Application of Sinter-Hardenable Materials for Advanced Automotive Applications
Such as Gears, Cams, and Sprockets
By
Michael C. Baran and Philip J. Winterton
Hoeganaes Corporation
1001 Taylors Lane
Cinnaminson, NJ 08077
Thomas E. Haberberger and Thomas J. Cornelio
Alpha Sintered Metals, Inc.
R.D. 1, Box 43D
Ridgway, PA 15853
Abstract: Recent demands within the automotive industry have been for
applications requiring high apparent hardness, high hardenability, and increased
mechanical performance. These often-conflicting requirements necessitated the
development of new materials that offer high as-sintered apparent hardness and
good static/dynamic mechanical properties without the added expense of a
secondary heat treatment. Traditionally, sinter-hardening materials have offered
acceptable apparent hardness but at the expense of mechanical properties and
sintered density. This paper will document the mechanical properties of a series of
sinter hardening materials that offer good compressibility, high apparent hardness
and enhanced mechanical properties. The discussion will focus on utilization of
these materials in automotive applications (within both the engine and
transmission) such as gears, cams and sprockets that are currently produced by
either the press, sinter, and heat treat process or by conventional machining of a
casting or wrought material. Enhanced processing through high temperature
sintering also will be discussed.
INTRODUCTION
The main focus of this paper is to present a process that eliminates steps in the manufacture of heattreated powder metallurgy (P/M) parts. Applications of particular interest for P/M part conversion
are gears and sprockets. This paper consists of two parts. The first deals with material development
and properties, while the second discusses part-specific automotive applications. These two items
are investigated concurrently, where applicable, in order to provide focus on application goals in
material development and conversion.
Density
The benefit of increased density on the mechanical properties of P/M materials has been thoroughly
investigated over the years. Double press/ double sinter processing and new technologies such as
warm compaction (ANCORDENSE®) processing have been shown to have a considerable effect on
P/M part density. Using warm compaction technology, it has been demonstrated that a density
increase of 3% resulted in a 30% increase in the transverse rupture strength (TRS) of a
Distaloy
4800A based material [1]. Further densification through high temperature sintering resulted in an
1
additional increase of 1.2% in density, and an additional 14% increase in TRS. The effect of
increased density on ductility measurements such as tensile elongation and impact properties was
even more pronounced. Understanding how to maximize the density of P/M components is an
important step toward producing parts for high performance applications.
Material Design
The aim of alloy design for sinter-hardenable materials is to increase hardenability by delaying the
austenite to ferrite plus carbide transition so that martensite forms during cooling. As hardenability
increases, martensite is capable of forming at progressively lower cooling rates. In ferrous
metallurgy, several predictors exist that foretell the effect of individual elements and combinations of
elements upon hardenability. Unfortunately, a qualitative ranking of alloying element effects, as
presented in Table I, indicates that, to a large degree, the alloys that are efficient in improving
hardenability tend to reduce compressibility and increase the oxygen content of the sintered part.
The table indicates that, if alloy design principles established for wrought alloys are employed,
efficient sinter-hardening alloys require significant chromium and manganese contents. However, the
stability of chromium and manganese oxides under conventional powder processing and sintering
conditions dictates that a high proportion of the alloy addition will not remain in solution in the alloy
matrix. Under these conditions, chromium and manganese will not contribute to hardenability and
their presence as particle and grain boundary oxides may reduce performance.
Hardenability
Effect on
Affinity for
Factor
Compressibility
Oxygen
Higher
Manganese
Copper
Manganese
Chromium
Nickel
Chromium
Molybdenum
Chromium
Nickel
Copper
Manganese
Molybdenum
Lower
Nickel
Molybdenum
Copper
TABLE I: Qualitative Ranking of Alloying Elements in Prealloyed Materials
Overall Alloying Effects Study
In an effort to discern the effects of individual alloying additions and combinations, a matrix study
was performed on over thirty prealloys with varying chromium, nickel, molybdenum, and manganese
contents and processed in straight graphite and copper-graphite mixes [3,4].
