosshead speed of 0.635 mm/min (0.025 in/min.). Elongation values were determined by utilizing an
extensometer with a range of 0-20%. The extensometer was attached to the samples through failure.
Table II: Test Matrix
Test
Residual Levels in
Ferro-Phosphorus
Powder
Oxygen Content of
Ferro-Phosphorus
Powder
LL
Low
Low
LH
Low
High
HL
High
Low
HH
High
High
Results and Discussion:
A test matrix was set up to examine the effect of residual elements as well as oxygen content in the
ferrophophorus powder. The criticality of residual element control in the ferrophosphorus on the impact
energies has been reported6. During the transient liquid phase sintering, phosphorus goes into solution
leaving behind the residual elements at the grain boundary. These residual elements are easily
oxidizable elements such as Si, Mn, Cr, Ti. The test matrix was set up as shown in Table II. Various
melts were made with low and high residuals. Oxygen levels were controlled during the grinding of
ferrophosphorus. Ferrophosphorus powder has a mean particle size (dm) of 10µm with about 4%
above 20 micron. Ferrophosphorus powders thus produced were mixed with iron powder (Ancorsteel
1000B), pressed to a density of 6.8 gm/cm³ and sintered at 1120 °C (2050 °F) for 15 minutes or 30
minutes at sintering temperature in a C.I. Hayes furnace and cooled in a water jacketed cold zone.
Effect of Oxygen:
To set a limit to oxygen content in the test matrix studies, an experiment was carried out to purposely
oxidize phosphorus powders by exposing the powder to air at 1500 °F and cooling carefully to avoid
powder fines from igniting. Impact bars were pressed with these ferrophosphorus powders admixed
with iron to end up with 0.45w/o P in Fe (Ancorsteel 45P). Table III shows the oxygen content and the
impact values measured on sintered bars.
Table III: Impact Energies Variation with Oxygen Content in Fe3P
(Sintered at 1120 °C [2050 °F] in 10 v/o H2)
Oxygen in Fe3P Impact Energy (ft-lbf)
0.7
22.0
1.1
6.0
1.2
7.3
1.7
4.0
Table IV: Chemical Analysis on the Test Matrix Lots
Bal
Mn (w/o)
Cr(w/o)
Ti (w/o)
Si (w/o)
O (w/o)
Total Oxidizable
Impurities (w/o)
Fe + Trace of
Other Elements
0.17
0.15
0.2
0.04
0.67
0.57
LH2B
"
0.31
0.27
0.1
0.07
1.18
0.75
HL3A
"
0.22
0.61
0.14
0.23
0.36
1.2
HL3B
"
0.31
0.42
0.19
0.016
0.53
0.94
HH4A
"
0.18
0.36
0.29
0.12
1.26
0.95
Heat #
LL1
LL = Low Residuals, Low Oxygen
HL = High Residuals, Low Oxygen
LH = Low Residuals, High Oxygen
HH = High Residuals, High Oxygen
Table V: Impact & Mechanical Properties of Test Matrix
(2050 ° F, 10v/o H2 – 30 min. sinter)
Heat #
Impact
ft-lbf
22
UTS
10³PSI
51
YS
10³PSI
37
11.9
Hardness
HRB
57
LH2B
3.8
47
33
10.3
53
HL3A
17.5
48
35
9.9
56
HL3B
23
48
34
10.8
56
HH4A
4.3
48
34
10.7
54
LL1
El %
Results of Test Matrix
Table IV lists the chemical analysis on the various materials made. Table V lists the mechanical
properties measured on these materials. It can be noted that lower oxygen powder exhibits higher
impact energies than the higher oxygen-containing powder.
Lot HH4A which gave low impact energies when sintered in 10v/o H2 atmosphere developed higher
impact energies when sintered in 75v/o H2 atmosphere (Table VI). It appears that increased hydrogen
content reduced the presence of grain boundary oxides, potential sites for crack initiation. The data
suggest that sintering in hydrogen rich atmosphere is beneficial in ensuring maximum impact energies.
The low oxygen in the ferrophosphorus powder, to a large extent, can allow sintering in hydrogen-lean
atmosphere without compromising impact energies.
Table VI: Impact Energy of Lots HL3B and HH4A Sintered in Various Hydrogen Levels
in the Sintering Atmosphere (Sintered @ 2050 °F-30 minutes)
Impact Energy (ft-lbf)
v/o Hydrogen in
Sintered
Atmosphere
v/o Nitrogen in
Sintered
Atmosphere
HL3B
HH4A
6
94
21
2.7
10
90
19
4.3
25
75
23
18
75
25
23
27
Effect of Graphite:
Since hydrogen gas is expensive, it is desirable to use lower amounts in the sintering atmosphere. The
effect of adding graphite during the processing of P/M parts was investigated. It is well known that
carbon can adversely affect the magnetic properties and hence magnetic data was also collected.
