Sinter Hardening Response Of A Cr-Si-Ni-Mo Containing Steel
Bruce Lindsley and W. Brian James
Hoeganaes Corporation, Cinnaminson, NJ 08077, USA
Abstract
A number of sinter-hardenable materials have been developed over the past several years.
Notable among these are Ancorsteel® 737 SH (prealloyed Mo, Mn, Ni steel), Ancorsteel
4600V (prealloyed Mo, Ni Steel) and hybrid alloys containing prealloyed molybdenum with
copper and admixed nickel. All these materials use copper to enhance hardenability.
Ancorsteel 4300 (Cr-Si-Ni-Mo containing steel) exhibits excellent hardenability without the
addition of copper, with as-sintered hardnesses greater than 20 HRC utilizing cooling rates
typically found in production sintering furnaces. A quantitative study to assess the
hardenability of this alloy system has been undertaken and a comparison will be made with
more traditional Mo-Ni-Cu alloys. Continuous sinter cooling transformation curves will be
presented along with apparent hardnesses and metallographic analysis of various phase
fractions.
Introduction
Sinter-hardenable P/M steel compositions allow the P/M industry to eliminate the expense of
austenitizing and oil quenching parts. These compositions are typically prealloyed Mo-Ni or
Mo based to which Cu is admixed. Ancorsteel 4300 utilizes chromium and silicon to enhance
the hardenability beyond Mo-Ni steels and does not require the addition of copper. Copper
additions have become a concern with respect to scrap recycling. The use of chromium in
wrought steels is a result of both its effectiveness as an alloying element and its relatively low
cost. Until recently, Cr had not been used in P/M steels due to oxidation concerns. Silicon
containing steels (Ancorloy® MD series) had been developed earlier that utilized the
strengthening contribution of silicon, but required high temperature (1260 °C) sintering (1,2).
Ancorsteel 4300 builds on the advantages of the silicon containing steels with the addition of
Cr, but does not require high temperature sintering. The hardenability of this steel is
examined within the range of cooling rates typically found in production sintering furnaces
and compared with P/M steels that contain more conventional alloying elements.
Experimental Procedure
The continuous sinter cooling transformation (CSCT) method is similar to the traditional
continuous cooling transformation (CCT) method for determining hardenability utilizing high
®
Ancorsteel and Ancorloy are registered trademarks of Hoeganaes Corporation
temperature dilatometry. The method used in this study is similar to that described by Semel
(3,4) with some minor modifications. A brief overview of the procedure follows.
The composition of Ancorsteel 4300 is given in Table I along with alloy FLC-4608. Samples,
with a length and width of 11.6 mm and thicknesses of 3.8, 6.4 and 10 mm, were pressed to a
green density of 7.1 g/cm3 and sintered in a ceramic belt furnace at 1120 °C (2050 °F) in a 90
vol% nitrogen – 10 vol% hydrogen atmosphere. An additional set of 4300 samples was
sintered at 1260 °C in the same furnace and atmosphere. The samples were then tempered at
650 °C to soften the alloys and allow a hole to be drilled for a thermocouple in the specimens.
Individual samples were then placed in a high temperature dilatometer and heated to 1120 °C
and held for 10 minutes.
The samples were cooled to 865 °C and held for 5 minutes to stabilize the temperature prior
to accelerated cooling. The samples were then cooled at different rates by changes in sample
thickness, furnace position and gas flow rate. The gas used in the dilatometer was 90 vol%
helium – 10 vol% hydrogen at a flow rate of 1.4 liters/min (3 cfh), except at the highest
cooling rates, when flow rates of 37 liters/min (80 cfh) and 28 liters/min (60 cfh) were used
during the cooling portion of the experiment. The cooling rate was defined as an average rate
between 850 °C and 315 °C.
The phase transformation temperature from austenite to the low temperature phase
(martensite, bainite, and/or ferrite/cementite) was determined from a plot of temperature vs.
dilation. An example of the temperature – dilation plot is shown in Figure 1. In this example,
the sample was cooled at 2.1 °C/s and transformations were detected at 470 °C, 354 °C and
241 °C. Above 470 °C, the sample is austenitic.
