Advanced Processing of Sinter-Hardening Materials
Thomas Haberberger,
Alpha Sintered Metals, Ridgway, PA
Francis Hanejko and Michael Baran,
Hoeganaes Corporation, Cinnaminson, NJ
ABSTRACT
The sinter-hardening process has been shown to provide excellent mechanical properties and
part-to-part size control. Previous work has indicated that exceptional mechanical properties
and high apparent hardness values can be achieved by sinter hardening in a high temperature
furnace with standard cooling. This work focuses on combinations of advanced techniques
intended to optimize mechanical properties for stringent applications. While warm compaction
is utilized to increase density, various material alloy combinations are blended together in an
effort to study green density variables. Where applicable, specific market opportunities are
identified.
INTRODUCTION
Production experience with the sinter-hardening process has demonstrated numerous
advantages to the P/M parts fabricator and end-users when compared to conventional
quenching and tempering. These documented advantages include lower processing costs,
reduced production time, and superior dimensional stability. The improved dimensional
stability translates into reduced distortion, tighter concentricity, and improved geometric
tolerances; enabling net shape P/M components in the hardened condition.
Previous work illustrated that sinter-hardening can be accomplished with alloyed P/M steels
and relative slow cooling rates combined with sintering at elevated temperatures, above 2200 0
F (1). Through the use of admixed elemental powders such as copper and nickel along with
varying percentages of graphite, substantial martensitic transformation was observed without
the need for a rapid cooling following sintering. This ability to high temperature sinter coupled
with a production sinter-hardening operation offers significant benefits to the P/M parts
manufacturer and end user. Specifically, the greater pore rounding inherent with higher
sintering temperatures often results in higher mechanical properties and higher impact
energies. More importantly, higher sintering temperatures increases the diffusion rates of
copper and nickel producing more homogenous microstructures with increased hardenability
and enhanced strength.
Earlier experimental work indicated that increasing density in conjunction with elevated
sintering temperatures would increase tensile and impact properties without sacrificing
hardness and / or dimensional change stability. However, increasing the green density of sinter
hardened components can be difficult because sinter-hardening materials have inherently
lower compressibility as a result of their higher alloy content. Overcoming this hurdle could
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expand the application possibilities for these sinter-hardening materials. The present
experimental work focused on enhancing mechanical performance, hardness and
microstructure through the use of warm compaction in combination with elevated temperature
sintering.
Last year’s experimental work reported by Haberberger and Baran (1) focused on
Ancorsteel 737 SH™ prealloy as the base material and subsequently adding different
percentages of various elemental powders. (1) In the present study, Ancorsteel 737 SH
premixed with 2% copper and 0.9% graphite is compared to Ancorsteel 85HP (FL-4400) and
Ancorsteel 150HP (FL-4900) premixed with copper, nickel, and 0.9% graphite. This matrix of
materials was chosen so to evaluate the effects of powder compressibility and sintered density
on the sinter-hardened mechanical properties, rolling contact fatigue (RCF) and
microstructures. This research represents the continuing effort towards advancing the
capabilities of the sinter-hardening process through improvements in materials and processing
with the objective of expanding the potential opportunities into more demanding applications
such as automotive gearing.
EXPERIMENTAL PROCEDURE
The compositions evaluated in the present study are presented in Table 1. The
Ancorsteel 737SH premixed with 2% copper and 0.90% graphite was incorporated as a
reference material for the two test materials. All the materials listed in Table 1 were premixed
using the ANCORDENSE premixing process, a 225 kg (~500 lb.) premix was made for each
composition. The copper used was ACuPowder 8081, the nickel used was INCO 123, and the
graphite was Asbury 3203.
Table I: Sinter-Hardening ANCORDENSE Material Compositions
BASE MAT'L
COPPER
NICKEL
MOLYBDENUN
CARBON
737 SH
2.00%
0.00%
0.00%
0.90%
150 HP
2.00%
1.50%
0.00%
0.90%
85 HP
2.00%
1.50%
0.65%
0.90%
Green density, green expansion, sintered density, and dimensional changes were determined
using the average of five compacted size change rings. These rings have a nominal outside
diameter of 38.10 mm (1.50 inches) with an inner diameter of 19.05 mm (0.75 inch) and an
overall length of 12.25 mm (0.50 inch). This geometry was chosen by Alpha Sintered Metals to
allow for dimensional evaluation of both O.D. and I.D. Tensile tests were conducted on
machined tensile specimens per MPIF spec (10) with a gauge length of 25.4 mm (1.0 inches)
and a nominal diameter of 5.08 mm (0.250 inches). Because of the high apparent hardness of
the material, tensile specimens were machined by a combination of CNC turning and grinding.
