Alloy Development Of Sinter-Hardenable Compositions
Bruce Lindsley
Hoeganaes Corporation
Cinnaminson, NJ 08077 USA
Presented at Euro PM2007
Toulouse, France 15-18 October
Market forces in the PM industry are challenging the traditional compositions typically used in
PM alloys. Mo, Ni and Cu are the predominant alloying elements used in ferrous PM due to
their low affinity for oxygen. In addition, Mo has little effect on compressibility and copper
rapidly alloys by way of a liquid phase at sintering temperatures. All three elements also
increase the hardenability of steels, allowing for sinterhardening of parts produced with a
combination of the elements. Price pressures are causing a reevaluation of powder
chemistries utilizing these elements, and the challenge of alloy development is to advance
alloy systems that optimize the balance between mechanical properties and overall
production cost. Sinter-hardenable compositions play a key role in this regard. This paper
will evaluate and discuss alternative alloys to those primarily used in the European market.
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 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, ensuring 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 is now 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
®
Ancorsteel is a registered trademark of Hoeganaes Corporation.
powder costs were relatively low compared to secondary heat treating steps. Ancorsteel
737SH (MPIF FL-4800) 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 cost prohibitive. However, 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 sinterhardening. The diffusion alloyed materials containing 4% Ni and
either 0.5% or 1.5% Mo also have 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 needn’t pay for extra
alloying when a leaner alloy would suffice. With that in mind, a developmental alloy is being
explored for lower cost sinter hardening. This paper discusses the alloy and the initial testing
that has been undertaken in its development.
Experimental Procedure
The nominal composition of the developmental alloy (Alloy 1) is given in Table I. This new
prealloyed composition was water atomized and annealed to obtain a particle size
distribution typical of that present in the industry. The developmental alloy was compared to
alloys FL-4600 and FL-4800 in premixes containing 0.7 to 0.9 % graphite, 0% to 2% admixed
Cu, and 0.75% EBS wax. TRS and ‘dogbone’ tensile specimens were compacted at either
690 MPa or to a green density of 7.0 g/cm3.
Table I. Nominal composition of the base alloys studied.
ID
Fe
(wt.%)
Mo
(wt.%)
Ni
(wt.%)
Mn
(wt.%)
Alloy 1
Bal.
0.9
0.5
0.4
FL-4600
Bal.
0.5
1.8
0.2
FL-4800
Bal.
1.2
1.4
0.4
The laboratory specimens were sintered in a belt furnace for 15 minutes at 1120 °C in an
atmosphere of 90N2-10H2 (vol.%). Temperature was measured using a thermocouple
embedded in a test bar. Time at temperature was measured when the sample was within
5 °C of the set temperature. Two average cooling rates, measured between 650 and 315 °C,
were obtained during the study: 0.7, and 1.6 °C/sec. A tempering temperature of 205 °C for
1 hour was used for all samples. Dimensional change was measured from die size.
Results and Discussion
Figure 1 shows the compressibility of the developmental alloy compared to the other alloys in
the study. FL-4400 (0.85% Mo) has also been included for comparison. Mixes of base alloy
with 0.7% graphite and 0.75% EBS wax were used. It can be seen that the new alloy has
improved compressibility compared with FL-4800, while both alloys exceed the
compressibility of alloy FL-4600. The compressibility of the FL-4400, which contains no
nickel, is the highest. Given that the new alloy is essentially a lean version of FL-4800, it
follows that its compressibility lies between that of FL-4400 and FL-4800.
Figure 1. The compressibility curves for the developmental alloy, FL-4400 (0.85% Mo), FL4600 and FL-4800. All mixes contain 0.7% graphite and 0.75% EBS wax.
Mixes of FL-4800 and the developmental alloy with 0.7% graphite and either 0 or 1% Cu
were tested at 690 MPa under conventional and accelerated cooling. The hardness and
tensile data for the mixes are shown in Table II. It is clear from the data that alloy FL-4800 is
a superior sinter-hardening alloy to the developmental alloy. Hardness values are higher for
the FL-4800 at the slower cooling rate and full hardenability can be achieved without the
addition of Cu at the faster cooling rate. Nevertheless, the developmental alloy is fully
hardened with the addition of 1% Cu and accelerated cooling. If a cooling rate of 1.6 °C/sec
can be achieved in a part, Alloy 1 + 1% Cu could be used in lieu of the more costly FL-4800.
In fact, it appears that the hardenability of the developmental alloy + 1% Cu is similar to that
of the FL-4800 with no admixed Cu. At a given hardness, the mechanical properties of the
two alloys are quite similar. The dimensional change of the alloys is also comparable.
Table II. Comparison of FL-4800 and the developmental alloy (Alloy 1).
