INTRODUCTION OF A NEW SINTER-HARDENING PM STEEL
Peter Sokolowski, Bruce Lindsley and Francis Hanejko
Hoeganaes Corporation
Cinnaminson, NJ 08077, USA
ABSTRACT
The initial sinter-hardenable alloys introduced into the marketplace relied on high concentrations of
alloying elements in the steel. As sinter hardening has become more commonplace, sinter furnace
producers have improved the cooling zones in the furnace. The increase in obtainable cooling rates
allows lower alloyed steels to be used to obtain sinter-hardened parts. This is especially pertinent to small
parts, where higher cooling rates can be achieved. Therefore, the need exists for an alloy with a
composition that optimizes the amount of price sensitive elements while maintaining the ability to sinter
harden in current belt furnaces. A new sinter-hardenable alloy is introduced and the physical properties
and hardenability of the alloy are discussed.
INTRODUCTION
Traditional sinter-hardening PM steel compositions utilize high levels of Mo, Ni and Cu along with high
carbon contents to achieve martensitic microstructures in the as-sintered condition. Historically, high
cooling rates could not be achieved in the sintering furnace, leading to only the most highly alloyed
materials being used for sinter hardening. With the advent of accelerated cooling zones and the adoption
of these technologies, lower alloy contents can be used. When alloy prices are low, it is easier to use
heavily alloyed materials, to ensure a martensitic microstructure forms regardless of section size and
cooling rate in the part. As price pressures force a re-assessment of alloy selection, it may be more cost
effective to invest in additional processing to reduce the content of high priced alloying elements. The
added processing may include longer sintering times to fully utilize admixed ingredients, accelerated
cooling, and an analysis of the actual cooling rate in each part to determine the lowest alloy content
necessary for sinter hardening.
There are several approaches that can be taken to address the raw material costs. The first is to introduce
lower cost alloying elements, such as Cr and/or Mn. These oxygen sensitive elements provide excellent
hardenability, but may lead to higher processing costs associated with powder production and sintering.
One benefit of these more effective alloying elements is that lower carbon levels can be used while
maintaining a martensitic microstructure. These lower carbon martensitic alloys provide lower
dimensional variation and enhanced mechanical properties [1]. The Cr-containing Ancorsteel® 4300 and
Ancorsteel 4300L (0.3% Mo) are examples of such alloys, where as-sintered martensitic microstructures
are common with sintered carbon contents less than or equal to 0.6 % (wt%) [2].
Another approach to lower cost sinter hardening is the development of alloys that have intermediate
levels of alloying elements. Earlier alloys used high levels of Mo and Ni as powder costs were relatively
low compared with secondary heat-treating steps. Ancorsteel 737 SH (MPIF FL-4800 [3]) has the
combination of good compressibility and excellent hardenability, but at current price levels, the 1.25%
Mo and 1.4% Ni prealloyed in the powder make it somewhat less attractive. Nevertheless, when
processing larger parts or where accelerated cooling is not an option, slow cooling rates within the part
require these high levels of alloying for sinter hardening. The diffusion alloyed materials containing 4%
Ni and either 0.5% or 1.5% Mo also have relatively high cost, and given that the Ni is not prealloyed, do
not take full advantage of the alloying elements present. Those parts producers that have the ability to
cool components at higher rates than conventional cooling need not pay for extra alloying when a leaner
alloy would suffice. With that in mind, a new alloy, Ancorsteel 721 SH, has been developed for lower
cost sinter hardening. This paper discusses the mechanical properties and hardenability of this new alloy.
EXPERIMENTAL PROCEDURE
The nominal compositions of Ancorsteel 721 SH and Ancorsteel 737 SH (FL-4800) are given in Table I.