Un-notched Charpy impact bars were also processed. All specimens were compacted at 690
MPa (50 tsi) and their corresponding green densities were recorded see Table IV.
Molding of the above specimens was done on a standard 200-ton compaction press utilizing a
commercially available heating unit. The processing temperature of the powder (Hoeganaes
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Corporation’s ANCORDENSE process) was maintained between 260 and 270 ºF (125 °C to
130 °C). All specimens were compacted to 50 TSI (690 MPa).
High temperature sintering of the selected test pieces was performed under production
conditions in a fully loaded furnace. The furnace used in this study was a CM 12“ pusher
furnace with a standard water-jacketed cooling section. The production sintering cycle for the
high temperature processed specimens was as follows:
Sintering Temperature:
Atmosphere:
Time in Hot Zone:
Cooling:
1260 ºC (2300 ºF)
50 v/o N2 , 50 v/o H2
50 minutes
Standard
In addition to the high temperature sintering, a second set of test specimens were sintered
under conventional production sintering hardening conditions. This was done to compare the
effectiveness of high temperature sintering and slow cooling to conventional sintering and
accelerated cooling. The conventional sintering conditions were:
Sintering Temperature:
Atmosphere:
Time in Hot Zone:
Cooling:
1163 ºC (2125 ºF)
95 v/o N2 , 5 v/o H2
45 minutes
Accelerated
After sintering, both the high temperature / slow cooled and conventional sintered / accelerated
cooled samples were tempered at 400 ºF (205 ºC) in air for 1 hour prior to testing or sample
preparation.
Apparent hardness measurements were performed on the surface of the tensile specimens
using a Rockwell hardness tester. All measurements were conducted on the Rockwell B scale
(HRB) for ease of comparison. Dimensional change from die size was measured on the
previously mentioned specimens. Tensile testing was performed on a 267,000 newton (60,000
pound) Tinius Olsen universal testing machine at a crosshead speed of 0.635
millimeters/minute (0.025 inches/minute). Elongation values were determined by utilizing an
extensometer with a range of 0 to 20%; the extensometer was attached to the tensile
specimens through failure.
Metallographic evaluation was performed on sections prepared from the non-machined tensile
bars. Photomicrographs showing typical microstructures were taken following a 2% nital / 4%
picral etch at an original magnification of 500X. The relative microstructural constituent
content was estimated by observing numerous representative fields.
Rolling Contact Fatigue (RCF) is also being examined utilizing the tensile bar specimens that
were specially machined to accommodate the RCF test fixture at Hoeganaes [Not completed
at time of publication].
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RESULTS AND DISCUSSION
The utilization of high temperature sintering was seen to substantially impact the properties of
three test compositions. Particular attention was paid to the key characteristics such as
dimensional change, apparent hardness, impact energy, and tensile properties. Comparisons
were made with the identical compositions from the same warm compacted run processed at
conventional sintering temperatures. The reader is referred to reference 5 for a more thorough
investigation of properties attained by conventional sinter-hardening processing.
Photomicrographs were analyzed in order to quantify martensite content and pore morphology.
Apparent Hardness
The data collected for all three-mix compositions are presented in Table IV. The data
suggested that the pre-alloyed Ancorsteel 737 SH blend was superior in hardenability at the
elevated sintering temperatures. However, the conventional sintering temperature with rapid
cooling reveals a slightly different comparison. The pre-mixed (Ancorsteel 150 HP) reveals a
moderate increase in hardness compared to it’s pre-mixed counterpart (Ancorsteel 85 HP) but
little difference compared to the pre-alloyed (Ancorsteel 737 SH). The higher apparent
hardness readings with conventional sintering & rapid cooling can be explained by the lack of
diffusion of the premixed nickel at the low sintering temperature. Both nickel and carbon
promote retained austenite, thus the high temperature sintered FL-4400 and FL-4900 materials
potentially have greater retained austenite because of the complete homogenization of the
nickel. The conventionally sintered materials have a heterogeneous microstructure with
islands of undiffused nickel. Ancorsteel 737SH with the premixed copper showed a consistent
hardness at both sintering temperatures because it is a completely prealloyed material and any
retained austenite as a result of the alloy composition will be consistent at both sintering
temperatures. This is not to imply that the high temperature sintering is undesirable; rather as
the following discussion will illustrate, the greater pore rounding and increased sintered density
improves the mechanical property response
Dimensional Change
Comparing the pre-alloyed Ancorsteel 737 SH to the two pre-mixed materials for dimensional
variation all samples demonstrated good dimensional stability. This continues to support the
superior dimensional stability of sinter-hard processing. It also compares well to previous sinter
hard work done at standard compaction pressures.