Overall, apparent hardness was seen to increase with alloy content. However, the most effective
alloying additions were found to be manganese and the combination of nickel and molybdenum.
Although chromium aided hardenability in straight graphite mixes when present in concentrations
less than 0.5 w/o, it had little effect in copper-graphite mixes.
Higher alloy contents generally led to lower compressibilities. Nickel tended to decrease
compressibility slightly, while manganese and molybdenum were very similar in their behavior and
caused only a moderate reduction in compressibility. The sharpest decrease in compressibility was
seen with increasing chromium content.
Selected Part Performance Requirements
Many automotive production parts / materials have been identified as candidates for conversion to
P/M [5,6]. Some applications include replacement of malleable iron castings and ductile iron
castings. Table II lists the properties of these castings [5]. As will be made clear,
Ancorsteel
737 SH can satisfy these property requirements. However, strength is not the only requirement, as
2
ductility and density may also be an issue. Material selection for specific applications requires close
interaction with design engineers.
Further development of materials will lead to P/M parts replacing transmission gears such as pinion
and ring gears. Stress requirements for typical pinion gears and ring gears, as calculated using von
Mises yield criterion, are shown in Table III. Other prerequisites, such as adequate fatigue
properties, also exist for these applications.
Material
UTS
(MPa / psi)
Ductile Iron Casting
(MPa / psi)
260 / 38,000 MIN
690 / 100,000 MAX
Ferritic / Pearlitic
550 / 80,000 MIN
Malleable Iron Casting
TABLE II: Physical Property Requirements of Materials Targeted for P/M Conversion
Type
σ 1*
(MPa / psi)
σ 2*
(MPa / psi)
Pinion
517 / 75,000
-1034 / -150,000
Ring
310 / 45,000
-827 / -120,000
* σ1 = bending stress in gear ; σ2 = compressive stress
Yield
Requirement
(MPa / psi)
668 / 97,000
724 / 105,000
TABLE III: Calculated Yield Requirements for Selected Gear Applications
By converting castings to sinter-hardened P/M parts in engine and transmission applications, some
processing steps and much of the machining scrap can be eliminated. In addition to this benefit,
sinter-hardening can offer the ability to tailor a part’s microstructure for a particular application by
varying admixed additions and effective furnace cooling rates. To date, sinter-hardened parts have
been embraced in every industry from automotive to small business machines to lawn and garden.
In order to meet the requirements of high strength part conversions, it became necessary to develop
new sinter-hardening grades. In the evolution of such P/M materials, a series of powder grades have
been introduced and utilized in a sinter-hardening capacity. These materials have included
Ancorsteel 2000, Ancorsteel 4600V, Ancorsteel 85 HP, and Ancorsteel 150 HP. Numerous
publications exist on the properties of these materials.
The most recent innovation in sinter-hardening powders, Ancorsteel 737 SH, was introduced in
1998. This new alloy provides improvements in hardenability and compressibility over the wellestablished FLC-4608 composition. These improvements will allow fabricators to reach higher
densities and mechanical performance under typical compaction and sintering conditions. The work
below illustrates the performance capabilities of Ancorsteel 737 SH.
EXPERIMENTAL PROCEDURE
It is generally well known that copper and graphite additions have a dramatic effect on the properties
of sinter-hardenable materials. Therefore, it becomes extremely important to fully characterize any
sinter-hardening base material by investigating a range of premix compositions. Due to the alloy
content’s tendency to shift the eutectoid carbon content of a system, each base powder is likely to
have its own ‘optimum’ premix composition(s) [7].
3
In an effort to screen premix compositions of Ancorsteel 737 SH for automotive applications, in
excess of 15 premix compositions were investigated. In addition, a concurrent study of high
temperature sintering (1260 ºC in 50 v/o N2 / 50 v/o H2 with standard cooling) was undertaken in
order to increase the properties of select Ancorsteel 737 SH premixes for use in more demanding
automotive applications. A 230 kg premix was made for each composition. The copper used was
ACuPowder 8081, the nickel was Inco 123, and the graphite was Asbury 3203 HSC. In all cases,
0.75 w/o Lonza Acrawax C was added to the mixes.