The new alloy, Ancorsteel 721 SH, contains the same prealloyed constituents as FL-4800, however with
0.3% less Mo and 0.9% less Ni. All mixes for this study were prepared with 0.75% EBS wax
(Acrawax® C) as the lubricant and varying amounts of Asbury type 3203H graphite. Admixed copper
was used to produce alloys with 1% and 2% Cu. Transverse rupture strength, dogbone tensile, and impact
bars were pressed at 415 MPa (30 tsi), 550 MPa (40 tsi) and 690 MPa (50 tsi) and sintered in
90% N2- 10% H2 (vol%) atmosphere at 1120 °C (2050 °F) for 15 minutes in an Abbott continuous-belt
furnace with an accelerated cooling system. Three average cooling rates were chosen for study: 0.7 °C/s
(1.3 °F/s), 1.6 °C/s (2.8 °F/s) and 2.2 °C/s (4.0 °F/s). The cooling rate was measured in the sample
between 650 °C (1200 °F) and 315 °C (600 °F). All samples were tempered at 205 °C (400 °F) for 1 hour
in a N2 atmosphere prior to testing. Measurements were performed in accordance with the relative
MPIF standards [4].
Table I. Nominal composition (in wt%) of the base alloys studied.
Alloy
Fe
Mo
Ni
Mn
Ancorsteel 721 SH
Bal.
0.9
0.5
0.4
Ancorsteel 737 SH (FL-4800)
Bal.
1.2
1.4
0.4
Hardenability of the new alloy was evaluated using the Jominy end-quench method. Cylindrical test
specimens were machined to a length of 100 mm (4 in.) and 25 mm (1 in.) diameter from blocks
compacted to a green density of approximately 7.0 g/cm3. All bars were initially sintered in
90% N2 - 10% H2 (vol%) atmosphere at 1120 °C (2050 °F) for 15 minutes in a continuous-belt furnace.
These samples were austenitized at 900 °C (1650 °F) for 30 minutes in 90% N2 - 10% H2 (vol%)
atmosphere prior to end-quenching. The Jominy end-quench test method and hardness measurements
were carried out according to ASTM standard A 255 [5] and MPIF standard 65 [4].
RESULTS
Figure 1 demonstrates the compressibility of Ancorsteel 721 SH in comparison to the more heavily
alloyed version, FL-4800. It is apparent that the new alloy has an improved compressibility with the
capability of achieving a green density of 7.1 g/cm3 or better at a compaction pressure of 690 MPa. The
increase in density at similar compaction pressures, in part, plays a significant role in the new alloy’s
capacity to achieve and even surpass the mechanical properties of FL-4800. As will be presented herein,
processing considerations (i.e. accelerated cooling zones) strongly dictate the effectiveness of the new
alloy in providing the necessary mechanical properties attributed to a viable sinter-hardening alloy.
Figure 1: Compressibility of Ancorsteel 721 SH and FL-4800.
Mechanical Properties
Mixes of Ancorsteel 721 SH and FL-4800 were prepared with additions of 1% Cu with 0.7% graphite
(Table II) and 2% Cu with 0.9% graphite (Table III). Compaction pressures of 550 MPa and 690 MPa are
presented which allow for an easy comparison of mechanical properties as a function of sintered density.
The hardness and tensile data are arranged according to conventional (0.7 °C/s) and accelerated cooling
(1.6-2.2 °C/s) conditions within a continuous-belt furnace. FL-4800 clearly displays superior mechanical
properties over Ancorsteel 721 SH at conventional cooling rates when only 1% Cu and 0.7% graphite is
added. This demonstrates the capability for a fully hardening FL-4800 under even the most difficult
processing condition. However, if the cooling rate can be increased beyond 0.7 °C/s during processing,
Ancorsteel 721 SH is capable of exhibiting comparable values to FL-4800 at a lower alloy cost. Though
not presented in this study, the new alloy even offers a sufficient hardenability with no admixed copper,
similar to that of FL-4800, given an increased cooling rate [6].
Section size and belt speed are increasingly more important factors when considering the usefulness of
accelerated cooling with a lean sinter-hardening alloy. Therefore, careful consideration as to the amount
of admixed constituents should be explored to fully utilize the alloy. With the addition of 2% copper and
0.9% graphite to Ancorsteel 721 SH, the mechanical properties appear to be less sensitive to cooling rate,
although reduced levels are still apparent at conventional cooling rates when compared with FL-4800.