As expected, the dimensional change of the compacts sintered at 1260 ºC (2300 º F) varied
from that of the specimens sintered at 1163 º C (2125º F). The high temperature sintered parts
increased in density and a higher degree of shrinkage from die size. On the other hand, the
conventional standard sintered compacts did not show the same die size capabilities because
of a lesser degree of shrinkage.
What is notable between the elevated and conventional sintering temperatures is that the part
to part variation was very tight for both processes. Thus, the potential distortion associated
with high temperature sintering is not a significant problem with these materials.
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Table II: Size Change
MATERIAL
PROCESS
D.C. (%)
O.D.
D.C. (%)
I.D.
SIZE
CHANGE
PART TO
PART
O.D.
SIZE
CHANGE
PART TO
PART
I.D.
737 SH
ELEVATED
0.27%
0.16%
0.20%
0.19%
150 HP
ELEVATED
0.11%
0.22%
0.05%
0.08%
85 HP
ELEVATED
0.40%
0.51%
0.18%
0.19%
737 SH
STANDARD
0.24%
0.32%
0.12%
0.10%
150 HP
STANDARD
0.24%
0.43%
0.18%
0.18%
85 HP
STANDARD
0.47%
0.61%
0.11%
0.08%
Impact Energy
The impact energies of specimens sintered at the elevated temperatures demonstrated
superior impact characteristics compared to the conventional sinter. The Ancorsteel 737 SH
material was again superior to the (FLNC-4408 and the FLNC-4908) pre-mix blends see Table
IV. It should be noted that the metallurgical disparities between the premix blends versus the
pre-alloyed material were most likely the difference.
Tensile Properties
The primary goal of the warm compaction combined with elevated temperature sintering was to
increase tensile properties in order to meet more stringent application requirements. As
evidenced by the data in Table IV. The pre-alloyed Ancorsteel 737 SH and Ancorsteel 150 HP
material were virtually identical whereas the Ancorsteel 85 HP was less. When comparing the
warm compacted Ancorsteel 737 SH material to the conventional pressed elevated sinter work
done previously, the warm compacted work shows a 13% increase in tensile properties which
is credited to the increased density from the warm compaction process.
The most notable element of this work is the nearly identical tensile properties at both the
elevated and conventional sintering conditions for the Ancorsteel 737SH prealloyed material.
Despite an approximate 0.10 g/cm³ difference in density, the conventionally sintered / rapid
cooled samples show nearly the same tensile strength and elongation. This we believe to be a
function of the warm compaction densification. However, the samples prepared using the FL4400 and FL-4900 base irons exhibited lower strength with conventional sintering. This
disparity results from the lack of nickel diffusion at the lower sintering temperature. Incomplete
diffusion will result in lower tensile strengths because of reduced ferrite strengthening and the
microstructure containing undiffused nickel particles.
Although the tensile properties of the Ancorsteel 737SH are the same regardless of the
sintering condition, it must be noted that the increase in impact energy with the high
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temperature sintering makes this a more robust material for gearing and the like. For the two
molybdenum-based materials, the high temperature sintering promoted both higher tensile
properties and improved impact toughness. Thus, the high temperature sintering combined
with standard cooling optimized both the tensile and impact properties of all the materials
evaluated in this study. See Table IV
Table III: Densities
BLEND
PROCESS
GREEN
DENSITY
(g/cm3)
737 SH
ELEVATED
7.15
7.3
150 HP
ELEVATED
7.2
7.24
85 HP
ELEVATED
7.16
7.16
737 SH
STANDARD
7.15
7.22
150 HP
STANDARD
7.2
7.23
85 HP
STANDARD
7.16
7.15
SINTERED
DENSITY
(g/cm3)
Table IV: Mechanical Properties
UTS
BLEND
YIELD
ELONG
PROCESS
3 PSI)
(10
3 PSI)
(10
( %)
IMPACT
APPARENT
ENERGY
(ft.- HARDNESS
lbf.)
(HRB)
737 SH
ELEVATED
174
136
1.10%
15
112
150 HP
ELEVATED
173
116
1.00%
12
105
85 HP
ELEVATED
156
136
1.10%
11
101
737 SH
CONV.
176
170
1.00%
8
110
150 HP
CONV.
114
108
1.00%
9
113
85 HP
CONV.
142
136
1.00%
6
104
NOTE: Properties given for specimens tempered at 400 ºF for 1 hour
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Metallography
Metallographic analysis of the three test materials and two sintering conditions is described
below.
a.) Ancorsteel 737SH
This material showed a tempered martensitic microstructure with
some evidence of retained austenite. The higher temperature sintered materials showed a
greater degree of pore rounding and a higher density. No significant difference between the
high temperature and low temperature microstructures was observed. The conventionally
sintered sample showed <1% pearlite / bainite in the core; the balance is tempered martensite.