100
Dilation (µ m)
80
60
40
20
o
470 C
0
o
o
241 C
354 C
-20
0
100
200
300
400
500
600
700
800
Temperature (oC)
Figure 1. Change in sample length during cooling at 2.1 °C/s of Ancorsteel 4300.
A reduction in slope represents the transformation to a lower density phase. The large change
in slope at 241 °C is due to martensite formation. The transformation temperature(s) were
correlated with the microstructure of the dilatometer specimens. The samples were crosssectioned, mounted, polished and etched using standard metallographic techniques. The
fraction of different microstructural constituents in the samples (excluding porosity) was
quantitatively determined using the point count method. The lower bainite morphology was
defined as bainite. The remaining ferrite + carbide microstructures were defined as pearlite.
Apparent hardness of the samples was measured in the center of the cross-sectioned face
opposite that used for metallography.
Results
Typical chemical compositions of the two alloys are shown in Table I. The carbon content of
each sample was measured after dilatometry testing. The average sintered carbon content is
given in the table. Carbon contents were within ±0.02 wt% of that listed. The carbon
contents of the thinnest samples (3.8 mm) contained the lowest carbon contents in this range
due to a small amount of decarburization.
Table I. Typical chemical compositions of the alloys tested (wt%).
Alloy
C
Mn
Si
Ni
Cr
Mo
Ancorsteel 4300
0.53
0.1
0.6
1
1
0.8
FLC-4608
0.8
0.1
1.8
0.5
Cu
2
The development of the continuous sinter cooling transformation diagram has been described
by Semel (3). The diagram is based on a linear time scale and traces of four cooling profiles
are shown along with their average cooling rates. The legend in each figure is as follows:
A – austenite, M – martensite, B – bainite, P – pearlite. The solid black lines represent the
transformation start temperatures for the various microstructures as measured in the
dilatometer. The grey transformation start lines were not detected in the dilatometer and are
interpretations based on the microstructures.
Ancorsteel 4300 Hardenability
Figure 2 shows the CSCT diagram for 4300. Three transformations are evident in the
diagram. Pearlite formed at temperatures close to 500 °C over the range of cooling rates
tested (0.6 °C/s to 3.1 °C/s). The pearlite in molybdenum containing P/M alloys, such as
4300, is not lamellar and is often referred to as divorced pearlite. Bainite and martensite were
detected at 410 °C and 175 °C, respectively, at the slowest cooling rate.
The pearlite and bainite start temperatures decreased as the cooling rate increased. The
martensite start temperature was, however, found to increase with increased cooling rate.
This result is consistent with earlier findings (4). The bainite transformation was not
measured in the dilatometer at the 2.8 °C/s and 3.1 °C/s cooling rates, although a small
amount of bainite was found metallographically. The bainite start line was extended (grey
portion) to the 3.1 °C/s profile to indicate the presence of bainite. The microstructure of the
sample cooled at 1.5 °C/s is shown in Figure 3. The microstructural constituents include
martensite (M - light etching), lower bainite (B - plate, brown), coarse pearlite (P) and
unresolved pearlite (UP).
This alloy was found to have good hardenability, as evident from the microstructural results.
More than half of the structure was martensitic at cooling rates > 1.5 °C/s, and the primarily
martensite/bainite microstructure produced as-sintered hardnesses of 70 HRA (39 HRC) and
higher. At cooling rates of 1 °C/s and below, one third of the microstructure remains
martensitic, resulting in good hardness even at slow cooling rates.
The CSCT diagram for 4300 sintered at 1260 °C was also generated, utilizing the same
sintering conditions as used in reference 5. No significant differences were found between the
two diagrams. The pearlite start temperature showed a slight decrease, about 25 °C, when the
samples were sintered at 1260 °C. The overall shape of the diagram remained unchanged,
along with the microstructures. At five of the seven cooling rates, the hardness values were
the same on both diagrams.