Beyond conventional cooling, Ancorsteel 721 SH with 2% Cu notably achieves the same, if not greater
properties over FL-4800.
Table II: Comparison of the new alloy Ancorsteel 721 SH and FL-4800 at cooling rates of 0.7-2.2 °C/s
with 1 wt% Copper and 0.9 % Graphite.
Base
Alloy
Copper
(wt%)
Graphite Cooling Compaction Density
(wt%) Rate (°C/s)
(MPa)
(g/cm³)
DC
(%)
Hardness YS
UTS Elong.
(HRA) (MPa) (MPa) (%)
721 SH
1
0.7
0.7
550
6.96
0.21
52
449
559
1.2
721 SH
1
0.7
0.7
690
7.11
0.23
54
540
644
1.3
FL-4800
1
0.7
0.7
550
6.83
0.31
65
726
783
1.0
FL-4800
1
0.7
0.7
690
6.98
0.33
67
777
875
1.0
721 SH
1
0.7
1.6
550
6.94
0.29
66
770
884
1.1
721 SH
1
0.7
1.6
690
7.10
0.31
68
858
955
1.1
FL-4800
1
0.7
1.6
550
6.83
0.33
67
767
785
0.8
FL-4800
1
0.7
1.6
690
6.99
0.35
69
849
906
0.9
721 SH
1
0.7
2.2
550
6.87
0.30
71
804
903
1.0
721 SH
1
0.7
2.2
690
7.03
0.33
71
873
963
0.9
FL-4800
1
0.7
2.2
550
6.83
0.34
67
732
823
0.8
FL-4800
1
0.7
2.2
690
6.99
0.37
69
891
945
0.8
Table III: Comparison of the new alloy Ancorsteel 721 SH and FL-4800 at cooling rates of 0.7-2.2 °C/s
with 2 wt% Copper and 0.9 wt% Graphite.
Base
Alloy
Copper
(wt%)
Graphite Cooling Compaction Density
(wt%) Rate (°C/s)
(MPa)
(g/cm³)
DC
(%)
Hardness YS
UTS Elong.
(HRA) (MPa) (MPa) (%)
721 SH
2
0.9
0.7
550
6.95
0.23
63
679
840
1.2
721 SH
2
0.9
0.7
690
7.09
0.27
66
733
912
1.2
FL-4800
2
0.9
0.7
550
6.85
0.15
68
654
818
1.2
FL-4800
2
0.9
0.7
690
7.00
0.22
69
693
996
1.4
721 SH
2
0.9
1.6
550
6.93
0.27
68
637
849
1.1
721 SH
2
0.9
1.6
690
7.08
0.31
71
701
954
1.2
FL-4800
2
0.9
1.6
550
6.85
0.16
69
634
829
1.2
FL-4800
2
0.9
1.6
690
7.00
0.22
70
685
983
1.4
721 SH
2
0.9
2.2
550
6.87
0.29
73
706
902
1.1
721 SH
2
0.9
2.2
690
7.00
0.32
74
759
919
1.0
FL-4800
2
0.9
2.2
550
6.85
0.18
69
618
944
1.3
FL-4800
2
0.9
2.2
690
6.99
0.23
70
649
914
1.1
Figures 2-6 highlight the mechanical properties of the alloys studied at a compaction pressure of 690 MPa
over the range of cooling rates tested. With the addition of 2% Cu and 0.9% graphite, the two alloys
perform similarly. Both alloys have sufficient hardenability, over the three cooling rates, to achieve 30
HRC and above, as shown in Figure 2. A major difference between Ancorsteel 721 SH and FL-4800 is
the dimensional change after sintering. Current die dimensions would require modification to account for
the increase in part expansion beyond that which is currently seen with FL-4800. However, one benefit
of the lower alloy content (especially Ni) in Ancorsteel 721 SH is less retained austenite in the as-sintered
microstructure. The lower retained austenite content will lead to better dimensional consistency in parts.
