A Comparison of FC-0208 to a 0.3% Molybdenum Prealloyed
Low-Alloy Powder with 0.8% Graphite
Francis Hanejko
Manager, Customer Applications
Hoeganaes Corporation
Cinnaminson, NJ 08077
Abstract
Iron copper steels are the most widely used structural PM materials. Although the
elemental copper addition melts during initial sintering, complete homogenization of the
copper is not achieved at conventional sintering temperatures and sintering times. This
lack of microstructural homogeneity coupled with the potential for large pores can often
lead to reduced strength and greater dimensional variability during sintering and
subsequent heat treatments, if required. A proposed alternative to the FC-02XX
materials is a 0.3% molybdenum prealloyed material. This material has nearly identical
compressibility compared with the iron copper materials but has the added benefit of a
homogeneous microstructure. This paper will focus on a comparison of FC0208 to a
0.3% molybdenum material with 0.8% added graphite. The materials will be compared
in both the as-sintered and heat-treated conditions. Additional testing will evaluate the
dimensional stability and variability of these two materials in both the as-sintered and
heat-treated conditions.
Introduction:
Iron-copper steels (FC-02XX) are the most widely used structural PM materials. Current
applications include main bearing caps, powder forged connecting rods, carrier
assemblies, etc. Although FC 02XX series are most frequently used in the as-sintered
condition, they are occasionally heat-treated to increase strength or improve wear
resistance. One intrinsic drawback of the iron-copper-carbon system is that copper is
added elementally. This elemental copper results in part growth during sintering
(positive dimensional change, DC) coupled with a potential for large voids after sintering.
[1] A review of the iron copper phase diagram shows that the room temperature solubility
of copper in iron is <2% [2]. Nevertheless, production experience shows that large free
copper particles are rarely seen in 2% copper steels. Copper contents above 2% show
copper precipitates in either the grain boundaries or at prior particle boundaries.
Although copper melts at 1083 °C (1981 °F), the diffusivity of copper into iron is such
that complete alloy homogenization is not achieved unless long sintering times or high
sintering temperatures are employed. Experimental work by Baran [3] and Lindsley [4]
illustrated high copper concentrations adjacent to prior particle boundaries with the
copper level in the center of the iron particle decreasing to almost zero, see Figure 1.
This copper gradient leads to variations in microstructure and a potential for variations in
dimensional change (DC) after sintering and heat-treating. Lindsley [4] demonstrated
that this localized gradient causes excessive retained austenite in sinter-hardening
grades that can transform to un-tempered martensite upon exposure to low temperature
treatments (or low climatic temperatures); thus resulting in additional growth of the
component.
a. SEM image of 0.50% molybdenum
steel with 2% copper added, sintered at
1120 °C (2050 °F) for 20 minutes at
temperature. The line illustrates where
both copper and nickel EDAX traces were
taken.
Figure 1:
b. Copper and nickel EDAX traces along
scan line in (a.). Note high concentration
of copper adjacent to prior particle
boundary. Copper trace is shown in green.
No nickel was added to this material
Copper gradient in a pre-alloyed molybdenum steel, sintered at 1120 °C
(2050 °F) for 20 minutes at temperature
The primary advantage of the FC-02XX materials is their unique combination of low
material cost coupled with ease of processing and good mechanical properties. [5]
Elemental copper additions do not degrade the base iron compressibility and compacted
densities of greater than 7.0 g/cm³ attainable. However, recent developments in the
manufacture of low molybdenum content steel powders presents the possibility of
substituting a prealloyed 0.3% molybdenum steel powder for the FC02XX materials.
With the recent downward trend of molybdenum pricing, the cost penalty of using
prealloyed molybdenum steels is lessened. If reductions in part-to-part dimensional
variation are possible, then any raw material cost differential will be offset by potential
reductions in out of specification parts. This paper will focus on comparing the
mechanical properties of an FC-0208 material vs. 0.3% molybdenum steel in both the
as-sintered and heat-treated condition. As a second part of this study, the effects of
variations in copper and graphite additions were studied to assess the impact of these
variables on dimensional change stability and sintered strength.