See Figure1for a photomicrograph of the conventionally sintered test sample.
b.) Ancorsteel 150HP
The high temperature sintered material was tempered martensite
with some evidence of retained austenite, there was no evidence of undiffused nickel. The
microstructure of the conventionally sintered material consisted of tempered martensite with
nickel rich areas and nickel rich martensite. Again, the higher temperature sintering showed
significant pore rounding. Figure 2 presents the microstructure of the conventionally sintered
material, it contains ~15% pearlite / bainite in the core; the balance is tempered martensite.
c.) Ancorsteel 85HP
The high temperature sintered material was tempered martensite
with some evidence of retained austenite, there was no evidence of undiffused nickel. The
microstructure of the conventionally sintered material consisted of tempered martensite with
nickel rich areas and nickel rich martensite. Again, the higher temperature sintering showed
significant pore rounding. Figure 3 shows the microstructure of the conventionally sintered
material, it contains ~40% pearlite / bainite in the core; the balance is tempered martensite.
CONCLUSIONS
Sinter-hardening materials processed via warm compaction techniques illustrated its ability to
improve mechanical properties when compared to conventional pressing work. The
Ancorsteel 737 SH + 2% copper blend was superior to the two premixes in achieving higher
mechanical properties and hardenability. High temperature sintering promotes a more
homogeneous microstructure and demonstrated the ability to slow cool from the elevated
temperature to produce a martensitic microstructure. The Ancorsteel 737SH showed
equivalent tensile properties for both sintering conditions but the high temperature sintering
produced higher impact toughness at the same level of tensile strength. The dimensional
stability of all materials was acceptable for both the conventionally sintered and high
temperature sintered processing routes.
Overall all tests blends support the concept that sinter-hard products illustrate superior
dimensional stability compared to the conventional heat-treated process.
FUTURE WORK
Although this body of work adds to a vast database of Ancorsteel 737 SH knowledge, further
work is warranted in several areas. Future investigations will be conducted on:
•
•
Develop accelerated cooling capabilities for elevated temperature sintering in an effort to
maximize apparent hardness and mechanical properties.
Analyze the Rolling Contact Fatigue data (RCF) currently being processed.
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Figure 1: Microstructure of a Ancorsteel 737 SH sintered at 2050 °F (1120 °C), material
contains 0.9 w/o graphite. Etched with 2% Nital / 4% Picral. Original Magnification
500X.
Figure 2: Microstructure of FL-4900 with 2 w/o copper and 1.5 w/o nickel and 0.9 w/o graphite
sintered at 2050 °F (1120 °C) . Etched with 2% Nital / 4% Picral. Original
Magnification 500X.
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Figure 3: Microstructure of FL-4400 with 2 w/o copper and 1.5 w/o nickel and 0.65 w/o
molybdenum and 0.9 w/o graphite sintered at 2050 °F (1120 °C). Etched with 2%
Nital / 4% Picral. Original Magnification 500X.
REFERENCES
1. Haberberger T., Cornelio T., Baran M., Winterton P., “Field Experience on A New SinterHardening Material”, 2000 International Conference on Powdered Metallurgy & Particulate
Materials, New York City, May 30 – June 3.
2. Davala, A.B., Graham, A.H., Causton, R.J., “Effect of Process Conditions Upon SinterHardening Response of FLC-4608 Materials”, Advances in Powder Metallurgy &
Particulate Materials – 1995, Vol. 2, Part 14, p.81, Metal Powder Industries Federation,
Princeton, NJ.
3. Davala, A.B., Graham, A.H., Causton, R.J., “Application of High Performance Materials and
Processes – Alloy System”, 1998 International Conference on Powder Metallurgy &
Particulate Materials, Las Vegas, May 31 – June 4.
4. Baran, M.C., Graham, A.H., Davala, A.B., Causton, R.J., Schade, C., “A Superior SinterHardenable Material”, 1999 International Conference on Powder Metallurgy &
Particulate Materials, Vancouver, June 20–24.
5. “Production Sintering Practices,” in ASM Handbook, Vol. 7, ASM International, Materials
Park, OH, 1998, p.474-476.
6. Baran, M.C., Murphy, T.F., “Metallographic Testing to Determine the Influence of Carbon
and Copper on the Retained Austenite Content in a Sinter-Hardening Material,” P/M
Science & Technology Briefs, Vol. 1, No. 3, 1999, p. 22-26.
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