Graphite was added in the amount of 0.02, 0.05, 0.08 and 0.1w/o to the 0.45 w/o P premix and pressed
into impact bars and dog bone tensile samples and sintered in 6 v/o H2, and 10 v/o H2 atmosphere.
The results are summarized in Table VII. As can be seen even for the high oxygen in the
ferrophosphorus, (1+ w/o) addition of 0.05 w/o graphite can dramatically improve the impact energies.
Additionally, the mechanical properties and dimensional change of mixes containing graphite were
measured and plotted in Figures 4, 5 and 6. In adding graphite, additional size changes were observed.
It is advisable that graphite additions be kept to a low enough level that the sintered carbon levels are
below 0.04%.
Magnetic properties were measured on mixes with and without graphite. Ring specimens were
sintered at 1120 °C (2050 °F) for 30 minutes and hysteresis loops were constructed. The coercive force
of a mix without graphite addition was 1.95 Oe and with the graphite addition (sintered carbon 0.04 w/o)
was 2.07 Oe, essentially an insignificant effect.
Table VII: Effect of Graphite Addition on Impact Energies and
Mechanical Properties of Ancorsteel 45P (FY4500) Mixes
Graphite
(Wt.w/o)
Oxygen in Fe3P
0.42
0.82
1.2
Impact Energy
(ft-lbf)
UTS
3
YS
3
(10 PSI) (10 PSI)
6 v/o H2 10 v/o H2
El
(%)
UTS
3
YS
3
(10 PSI) (10 PSI)
6 v/o H2
El
(%)
10 v/o H2
0
23
23
50
35
12.7
50
35
12.0
0.02
25
26
51
36
11.4
51
37
10.9
0.05
22
22
51
36
9.5
52
38
9.0
0.08
16
16
53
39
8.9
52
39
8.5
0
11
14
49
34
11.7
49
34
11.9
0.02
25
28
51
35
11.7
50
35
10.9
0.05
24
22
50
36
9.4
51
37
9.2
0.08
21
20
51
37
8.9
51
38
8.5
0
3
4
47
35
8.9
47
34
9.8
0.02
6
5
49
35
11.4
50
35
11.8
0.05
25
23
51
36
10.8
51
36
10.7
0.08
22
20
50
36
9.9
51
38
9.6
0.35
0.42 w/o - O2 in Fe3P - 10 v/o H2 / Bal N2
0.3
0.42 w/o - O2 in Fe3P - 6.0 v/o H2 / Bal N2
1.2 w/o - O2 in Fe3P - 10 v/o H2 / Bal N2
Dimensional Change %
0.25
1.2 w/o - O2 in Fe3P - 6.0 v/o H2 / Bal N2
0.2
0.15
0.1
0.05
0
-0.05
-0.1
-0.01
0.01
0.03
0.05
0.07
0.09
0.11
0.13
0.15
0.17
Graphite w/o
Figure 4: Dimensional Change as a Function of Added Graphite, Sintering Atmospheres are shown in
the Legend
30
25
Impact Energy (ft-lbf)
20
15
10
0.42 w/o - O2 in Fe3P - 10 v/o H2 / Bal N2
0.42 w/o - O2 in Fe3P - 6.0 v/o H2 / Bal N2
5
1.2 w/o - O2 in Fe3P - 10 v/o H2 / Bal N2
1.2 w/o - O2 in Fe3P - 6.0 v/o H2 / Bal N2
0
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
Graphite w/o
Figure 5: Impact Energy as a Function of Added Graphite, Sintering Atmospheres are shown in the
Legend
30
25
Impact Energy (ft-lbf)
20
15
0.42 w/o - O2 in Fe3P - 10 v/o H2 / Bal N2
10
0.42 w/o - O2 in Fe3P - 6.0 v/o H2 / Bal N2
1.2 w/o - O2 in Fe3P - 10 v/o H2 / Bal N2
5
1.2 w/o - O2 in Fe3P - 6.0 v/o H2 / Bal N2
0
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
Sintered Carbon
Figure 6: Impact Energy as a Function of Sintered Carbon, Sintering Atmospheres are shown in the
Legend
Impact as a Function of Temperature:
The impact energy of the ferrophosphorus mixes was measured with low oxygen-containing ferrophos
at various temperatures. As evident in Table VIII, good impact energies are achieved even at low
temperatures.