Experimental Procedure:
This study was divided into two segments. Part one of the study investigated an
FC0208 vs. a prealloyed 0.30% molybdenum steel with 0.8% graphite addition. Part two
investigated the sensitivity of variations in copper and graphite additions on the
dimensional change of the FC-0208 in relation to variations of graphite additions on the
prealloyed 0.3% molybdenum steel.
Part 1: Comparison of FC-0208 to 0.3% molybdenum with 0.8% graphite
Four premixes were prepared as shown in Table 1. The mixes containing atomized
Acrawax C were conventional premixes prepared via double cone blending. The
premixes containing the warm die lubricant (AncorMax 200™) were prepared via a
proprietary premixing process.
Table 1
Composition of Premixes Prepared for Part 1
Mix
1
2
3
4
Base Iron
Unalloyed iron powder
Unalloyed iron powder
Prealloyed 0.3%
Molybdenum
Prealloyed 0.3%
Molybdenum
% Copper
2
2
% Graphite
0.8
0.8
0.8
NA
0.8
% Lube & Type
0.75% Acrawax
0.4% warm die
lubricant
0.75% Acrawax
0.4% warm die
lubricant
The following test specimens were compacted from the four premixes listed in Table 1.
¾ Compressibility test specimens at 415, 550, and 690 MPa (30, 40, and 50 tsi).
For the warm-die premixes additional compressibility data was generated at 830
MPa (60 tsi). The conventional premixes were compacted in dies at room
temperature; whereas, the AncorMax 200 premixes were compacted at 93 °C
(200 °F), no powder preheating.
¾ Transverse rupture strength bars for both as-sintered and heat treated strength
at the various compaction pressures.
¾ MPIF standard dog-bone type tensile specimens for both as sintered and heattreat conditions (MPIF standard 10).
Sintering of all samples was done at 1120 °C (2050 °F) in a continuous belt furnace in
an atmosphere of 90% nitrogen / 10% hydrogen for ~20 minutes at temperature; the
cooling rate after sintering was ~0.6 °C per second (no accelerated cooling). All
samples were sintered on ceramic trays. Heat-treated samples were re-austenitized at
815 °C (1500 °F) in a 75% hydrogen / 25% nitrogen atmosphere for 1 hour at
temperature. They were oil quenched in 65 °C (150 °F) oil and subsequently tempered
at 205 °C (400 °F) for 1 hour in a nitrogen atmosphere.
These premixes were utilized to evaluate the dimensional change variability during the
heat-treat response of a prototype gear. The gear geometry used is shown in Figure 1.
Approximately 1,000 gears were made from each premix, the two conventional doublecone premixes were run in unheated tooling with a target part density of 7.1 g/cm³ and
the warm-die lubricant premixes were run in a die heated to 93 °C without top punch
heating and a target part density of ~7.1 g/cm³. The compacted gears were sintered at
1120 °C (2050 °F) for 20 minutes at temperature in a 90% nitrogen / 10% hydrogen
atmosphere in a 6-inch ceramic belt furnace without accelerated cooling. From these
sintered gears 50 were randomly chosen and sent to a commercial heat-treat facility for
a quenching and tempering operation. The heat treat cycle used was to austenitize at
870 °C (1600 °C) for 1 hour in a 0.8% carbon atmosphere followed by oil quenching and
tempering. From these 50 heat-treated gears ~20 were then measured for part-to-part
dimensional variation via a measurement over wire (MOW) of the gear form.
Spur Gear for Evaluation
–
–
–
–
–
–
–
Figure 2:
Major OD
1.10 in (28.3 mm)
Minor OD
0.85 in (21.6 mm)
Pitch Diameter 0.96 in (24.5 mm)
Pressure angle 20°
ID
0.38 in (9.5 mm)
# Teeth
16
Module
1.66
Spur gear produced to evaluate part-to-part consistency in the heattreated condition
Part 2: Effect of copper and graphite variations on FC0208 and effect of graphite
variations on 0.3% molybdenum prealloy
Shown in Table 2 is a list of premixes prepared with the objective of determining the
effects of copper and graphite variations on an FC-0208 material and the effects of
graphite variations on a 0.3% molybdenum prealloyed steel powder.