Tensile tests were conducted on machined, threaded tensile specimens with a gauge length of 25.4
mm and a nominal diameter of 5.08 mm. Due to the apparent hardness of the material, tensile
specimens were machined by grinding. All conventionally sintered specimens were compacted at
415 MPa, 550 MPa, and 690 MPa, while high temperature sintered specimens were pressed to a
nominal 7.0 g/cm3 .
All test pieces were sintered under production conditions. An Abbott furnace was used in the study
of all conventionally sintered specimens. The furnace was equipped with a VARICOOL post
sintering cooling system, which combines radiant and convective cooling to accelerate the cooling
capabilities of the continuous belt furnace. The conventional production cycle was as follows:
Sintering Temperature:
1140 ºC
Atmosphere:
90 v/o N2 , 10 v/o H2
Belt Speed:
12.5 cm/min
VARICOOL Setting:
60 Hz
Time at Temperature:
30 minutes
Post-Sinter Temper:
205 ºC for 1 hour in air
All high temperature sintering was conducted in a 305 mm CM pusher furnace according to the
following production conditions:
Sintering Temperature:
1260 ºC
Atmosphere:
50 v/o N2 , 50 v/o H2
Cooling:
Standard Cooling Section
Time in Hot Zone:
42 minutes
Post-Sinter Temper:
205 ºC for 1 hour in air
Apparent hardness measurements were performed on the surface of the specimens using a Rockwell
hardness tester. All measurements were conducted on the Rockwell C scale (HRC) for ease of
comparison. Transverse rupture strength and dimensional change from die size were measured
according to ASTM B 528 and B 610. Tensile testing was performed on a 267 kN Tinius Olsen
universal testing machine at a crosshead speed of 0.635 mm / minute. Elongation values were
determined by utilizing an extensometer with a range of 0 to 20%. The extensometer was left on
until failure.
CANDIDATES FOR AUTOMOTIVE APPLICATIONS
Conventional (1140 ºC) Sintering with Accelerated Cooling
In the scope of the initial premix screening, only minor increases in apparent hardness were observed
at admixed graphite levels beyond 0.7 w/o. This lack of apparent hardness gain seemed to show the
robustness of Ancorsteel 737 SH and the presence of an ‘apparent hardness plateau’ under the
production conditions considered. The effect of carbon content on mechanical properties was
especially evident in the TRS data trend shown in Figure 1. Irrespective of copper content,
transverse rupture strength was seen to peak at the 0.7 w/o admixed graphite level (0.63 w/o
sintered carbon).
4
215
1450
2 w/o Copper
200
1350
1250
185
1150
170
1050
155
TRS (10 3 psi)
TRS (MPa)
1 w/o Copper
No Copper
140
950
850
0.35
0.45
0.55
0.65
0.75
Sintered Carbon (w/o)
0.85
125
0.95
FIGURE 1: Transverse Rupture Strength as Function of Sintered Carbon Content for
Specimens Compacted at 550 MPa, Sintered at 1140 ºC, and Tempered at 205 ºC
Beyond the peak at 0.7 w/o graphite content, transverse rupture strength was thought to decrease
due to lower Ms temperatures (with increasing graphite content) and more retained austenite.
Although a similar peak was seen in yield strength data, ultimate tensile strength values did not
follow this trend.
The effect of copper can clearly be seen in Figure 1. A 1 w/o copper addition produced a marked
increase in transverse rupture strength values over the entire range of carbon contents. However,
when copper was increased to 2 w/o, no appreciable increases in transverse rupture strength were
observed. A similar effect was seen with yield strength. This result seemed to suggest that, for
strength and economy, a 1 w/o copper addition was optimum under the production conditions
studied.