Base
Alloy
FL-4800
FL-4800
Alloy 1
Alloy 1
FL-4800
FL-4800
Alloy 1
Alloy 1
Copper Graphite Cooling
Density
(wt%)
(wt%) Rate (°C/s) (g/cm3)
0.7
0.7
7.07
1
0.7
0.7
7.05
0.7
0.7
7.11
1
0.7
0.7
7.08
1
1
0.7
0.7
0.7
0.7
1.6
1.6
1.6
1.6
7.06
7.04
7.10
7.06
DC
(%)
0.19
0.28
0.21
0.31
Hardness
(HRA)
55
63
51
57
YS
(MPa)
504
684
440
542
UTS
(MPa)
621
828
511
662
El
(%)
1.4
1.2
1.1
1.2
0.24
0.32
0.24
0.39
70
69
63
70
857
865
805
902
883
977
848
937
0.8
1.1
0.9
0.9
A common sinterhardening alloy in North America is MPIF FLC-4608, which contains the FL4600 base material (Table I) and 2% admixed Cu and 0.9% graphite. The developmental
alloy compares favorably to the FL-4600 in mixes with 2% Cu (Table III). The apparent
hardness is quite similar for the two alloys at the different carbon contents and cooling rates.
The growth is significantly higher for the FL-4600 alloys, in part due to the higher compaction
pressure used to achieve the target green density of 7.0 g/cm3. The tensile properties were
higher for Alloy 1 compared with the FL-4600, indicating that it may be possible to reduce the
amount of admixed Cu and/or C with Alloy 1 to achieve the same strength properties as the
FLC-4608. Such a modified composition could also be used to produce similar dimensional
change vaues as the FLC-4608 as well. As the two base alloys appear to provide similar
mechanical properties, they could likely be substituted for each other in applications
depending upon cost of the alloy system. Alloy 1 contains 0.4% more molybdenum and
1.3% less Ni than FL-4600, so the alloys will have similar costs when the Mo is roughly three
times the price of Ni. The high price and volatility of alloying elements have hurt the PM
industry, as many of the alloy surcharges can not be passed onto the end user. In high
volume applications, it would therefore be beneficial to approve multiple alloy compositions
so that the part producer could choose the alloy and process that results in the lowest cost
production. This would help to mitigate the unpredictable alloy costs.
Table III. Comparison of FLC-4608 and the developmental alloy (Alloy 1). Samples were
pressed to a 7.0 g/cm3 green density.
Base
Alloy
FL-4600
FL-4600
Alloy 1
Alloy 1
FL-4600
FL-4600
Alloy 1
Alloy 1
Copper Graphite Cooling
Density
(wt%)
(wt%) Rate (°C/s) (g/cm3)
2
0.7
0.7
6.89
2
0.9
0.7
6.92
2
0.7
0.7
6.89
2
0.9
0.7
6.95
2
2
2
2
0.7
0.9
0.7
0.9
1.6
1.6
1.6
1.6
6.88
6.92
6.89
6.93
DC
(%)
0.54
0.35
0.39
0.20
Hardness
(HRA)
57
62
58
62
YS
(MPa)
517
550
586
625
UTS
(MPa)
608
743
757
773
El
(%)
1.0
1.2
1.4
1.1
0.59
0.37
0.39
0.25
70
73
70
73
664
577
701
584
738
595
769
620
0.9
0.7
1.0
0.8
The microstructure of the FL-4800 alloy with 0.7% graphite (FL-4805) is fully martensitic
(Figure 2a) when the sample was cooled at 1.6 °C/sec, which agrees with the hardness
value of 70 HRA (39 HRC) in Table II. The developmental alloy is fully bainitic under the
same conditions, Figure 2b, while with the addition of 1% admixed Cu, the microstructure is
predominately martensitic with a small amount of bainite, estimated to be 5% of the
microstructure. The bainitic microstructure results in good tensile properties at an
intermediate hardness level. Although all samples were tempered in this study, the bainitic
structure with the absence of any martensite may allow this alloy and cooling rate to be used
in the untempered condition.
(a)
(b)
(c)
Figure 2. Microstructures of (a) FL-4805, (b) Alloy 1 with 0.7% admixed graphite, and (c)
Alloy 1 with 1% admixed Cu and 0.7% admixed graphite. All samples were sintered at 1120
°C and cooled at 1.6 °C/s.
The microstructure of the FLC-4608 and Alloy 1 with 2% Cu and 0.9% graphite are quite
similar at both cooling rates. In Figure 3, both alloys are shown in the conventionally cooled
(0.7 °C/s) condition. The resulting microstructure is a mixture of high carbon martensite with
retained austenite and pearlite / bainite. The amount of martensite is similar in both alloys,
suggesting the hardenability is comparable. A more in depth study is required to quantify the
hardenability of the developmental alloy.
(a)
(b)
Figure 3. Microstructures of (a) FLC-4608 and (b) Alloy 1 with 2% admixed Cu and 0.9%
admixed graphite. The samples were sintered at 1120 °C and cooled at 0.7 °C/s.
Conclusions
A new alloy system is being explored that provides good hardenability at a lower cost than
many sinter-hardenable alloys currently on the market. The developmental alloy presented
above balances alloy cost with hardenability, and gives parts producers another option in
alloy selection. Those producers that achieve high cooling rates in parts, be it through
accelerated cooling and/or thin section sizes, can utilize such an alloy to produce either
bainitic or martensitic microstructures. Martensitic structures were formed with as little as 1%
admixed Cu and 0.7% graphite and, in this condition, the properties were similar to the more
heavily alloyed FL-4800. The mechanical properties of the new alloy match those of FL-4600
in copper-carbon mixes at both conventional and accelerated cooling rates.
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.