Table 2
Material Combinations Evaluated to Determine Effects
of Copper and Graphite Variations
Base Iron
Un-alloyed iron powder
Prealloyed 0.3%
molybdenum
w/o Copper
2.0
2.0
1.8
1.8
2.2
2.2
1.5
2.5
N/A
w/o Graphite
0.7
0.9
0.8
0.7
0.8
0.9
0.8
0.8
0.7
0.9
The range shown in Table 2 was selected to cover standard variations in alloy additions
that are traditionally used within the FC0208 specification range. All premixes shown in
Table 2 were prepared using conventional premixing techniques with 0.75% added
Acrawax C. From these premixes, standard transverse rupture strength bars were
compacted at 415, 550, and 690 MPa (30, 40, and 50 tsi). Sintering was done under the
conditions previously listed. The TRS bars were evaluated for dimensional change and
as-sintered strength.
Results and Discussion:
A. Property comparison of FC-0208 vs. 0.3% prealloyed molybdenum steel with
0.8% graphite
Figure 3 presents the compressibility curves of the four materials listed in Table 1. As
previously noted, compaction of the Acrawax containing premixes took place in
unheated dies at room temperature; whereas, compaction conditions of the warm-die
powder premixes was in dies heated to 93 °C (200 °F) without any prior powder heating.
The compressibility of the prealloyed 0.3% molybdenum steel is identical to the FC-0208
material at both room temperature and at die temperatures of 93 °C. Utilizing warm die
processing results in a 0.05 to 0.10-g/cm³ increase in green density relative to
conventional lubricants for both material options. The optimal benefit with warm-die
compaction is observed at compaction pressures greater than 550 MPa (40 tsi).
Although not shown, warm die processing also promotes higher green strength with
reduced green expansion and reduced ejection forces. [6]
The as-sintered and heat-treated transverse rupture strengths as a function of sintered
density for the FC0208 and 0.3% molybdenum prealloy with 0.8% added graphite are
presented in Figure 4. Warm-die processing did not affect the sintered or heat-treated
strength. As anticipated, in the as-sintered condition, the FC-0208 material possesses
higher strengths when compared to the prealloyed 0.3% molybdenum material.
However, once heat-treated the strengths of the two materials are nearly equivalent.
7.40
Green Density, g/cm³
7.30
7.20
7.10
7.00
FC0208
6.90
AM 200 FC0208
6.80
0.30% Mo with .8% Gr
6.70
AM 200 0.30% Mo
with .8% Gr
6.60
300
400
500
600
700
800
900
Compaction Pressure, MPa
Figure 3:
Compressibility of the four initial premixes
1300
1200
TR Strength, MPa
1100
1000
900
800
FC0208 as sintered
FC 0208 HT
A30 HP as sintered
A30 HP HT
700
600
500
6.60
Figure 4:
6.70
6.80
6.90
7.00
Sintered Density, g/cm³
7.10
As-sintered and heat-treated transverse rupture strengths
7.20
As-sintered tensile properties and heat-treated tensile properties for the 0.8% graphite
additions are presented in Table 3. Similar to the trend observed with transverse rupture
strength testing, the as-sintered yield and ultimate tensile strengths of the FC-0208 are
greater than the prealloyed 0.3% molybdenum steel. Elongation values are nearly the
same. Presented in Table 3 are the ultimate tensile strengths of the FC-0208 vs. the
0.3% molybdenum steel in the quenched and tempered condition. Similar to what was
found in TRS testing, in the heat-treated condition the prealloyed 0.3% molybdenum
steel with 0.8% graphite and heat-treated FC-0208 have nearly identical ultimate tensile
strengths. The data presented Table 3 represent compaction at 415, 550, and 690 MPa.
The prealloyed 0.3% molybdenum material has lower growth relative to the FC-0208,
thus the sintered densities at the same compaction pressure are higher for the
prealloyed molybdenum material.