When all premixes were compared, a 1 w/o copper + 0.7 w/o graphite composition presented the
most interesting combination of apparent hardness, strength, and ductility. The mechanical
properties attainable with this composition were seen to closely mirror those achieved by the
commonly used 2 w/o copper + 0.9 w/o graphite composition. In fact, the yield and tensile strengths
of the 1 w/o copper + 0.7 w/o graphite premix tended to be 10-15% higher than that of the 2 w/o
copper + 0.9 w/o graphite material. Sintered carbon levels above the 0.65 - 0.75 w/o range were
found to adversely affect the mechanical properties of Ancorsteel 737 SH without substantially
boosting properties in other key areas (i.e. apparent hardness).
5
The yield and ultimate tensile strengths of Ancorsteel 737 SH specimens are presented in Figure 2.
The balance of data for these premixes, collected at a compaction pressure of 690 MPa, is shown in
Table IV.
200
0.2% YS
150
1035
100
690
50
345
0
0
0.8w/o Gr
1w/o Cu +
0.7w/o Gr
1.5 w/o Cu +
0.8w/o Gr
Strength (MPa)
Strength (103 psi)
UTS
2 w/o Cu +
0.9w/oGr
FIGURE 2: Strengths of Ancorsteel 737 SH Specimens Compacted at 690 MPa , Sintered at
1140 ºC with Accelerated Cooling, and Tempered at 205 ºC
Once again, it was observed that increasing copper and graphite additions beyond 1.0 w/o and
0.7
w/o, respectively, led to only minor changes in apparent hardness values. Furthermore,
1
w/o Cu + 0.7 w/o graphite and 1.5 w/o Cu + 0.8 w/o graphite premixes exhibited nearly identical
mechanical properties while higher graphite additions in the 2 w/o Cu + 0.9 w/o graphite material, a
commonly used sinter-hardening composition, caused a decline in mechanical performance. Further
work is in progress studying the rotating bending fatigue and rolling contact fatigue of these
materials.
Premix
1
2
3
4
Cu
(w/o)
Gr
Green
Sint.
Dimensional Apparent UTS
0.2% YS
(w/o) Density Density Change (%) Hardness (MPa)
(MPa)
(g/cm3) (g/cm3)
(HRC)
-0.8
7.04
7.01
+0.22
39
1000
890
1.0
0.7
7.05
7.02
+0.27
37
1215
985
1.5
0.8
7.06
7.01
+0.26
39
1195
915
2.0
0.9
7.06
7.03
+0.15
39
1105
795
TABLE IV: Tempered Properties of Ancorsteel 737SH Premix Candidates
6
High Temperature (1260 ºC) Sintering with Standard Cooling
The employment of high temperature sintering was seen to impact all properties of the sinterhardenable premixes. In this effort, particular attention was paid to apparent hardness and tensile
properties. As previously shown, a 1 w/o Cu – 0.7 w/o graphite premix processed at conventional
sintering temperatures with accelerated cooling yielded the best combinations of strength and
apparent hardness. However, given the extremely slow cooling rates inherent in high temperature
furnaces employing standard cooling sections, it was hypothesized that lower graphite contents
would be detrimental to apparent hardness during high temperature sintering. For this reason,
graphite contents below 0.9 w/o were only utilized in premixes containing 2 w/o nickel or copper.
The data collected for all the “high temperature study” premix compositions are presented in Table
V. Not surprisingly, premix compositions were found to have a tremendous effect on the apparent
hardness values obtained. More specifically, the data suggested that admixed copper and/or nickel
additions were vital for a significant sinter-hardening response under the conditions considered. In
contrast, previous work illustrated the ability to achieve high apparent hardnesses at lower sintering
temperatures with graphite alone [8]. Given the inherently slow cooling rates in high temperature
furnaces, the thorough diffusion of copper and/or nickel was thought to increase the hardenability of
the compact and allow for significant martensitic transformation.