The effect of additional nickel on the hardenability of 4300 is dramatic. Results published in
ref. 5 show alloy 4300 with the addition of 2 wt% Ni sintered at 1260 °C. The martensite
content at 0.5 °C/s was 51%, with greater than 80% martensite at cooling rates of 1.5 °C/s and
higher. The results are similar to those for Ancorloy MDC at a sintered carbon content of
0.6 wt% (4). The total alloy content of the 4300 + 2 wt% Ni is lower than MDC, showing the
improvement in hardenability with the presence of chromium.
3
Temperature (C)
4300 - 0.53wt% Sintered Carbon at 7.1g/cm
Continuous Sinter Cooling Transformation Diagram
900
800
700
600
500
400
300
200
100
0
Microstructure and Apparent Hardness
Cooling Rate
o
A
C/s
3.1
2.8
2.1
1.9
1.5
1.0
0.6
Ps
Bs
A+P
o
F/s
5.6
5.0
3.8
3.4
2.6
1.7
1.1
M
B
P
Hardness
%
83
75
67
56
48
34
30
%
12
12
19
20
24
26
11
%
5
13
14
24
28
40
59
HRA
73
73
71
70
68
67
64
A+B+P
Ms
M+B+P
o
3.1 C/s
0
5
10
o
2.1 C/s
15
o
0.6 C/s
o
1.5 C/s
20
25
Time (min)
30
35
40
45
Figure 2. CSCT Diagram for Ancorsteel 4300 + 0.53 wt% sintered carbon
Figure 3. 4300 cooled at 1.5 °C/s. Martensite (M), bainite (B), pearlite (P) and unresolved
pearlite / upper bainite (UP) morphologies are labeled.
3
Temperature (C)
FL-4600 + 2%Cu + 0.8% Sintered Carbon at 7.0 g/cm
Continuous Sinter Cooling Transformation Diagram
900
800
700
600
500
400
300
200
100
0
Microstructure and Apparent Hardness
Cooling Rate
o
C/s
2.4
2.2
1.9
1.4
1.0
0.6
A
Ps
o
F/s
4.2
3.9
3.4
2.5
1.8
1.1
M
B
P
Hardness
%
93
93
86
69
44
12
%
7
7
14
28
51
54
%
0
<1
<1
3
5
34
HRA
72
73
73
70
67
60
A+B+P
A+B
Ms
M+B
o
1.4 C/s
o
2.4 C/s
0
5
10
15
M+B+P
20
25
Time (min)
o
0.6 C/s
30
35
40
45
Figure 4. CSCT diagram of 4600 + 2 wt% Cu + 0.8 wt% sintered C (MPIF FLC-4608).
Figure 5. Microstructures of FLC-4608 at 1.4 °C/s. Martensite (M) and bainite (B)
morphologies are labeled.
MPIF FLC-4608 is a standard prealloyed Ni-Mo sinter-hardening alloy, to which 2 wt% Cu is
added. The CSCT diagram for this alloy is shown in Figure 4. This alloy has high martensite
contents at the faster cooling rates, making it suitable for sinter-hardening. At 1.9 °C/s, the
4608 has 86% martensite and a hardness of 73 HRA (45 HRC), compared with 4300, which
has 56% martensite and a hardness of 70 HRA (39 HRC) at the same cooling rate. At the rate
of 1.4 °C/s, significantly more martensite is present in FLC-4608 (Figure 5) than Figure 3.
Interestingly, at slow cooling rates (0.6 °C/s), the 4300 has better hardenability with 30%
martensite and a hardness of 64 HRA compared with the 4600 with 12% martensite and 60
HRA. Overall, the FLC-4608 with 0.8 wt% sintered carbon has better hardenability than
4300 with 0.53 wt% sintered carbon. However, the higher carbon content required for this
hardenability in the FLC-4608 alloy comes at a cost. The high carbon levels reduce
mechanical properties in the material other than hardness. The ability to produce martensite
at lower carbon contents is beneficial for properties such as toughness and fatigue. The good
hardenability of the 4300 alloy without Cu and at lower carbon contents results in excellent
overall properties (6).