Table 3
Summary of Tensile Properties of FC0208 and Prealloyed 0.3% Molybdenum Steel
Material
FC0208
0.30% Mo
Pre-alloy
with
0.80%
Graphite
Sintered
Density,
g/cm³
6.67
6.90
7.04
6.67
6.90
7.04
6.75
7.00
7.14
6.75
7.00
7.14
Heat
Treated
No
0.2%
YS
(MPa)
393
448
467
Yes
NA
No
293
333
359
Yes
NA
UTS,
(MPa)
470
564
597
672
771
870
334
415
435
705
805
855
Total
Hardness,
Elongation
(HRA)
(%)
1.4
47
1.8
50
1.9
53
64
<1.0
69
69
1.2
40
1.6
45
1.7
47
64
<1.0
69
71
B. Dimensional Change and How it is Affected by Variations in Chemistry
The dimensional change variations resulting from variations in graphite additions to the
two materials studied are studied in Figures 5 and 6. Lowering the graphite addition had
an opposite effect for the two materials evaluated. For the FC0208, lowering the
graphite to 0.7% had little effect on the DC, remaining approximately the same as the
0.8% additions. However, the 0.30% molybdenum pre-alloyed material with a 0.7%
graphite addition showed reduced the DC after sintering. Increasing the graphite to
0.9% resulted in less growth for the FC-0208 relative to 0.8% graphite and
approximately the same growth as the 0.8% graphite for 0.3% molybdenum prealloyed
steel. The trend observed for the FC-0208 material is consistent with data reported in
the literature. [7] Higher carbon contents in FC0208 material decrease the solubility of
copper in iron, thus reducing the growth and increasing the amount of fine copper
precipitates. [1] Conversely, graphite additions above 0.7% result in greater growth for
the 0.3% molybdenum prealloyed steel (greater volume expansion of the iron lattice).
0.45
Sintered DC, %
0.40
0.35
0.30
2% Cu, 0.80% Gr
2% Cu, 0.70% Gr
2% Cu, 0.9% Gr
0.25
0.20
6.70
6.75
6.80
6.85
6.90
6.95
7.00
7.05
7.10
7.15
Green Density, g/cm³
Figure 5:
Effect of Graphite additions on the DC of FC-0208
0.30
Sintered DC, %
0.25
0.20
0.15
0.80% Graphite
0.70% Graphite
0.90% Graphite
0.10
0.05
0.00
6.70
Figure 6:
6.80
6.90
7.00
Green Density, g/cm³
7.10
7.20
Effect of Graphite additions on the DC of 0.3% Molybdenum steel
The decrease in growth for FC-0208 with 0.9% graphite was approximately 0.07% to
0.08% over the green density range of 6.75 to 7.10 g/cm³ compared with the material
that contained 0.7 or 0.8% graphite. Reviewing the data for the 0.3% molybdenum
prealloyed steel, the increase in growth with varying the graphite addition from 0.7% /
0.8% to 0.9% was ~0.05% over the same green density range. Along with reduced
scatter, the molybdenum prealloyed steel exhibits reduced growth relative to the
FC-0208 material. Another feature of the prealloyed steel material was the reduced
growth as a function of increasing green density. The molybdenum steel showed about
a 0.05% increase in DC as the density increased from 6.75 to 7.10 g/cm³; while, the
FC-0208 material showed ~0.07% increase in growth over the same green density
range. Implications of these data are that the molybdenum steel is potentially less
sensitive to variations in green density and carbon variations part-to-part. This reduced
sensitivity can give reduced part-to-part dimensional scatter and lower heat-treat
variations. Interesting for both materials is the overlap of DC for the 0.7% and 0.8%
graphite additions for F-C0208 and 0.8% and 0.9% graphite for the 0.3% molybdenum
steel. The inference of this is that reduced growth variations can be achieved by
maintaining graphite levels in specific (albeit different) ranges for both materials.