Sintered
Apparent
Y.S.
UTS
Elong.
Density
Hardness
(MPa)
(MPa)
(%)
3
(g/cm )
(HRC)
--0.9
7.00
580
765
2.9
13
0.5
-0.9
6.98
605
815
2.4
16
1.0
-0.9
6.99
625
840
2.3
23
1.5
-0.9
6.99
760
1040
1.5
40
2.0
-0.9
6.99
780
1045
1.4
41
2.0
-0.7
6.95
850
1165
2.0
31
-1.0
0.9
7.04
660
915
2.1
24
-2.0
0.7
7.07
745
1055
2.4
30
-2.0
0.9
7.08
695
1100
1.7
38
3
TABLE V: Properties of Ancorsteel 737 SH Specimens Compacted to 7.0 g/cm , Sintered at
1260 ºC, and Tempered at 205 ºC
Copper
(w/o)
Nickel
(w/o)
Graphite
(w/o)
The primary goal of the high temperature sinter-hardening effort was to increase tensile properties in
order to meet more stringent application requirements. Specifically, it was hoped that elongations of
at least 2% could be attained at strength levels greater than or equal to those seen in conventional
processes. As evidenced by the data in Table V, this effort was somewhat successful. Ductility
goals were met in specimens whose apparent hardnesses ranged from 13 to 30 HRC.
The most significant results were seen in both the 2 w/o copper + 0.7 w/o graphite and 2 w/o nickel
+ 0.7 w/o graphite mixes. In addition to achieving high strength levels and adequate hardnesses for
many existing applications, elongation values were 2% or higher. Furthermore, it was determined
that adding 0.7 w/o graphite instead of 0.9 w/o in a 2 w/o copper mix resulted in higher strength and
ductility. Similar results were observed in conventionally processed premixes of the new sinterhardening material containing copper [8]. However, the effect of carbon level in 2 w/o nickel mixes
was mixed since yield strength and elongation increased, but ultimate tensile strength decreased. A
more extensive investigation is planned to optimize premix compositions for high temperature
sintering.
7
It is extremely important to recognize that a vast array of processing options and properties still
remain unexplored. Both rolling contact fatigue and rotating bending fatigue behaviors are likely to
respond favorably to the increase in sintering temperature as pore morphologies change and the
degree of sinter increases. Finally, this material system and the P/M industry may benefit immensely
from the commercial introduction of accelerated cooling / conveyance systems for high temperature
furnaces.
CONCLUSIONS
When conventional sintering temperatures (1140 ºC) are coupled with accelerated cooling, an
Ancorsteel 737 SH premix containing 1 w/o copper + 0.7 w/o graphite was found to provide the
best combination of tensile properties and apparent hardness. When carbon levels were increased
beyond 0.7 w/o, a decrease in strength was observed due to the formation of retained austenite.
Hence, under the production conditions considered, 2 w/o copper + 0.9 w/o graphite specimens
exhibited lower tensile and yield strengths than 1w/o copper + 0.7 w/o graphite specimens.
Sinter-hardening was achieved by high temperature sintering (1260 °C) Ancorsteel 737 SH premixes
containing either graphite and copper or graphite and nickel. However, as a consequence of slow
cooling rates, copper or nickel additions were imperative to achieving an acceptable sinter-hardening
response. Lowering graphite additions from 0.9 w/o to 0.7 w/o in the presence of 2 w/o copper
caused an increase in both strength and ductility. Although a mixed effect was observed in a 2 w/o
nickel mix, higher ductility resulted when graphite was reduced from 0.9 w/o to 0.7 w/o.
ACKNOWLEDGMENTS
The authors wish to thank George Fillari, Ronald Fitzpatrick, Gerry Golin, Steve Kolwicz, and
Thomas Murphy for preparing, packing, testing, and analyzing specimens. In addition, thanks go to
John Butterfuss of Alpha Sintered Metals, Inc. for imparting his processing expertise to this effort.
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