Additionally, elongation, ultimate tensile strength, and yield strength are comparable in level and
observed trend for parts admixed with 2% Cu and 0.9% graphite for both alloys. These results suggest
that Ancorsteel 721 SH is a feasible sinter-hardening alternative to FL-4800.
At 1% Cu and 0.7% graphite, the new alloy exhibits a reduced level of hardness than that of FL-4800 at
the lower cooling rates as a result of diminished hardenability. Nevertheless, Ancorsteel 721 SH is
capable of demonstrating an improved hardness at the highest cooling rate, 41 HRC, exceeding FL-4800.
Under conventional cooling rates, Ancorsteel 721 SH is clearly inferior to FL-4800 in hardenability and
therefore mechanical properties. Accelerated cooling in conjunction with enhanced compressibility of the
alloy does however, provide the capability to greatly enhance these properties to comparable values.
Figure 2: Apparent hardness of Ancorsteel 721 SH and FL-4800.
Figure 3: Dimensional change of
Ancorsteel 721 SH and FL-4800.
Figure 4: Elongation of Ancorsteel 721 SH and
FL-4800.
Figure 5: Ultimate Tensile Strength of
Ancorsteel 721 SH and FL-4800.
Figure 6: Yield Strength of Ancorsteel 721 SH and
FL-4800.
Microstructure
The microstructure of Ancorsteel 721 SH admixed with 1% Cu and 0.7% graphite is increasingly
martensitic when processed at higher cooling rates, Figure 7. When slow cooled at 0.7 °C/s, this new
alloy largely transforms into a pearlitic (with some bainite) microstructure, which agrees well with the
lower mechanical properties already presented. At increased cooling rates however, the microstructure
almost fully transforms into martensite; increasing from approximately (a) 15% martensite, 85%
bainite/pearlite, to (b) 72% martensite, 28% bainite/pearlite, and (c) 87%martensite, 13%bainite/pearlite
at the highest cooling rate; based on a point-count method [7]. This clearly displays the significance and
necessity of using accelerated cooling rates in order to take full advantage of this lean
sinter-hardening alloy.
When admixed with 2% Cu and 0.9% graphite, the microstructural development of Ancorsteel 721 SH
follows a similar trend when processed with increasing cooling rates, Figure 8. With the increased
amount of admixed constituents, the slow cooled microstructure transforms to a greater fraction of
martensite compared with the 1% Cu version; starting at approximately (a) 45% martensite, 55%
bainite/pearlite for a cooling rate of 0.7 °C/s, to (b) and (c) 97% martensite, 3% bainite/pearlite for
cooling rates of 1.6 to 2.2 °C/s, respectively. A small amount of retained austenite was observed in the
martensitic microstructure. As mentioned previously, the increase in copper and graphite content
generally leads to the formation of retained austenite, typically around the prior particle boundaries [7],
potentially generating a negative impact on properties. The amount of retained austenite appeared to be
lower in the Ancorsteel 721 SH alloy compared with FL-4800. This alloy is most effective when
sintering furnaces are equipped with accelerated cooling zones.
(a) 0.7 °C/s
(b) 1.6 °C/s
(c) 2.2 °C/s
Figure 7: Microstructures of Ancorsteel 721 SH with 1% Cu and 0.7% graphite compacted at 690 MPa,
sintered at 1120 °C for 15 minutes, and cooled at (a) 0.7 °C/s (b) 1.6 °C/s and (c) 2.2 °C/s.
(a) 0.7 °C/s
(b) 1.6 °C/s
(c) 2.2 °C/s
Figure 8: Microstructures of Ancorsteel 721 SH with 2% Cu and 0.9% graphite compacted at 690 MPa,
sintered at 1120 °C for 15 minutes, and cooled at (a) 0.7 °C/s (b) 1.6 °C/s and (c) 2.2 °C/s.