0.50
0.45
Sintered DC, %
0.40
0.35
0.30
0.25
0.20
1.8% Cu, 0.8% Gr
2.2% Cu, 0.80% Gr
1.5% Cu, 0.80% Gr
2.5% Cu, 0.80% Gr
2.0% Cu, 0.8% Gr
A30HP, 0.8% Gr
0.15
0.10
0.05
0.00
6.70
Figure 7:
6.80
6.90
7.00
Green Density, g/cm³
7.10
7.20
Effect of Copper Variations on DC of FC0208
Shown in Figure 7 is the effect of copper variations on the FC-0208 again comparing the
data with the 0.8% graphite molybdenum steel. The data presented in Figure 7 suggest
that variations in copper content result in dimensional change variations up to 0.07%
over the copper range evaluated. As anticipated variations in both graphite and copper
additions result in variability in growth of the FC-0208 material. The absolute magnitude
of these variations is larger than the DC variation of the 0.3% molybdenum steel with the
potential variability of graphite only.
Table 4
Summary of DC Variations for FC-0208 and Pre-alloyed 0.3% Molybdenum
Material
FC-02XX
with 2%
Copper
0.3% Mo
Prealloy
Sintered
Density,
g/cm³
6.67
6.90
7.04
6.67
6.90
7.04
6.75
7.00
7.14
6.75
7.00
7.14
Heat
Treated
No
Yes
No
Yes
0.8% gr
0.9% gr
0.7% gr
+0.35
+0.40
+0.43
+0.20
+0.28
+0.31
+0.19
+0.20
+0.22
+0.08
+0.10
+0.15
+0.28
+0.32
+0.34
+0.19
+0.23
+0.28
+0.24
+0.25
+0.26
+0.09
+0.14
+0.17
+0.36
+0.41
+0.43
+0.26
+0.33
+0.40
+0.16
+0.18
+0.19
+0.05
+0.12
+0.14
Table 4 presents the maximum variation in DC with variations in graphite content for the
alloys tested. A maximum DC variation of ~0.09% can be expected at a given green
density with the as-sintered FC-0208 material and the range of graphite additions
evaluated in this study. A variation of up to 0.12% was observed for the FC-0208
material in the heat-treated condition. The greatest effect on DC for FC0208 materials
results from variations in graphite additions. For the molybdenum pre-alloyed steel at a
given density level; potential variations in carbon content result in a maximum DC
variation in the as-sintered condition of ~0.08%. Heat-treating the molybdenum steel
reduces this maximum DC variation to ~0.04% (approximately one third of the variation
observed with the FC-0208).
Sintered and heat treated TRS testing of materials with the possible variations in copper
/ graphite for the FC-0208 and for graphite only for the 0.3% molybdenum steel are
shown in Table 5. Focus is given to TRS results only. Copper has the strongest
influence on TRS properties for copper containing steels. Higher copper contents
increase TRS strength; this results from additional precipitation hardening effects.
Results for the 0.3% molybdenum steel show the highest heat-treated strength with a
graphite addition of 0.7%. Additional graphite decreases the strength. A possible
explanation for these phenomena is that molybdenum is a ferrite stabilizer and it
decreases the eutectoid carbon level. At graphite additions of 0.8% and 0.9% the
occurrence of grain boundary carbides are possible.
Table 5
Range of Sintered and Heat-Treated TRS Test Results for
Copper and Graphite Variation
Iron Base
% Cu
2.0
Unalloyed
Iron
2.5
1.5
0.30%
molybdenum
prealloy
steel
NA
Sintered
Density,
% Gr g/cm³
6.65
0.8
6.90
7.03
6.64
0.8
6.89
7.02
6.66
0.8
6.89
7.02
6.76
0.7
7.00
7.13
6.75
0.8
7.00
7.12
6.74
0.9
6.98
7.10
Sintered TRS,
MPa (psi)
850 (133,700)
1035 (150,500)
1153 (167,600)
850 (123,600)
1055 (153,100)
1175 (170,500)
780 (113,000)
960 (139,300)
1075 (156,100)
675 (97,900)
820 (119,000)
910 (132,200)
675 (98,000)
820 (119,000)
910 (132,000)
720 (104,600)
860 (124,700)
925 (134,100)
Heat Treated TRS,
MPa (psi)
1100 (160,000)
1305 (189,600)
1325 (192,600)
1100 (160,000)
1340 (194,600)
1450 (210,500)
990 (143,800)
1195 (173,500)
1240 (180,500)
1055 (153,100)
1370 (198,900)
1480 (215,350)
1120 (163,000)
1205 (175,300)
1295 (187,900)
940 (136,100)
1130 (163,800)
1215 (176,900)
C. Results of heating treating gear shown in Figure 2.
Prototype gears were produced using the four materials discussed in part one of this
study. They were made on a Dorst 140 metric ton compaction press at a rate of 10 per
minute. The gears were sintered in a continuous belt sintering furnace at 1120 °C (2050
°F) in a 90% nitrogen / 10% hydrogen atmosphere with a time at temperature of ~20
minutes. After sintering approximately 50 gears from each material grouping were heattreated by quenching and tempering utilizing a commercial heat-treat cycle for FC0208.