Another sinter-hardenable alloy system is based on FL-4800, which contains 1.4 wt% Ni,
1.25 wt% Mo and 0.4 wt% Mn. The FL-4800 alloy is specifically designed for sinterhardening applications, and the hardenability of this alloy system with varying levels of
copper and carbon has been described by Semel (3). The 2 wt% Cu, 0.8 wt% sintered carbon
composition (FLC2-4808) is extremely hardenable, with 90% martensite at a cooling rate of
just 1.2 °C/s. The hardenability of the FL-4800 with 1 wt% Cu and 0.6 wt% sintered carbon is
similar to the FLC-4608 shown above. The role of Mo and Mn on hardenability is apparent,
as less copper and graphite are required to achieve similar levels of hardness in the FL-4800
system. The hardenability of FL-4800 base with 0.5 wt% C is much less than the 4300 alloy
with a similar level of carbon (0.53 wt%). The hardness of FL-4805 at the maximum cooling
rate (58 HRA) is lower than that of 4300 at the slowest cooling rate (64 HRA). The beneficial
effect of Cr and Si on hardenability outweighs the reduction in Mo and Ni content.
Conclusions
Typical sinter-hardenable P/M steels are based on prealloys of Mo and Ni, to which Cu and
graphite are admixed. Ancorsteel 4300 is a Cr and Si containing steel that does not require
copper to enhance hardenability. The hardenability of 4300 was compared with a standard
sinter-hardening grade (FLC-4608) utilizing continuous sinter cooling transformation
diagrams. The lower carbon 4300 alloy exhibits good hardenability, with as-sintered
hardnesses greater than 20 HRC at cooling rates typically found in production sintering
furnaces. The good hardenability at lower carbon contents allows this alloy to be used in high
performance applications. FLC-4808 has better hardenabilility with the addition of 2 wt% Cu
and 0.8 wt% carbon, and is appropriate for traditional sinter-hardening applications.
References
1. James, W.B., Causton, R.J., Baran, M.C., and Narasimhan, K.S., “New High Performance P/M
Alloy Substitutes for Malleable and Ductile Cast Irons,” Advances in Powder Metallurgy &
Particulate Materials, compiled by H. Ferguson and D. Whychell, MPIF, Princeton, NJ, 2000,
part 13, p.123.
2. Baran, M.C., Chawla, N., Murphy, T.F., and Narasimhan, K.S., “New High Performance P/M
Alloys for Replacing Ductile Cast Irons,” Advances in Powder Metallurgy & Particulate
Materials, compiled by H. Ferguson and D. Whychell, MPIF, Princeton, NJ, 2000, part 13, p.133.
3. Semel, F. J., “Cooling Rate Effects on the Metallurgical Response of a Recently Developed Sinter
Hardening Grade”, Advances in Powder Metallurgy & Particulate Materials, Compiled by V.
Arnhold, C-L. Chu, W. Jandeska and H. Sanderow, MPIF, Princeton, NJ, 2002, part 13, p. 102.
4. Semel, F. J., “Ancorloy Hardenability”, Advances in Powder Metallurgy & Particulate Materials,
Compiled by R. A. Chernenkoff and W. B. James, MPIF, Princeton, NJ, 2004, part 7, p. 50.
5. Lindsley, B., “Effects of Cooling Rate on the Hardenability of Chromium Containing P/M Steels”,
Advances in Powder Metallurgy & Particulate Materials, Compiled by R. A. Chernenkoff and W.
B. James, MPIF, Princeton, NJ, 2004, part 7, p. 62.
6. King, P., “Chromium Containing Materials for High Strength – High Fatigue Applications”, Euro
PM2004, Ed. By H. Danninger and R. Ratzi, EPMA, Vol. 3, 2004, p. 165.