Jominy End-Quench Hardenability
The hardenability of ferrous alloys has been well documented using the Jominy end-quench test. This
method is used as an indicator of the depth, or thickness to which an alloy is capable of being transformed
to martensite. Figure 9 compares the hardness profiles along the length of water end-quenched bars of
Ancorsteel 721 SH to another commonly used sinter-hardenable alloy, Ancorsteel 4600V
(MPIF FL-4600 [3]), which has 0.5% Mo, 1.8% Ni, and 0.2% Mn. Notably, Ancorsteel 721 SH with 2%
Cu and 0.9% graphite through-hardens in excess of 75 mm (3 in.), maintaining a hardness above 45 HRC
throughout the majority of the bar length. The higher content of carbon in solution is generally a
significant contributing factor in maximum attainable hardenability [8]. Nevertheless, the difference
between the two alloys is clearly a result of other alloy additions and their relative alloying behavior [2].
It is well known that the hardenability factors for Mn and Mo are greater than that of Ni, and as
Ancorsteel 721 SH contains more Mn and Mo than FL-4600, the Jominy results represent the expected
performance of the alloy constituents. This material feature, in connection with accelerated cooling,
should allow for parts with thick cross-sections to be constructed from this new alloy. Even with a
reduced level of admixed copper and graphite, the alloy retains an appreciable level of hardenability, not
dropping below the J Depth (65 HRA) until approximately 57 mm (36/16 in.). In comparison with FL4600, Ancorsteel 721 SH is a superior sinter-hardening grade at similar levels of admixed copper and
graphite. The amount of admixing can be tailored to meet customer specific requirements based on part
dimension, sinter furnace design, and required mechanical constraints.
Figure 9: Jominy end-quench hardenability of Ancorsteel 721 SH and Ancorsteel 4600V.
CONCLUSIONS
A new sinter-hardening alloy, Ancorsteel 721 SH, has been developed as a viable alternative to address
the continuing trend in metal price volatility. The reduced level of alloying, in comparison with its
predecessor Ancorsteel 737 SH (FL-4800), can lead to insufficient hardening under conventional cooling
parameters (~ 0.7 ºC/s). Nonetheless, with the incorporation of advanced cooling systems in modern
sintering furnaces, accelerated cooling rates of 1.6 to 2.2 °C/sec are capable of yielding almost fully
martensitic microstructures within the new alloy. The relative amount of martensitic transformation
under accelerated cooling, of Ancorsteel 721 SH, provides suitable mechanical properties rivaling those
of FL-4800 at a reduced cost. In the event part manufacturers are willing to utilize elevated cooling
systems along with judicious tailoring of the premix, Ancorsteel 721 SH provides the appropriate level of
strength and mechanical response required for selective components.
ACKNOWLEDGEMENTS
The authors would like to thank Gerard Golin for his assistance in providing the microstructures and
analysis thereof as well as Andrew Chan and Paul Fallis in collecting the data found within this paper.
REFERENCES
1. B. Lindsley and T. F. Murphy, “Effect of post sintering thermal treatments on dimensional precision
and mechanical properties in sinter-hardening PM steels”, Advances in Powder Metallurgy &
Particulate Materials, MPIF, Princeton, NJ, 2007.
2. P. King and B. Lindsley, “Performance Capabilities Of High Strength Powder Metallurgy Chromium
Steels With Two Different Molybdenum Contents” Advances in Powder Metallurgy & Particulate
Materials, MPIF, Princeton, NJ, 2006.
3. MPIF Standard 35, Materials Standards for PM Structural Parts, 2007 Edition.
4. MPIF Standard Test Methods for Metal Powders and Powder Metallurgy Products, 2007 Edition.
5. ASTM A 255, Standard Method of End-quench test for Hardenability of Steel.
6. B. Lindsley, “Alloy Development of Sinter-Hardenable Compositions”, EuroPM Proceedings, 2007.
7. T.F. Murphy, M. Baran, “An Investigation Into the Effect of Copper and Graphite Additions to Sinter
Hardening Steels”, Advances in Powder Metallurgy & Particulate Materials – 2004, compiled by
W.B. James and R.A. Chernenkoff, Metal Powder Industries Federation, Princeton, NJ, Part 10, pp.
266-274.
8. W.B. James, “What is Sinter-Hardening?” Presented at PM2TEC 1998.