After quenching, the gears were then tempered at 205 ° C (400 °F). The gears were
checked for dimensional variation using 3.162 mm (0.1245-inch) diameter pins.
Measurements were taken at four locations per gear starting with the front of the gear
(as compacted) and then measuring three additional locations around the outside
diameter. The results of the measurement over wires (MOW) are presented in Table 6.
Table 6
Dimensional Variations of Heat Treated Gears
Material
FC-0208
(regular
premix)
0.3% Mo
with 0.8% gr
(regular
premix)
FC-0208
(bonded
premix
0.3% Mo
with 0.8% gr
(bonded
premix)
Av. MOW
Inches
(mm)
Max MOW
Inches
(mm)
Min MOW
Inches
(mm)
Max – Min
Inches
(mm)
St Dev
MOW
1.2096
(30.72)
1.2118
(30.78)
1.2075
(30.67)
0.0043
(0.11)
0.0013
(0.03)
1.2090
(30.71)
1.2096
(30.72)
1.2081
(30.69)
0.0015
(0.04)
0.0003
(0.01)
1.2099
(30.73)
1.2117
(30.78)
1.2082
(30.69)
0.0036
(0.09)
0.0008
(0.02)
1.2084
(30.69)
1.2089
(30.71)
1.2075
(30.67)
0.0014
(0.04)
0.0003
(0.01)
Table 7
Apparent Hardness Values of Gears in the Heat-Treated Condition
Material
FC-0208 (regular
premix)
0.3-0% Mo with
0.8% gr (regular
premix)
FC-0208 (bonded
premix
0.3% Mo with 0.8%
gr (bonded premix)
Sintered Density,
g/cm³
Average Hardness,
HRA
Range of
Hardness, HRA
7.01
68.1
64.0 / 71.4
7.05
70.4
67.1 / 72.7
7.04
69.3
64.2 / 73.5
7.10
71.0
67.5 / 73.5
The data presented in Table 6 clearly demonstrate the reduced scatter realized with the
prealloyed 0.3% molybdenum steel relative to a standard FC-0208 material. The
absolute magnitude of the MOW is different for the two material combinations; this is a
result of the absolute DC differences between an FC-0208 and a 0.3% molybdenum
prealloyed steel with 0.8% graphite. Thus, it can be clearly seen that a molybdenum
steel gives greater dimensional stability in the heat-treated condition. No green or
sintered gears were measured because the focus of this report was to investigate heattreated results. One additional observation is the comparison of the warm-die powder
premixes vs. ‘regular premixes’. For the FC-0208 material, the binder treated warm-die
powder gave a reduced overall scatter as measured by the maximum measurement
minus the minimum measurement and also the standard deviation of the measurement
differences. Thus, the binder treatment does result in reduced part-to-part scatter even
for the copper-containing steel. No difference in dimensional variation was observed for
the binder treated 0.3% molybdenum steel.
Apparent hardness data collected on the gears are presented in Table7. The gears
were initially compacted to a 7.1 g/cm³ green density; the densities given in the table are
heat-treated densities. Another benefit of the low alloy steel powder is the greater
uniformity of heat-treated hardness as measured by the range of hardness readings.
One potential benefit of eliminating the copper may be a reduced tendency for cracking
during induction hardening. Induction hardening of copper containing PM steels has
shown sensitivity to cracking. A possible explanation is the non-uniform microstructure
inherent with copper containing PM steels. Eliminating the copper would eliminate one
potential for crack initiation during induction hardening of the PM materials.