Table VIII: Impact Energy as a Function of Temperature of Ancorsteel 45P (FY 4500) Mixes
(Sintered at 2050 °F in 10 w/o H2 Atmosphere)
Temp.
(°F)
Density
(g/cm³)
DC (%)
Hardness
HRB
Impact (ft-lbf)
Avg. of 3
samples
RT
7.01
0.01
54
25
32
7.00
0.01
54
20
0
7.01
0.01
54
19
-40
7.01
0.00
54
17
Fractography and Dicussion:
Fractographic examination was made on specimens with both low impact and high impact energies.
Fractography of samples with graphite added to low impact compositions were examined as well.
Figures 7, 8, 9 are the fractographs from the three graphs respectively. As can be seen, ductile
fracture is associated with high impact energies. Transgranular cleavage is also seen in all three
specimens. However, intergranular cracking appears to be evident in low impact energy specimens.
Figure 7: Fractographs of Low Impact Energy Sample (Impact Energy – 4.3 ft-lbf)
Fig. 8: Fractographs of Low Impact Energy Mix with 0.07 w/o Gr Addition, (Impact energy – 23.5 ft-lbf)
Figure 9: Fractographs of Low Oxygen Fe3 P Mix, High Impact Values (Impact Energy 25 ft-lbf)
Auger analysis was performed on all three types of fracture surfaces in the vacuum chamber. Atomic
percent of elements detected are tabulated in Table VIII. Atomic percent of 5 for P in grain boundaries
is equivalent to 4 w/o and this is much higher than the overall P level of 0.45 w/o by weight. The
porosity surface has a lot lower level of iron (19.3w/o) when compared to that for grain boundary (58.9
a/o) or for overall (95+ a/o)
Table VIII: In-Situ Chemical Composition of Grain Boundary and Porosity Surface
Element
Oxygen
Carbon
Iron
Sulfur
Phosphorus
Atomic Percent
Porosity Surface
Grain Boundary
33.5
4.2
46.4
31.9
19.3
58.9
0.7
5
Auger elemental maps show that grain boundaries are rich in phosphorous and iron. The neighboring
porosity surface was found to be rich in oxygen and free of any Fe/P segregation. The individual wide
energy spectra for grain boundary and porosity surface are shown in Figures 10 and 11. Figure 13,
from the low impact energy group, shows grain boundary cracking mixed with original porosity surface.
Figure 10: The Wide Energy Spectrum for the Porosity Surface
Figure 11: The Wide Energy Spectrum for the Grain Boundary
Figure 12: Auger Analysis – Low Impact Energy Group – Showing Cracking Along Grain Boundaries
which are Rich in P and Fe when Compared to the Adjacent Porosity Surface
The segregation of phosphorus to grain boundaries, conceivably in the form of Fe3P, is believed to
degrade the bond strength. This is consistent with the observed intergranular cracking due to the
weakened grain boundaries. It is also known in the wrought steel industry that under certain conditions
(e.g., slow cool through certain temperature range) P will segregate to the grain boundaries resulting in
temper embrittlement. Multiple thin layers were sequentially sputtered away from the fracture surface
to determine the thickness of the Fe-P rich region and the results are presented in Figures 13 and 14.
The P-rich layer was estimated to be about 60 to 100 Ångstroms thick.
Figure 13: Result of Sputtering on Grain Boundary Layer Chemistry
Figure 14: Grain Boundary Chemistry upon Sputtering (Finer Scale)
Conclusion
A study of factors controlling the impact energy of mixes containing ferrophosphorus was undertaken to
pinpoint occasional brittle failures observed when parts are sintered in low hydrogen atmospheres.
While the control of easily oxidizable elements is essential, this study points to a need to maintain lower
oxygen in the ferrophosphorus powder to maintain acceptable impact energies. A study was also
conducted adding graphite in small amounts to offset potential oxygen pick-up during the processing of
parts in the sintering furnace. The results suggest that up to 0.04w/o sintered carbon can be tolerated
with no detrimental effects on the magnetic properties with only a small effect on dimensional change.
Acknowledgement
Many thanks to Dr. Harry Myer of Materials Analysis User Center in Oak Ridge National Laboratories
(ORNL) for the Auger analysis work. The coordination between Dale Wetzel of DaimlerChrysler
Materials Characterization Laboratories and ORNL is appreciated.
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1.
2.
3.
4.
5.
6.
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