Discussion:
The objective of this study was to evaluate the feasibility of using a 0.3% molybdenum
steel in lieu of FC-2080. In the as-sintered condition, the TRS and tensile properties of
the 0.3% molybdenum steel with 0.8% graphite are inferior to the FC-0208 steel. The
molybdenum steel does experience less growth during sintering compared with the
FC-0208. Additionally, the molybdenum prealloyed steel has reduced DC variation
resulting from variations in compacted density and premixed graphite variations when
compared to the copper steel. Once heat-treated the 0.3% molybdenum steel with 0.8%
graphite has nearly equivalent TRS and ultimate tensile strength compared to the copper
steel. Dimensional change of the heat-treated TR specimens using the molybdenum
steel exhibited reduced dimensional change variation with variations in density and
variations in graphite additions. An implication of this data suggests that reduced scatter
is possible in actual components that required a heat-treatment.
Measurement over wire measurements of compacted, sintered, and heat-treated gears
showed a significant reduction in the variability of the MOW within a gear and gear
measurements made over 20 measured gears. This reduced variation as measured by
the standard deviation of the MOW was greater than 50%.
One possible implication of this data is the potential to use a 0.3% molybdenum
prealloyed steel in those applications possessing tight dimensional control coupled with
a heat-treated microstructure. PM is continually being challenged to produce evertighter DC along with higher densities. Using a prealloyed molybdenum steel reduces
variations in DC with variations in density and graphite levels yet provides heat-treated
strengths equivalent to heat-treated FC-0208. Additionally, the uniform microstructure
promotes greater uniformity of hardness with the potential for reduced cracking if
induction hardening is required.
Conclusions:
Results from this study showed the following trends:
1. In the as-sintered condition an FC-0208 has superior mechanical properties
when compared with a 0.3% molybdenum prealloyed steel with 0.8% added
graphite.
2. In the heat-treated condition, the 0.3% molybdenum prealloyed steel with 0.8%
added graphite has nearly identical strength to a heat treated FC-0208.
3. Variations in graphite additions have opposite effects on both material. Higher
graphite additions result in less growth for the FC-0208 and approximately the
same growth for a 0.3% molybdenum prealloyed steel.
4. Reducing the graphite addition to 0.7% results in more growth for the FC-0208
and less growth for the 0.3% molybdenum prealloyed steel.
5. Carbon has the greatest effect on DC variation for the copper steels.
6. Variations of part density result in lower DC variations for the 0.3% molybdenum
pre-alloyed material when compared with the FC0208.
7. Heat treatment of gears made from both materials showed greater dimensional
scatter for a FC-0208 relative to a 0.3% molybdenum prealloyed steel.
8. Using a binder treated FC-0208 resulted in reduced scatter relative to a regular
premix. The rationale for this is the reduced potential for graphite segregation.
References:
1. ASM Metals Handbook, Volume 7 Ninth Edition, published by ASM International,
Materials Park, Ohio, 1998, pp.472-484.
2. ASM Metals Handbook, Volume 8 Eight Edition, published by ASM International,
Materials Park, Ohio, 1973, p. 293.
3. T. Murphy, M. Baran, “An Investigation into the Effect of Copper and Graphite
Additions to Sintering Hardening Grades” Advances in Powder Metallurgy and
Particulate Materials --, Compiled by W. Brian James and Russell A.
Chernenkoff, Metal Powders Industries Federation, 2004, pp.
4. B. Lindsley, T. Murphy, “Effect of Post Sintering Thermal Treatments on
Dimensional Precision and Mechanical Properties of Sinter Hardening PM
Steels”, Advances in Powder Metallurgy and Particulate Materials -- 2007,
Compiled by John Engquist and Thomas F. Murphy, Metal Powders Industries
Federation, 2007, pp. .
5. Materials Standards for PM Structural Parts, MPIF standard 35, 2007 Edition,
published by Metal Powders Industry Federation, Princeton, NJ, 2007, pp. 1213.
6. F. Hanejko, High Density via Single Pressing / Single Sintering, Presented at
Euro PM 2007.
7. Hoeganaes Corporation Technical Literature at www.hoeganaes.com, product,
unalloyed steels.