SINTER-HARDENING RESPONSE OF LEANER ALLOY SYSTEMS
Bruce Lindsley
Hoeganaes Corporation
Cinnaminson, NJ 08077, USA
ABSTRACT
With the recent volatility in raw material prices, powder producers have responded by introducing leaner
alloy systems into the marketplace. Examples of these new developments include Ancorsteel® 721 SH
and Ancorsteel 4300L. While leaner alloys are cost effective, the reduced alloy content suggests that
hardenability will be reduced. It is important to understand how these leaner alloy systems behave under
conditions typically found in sintering furnaces. A quantitative study to assess the hardenability of these
alloy systems has been undertaken and a comparison made with more traditional Mo-Ni-Cu alloys.
Continuous sinter cooling transformation curves are presented along with apparent hardness and
metallographic analysis of various phase fractions.
INTRODUCTION
Sinter hardening refers to the ability to produce predominantly martensitic (hardened) as-sintered
compacts. Sinter-hardenable PM steel compositions allow the PM industry to eliminate the time and
expense of austenitizing and oil quenching parts. In addition to MPIF alloy designation FLC-4608,
several sinter-hardenable prealloy grades were introduced in the 1990’s [1-3]. These compositions
contain prealloyed Mo-Ni-Mn(-Cr) to which Cu is admixed. The recent volatility in the cost of these
alloying elements forced part makers to re-evaluate the alloying systems that are being used. With a
continued desire for improved properties for new applications and the price pressures associated with
traditional alloying elements of Mo, Ni and Cu, powder producers have turned to alternative alloying
elements such as chromium and manganese and have worked to optimize alloy compositions for current
sinter-hardening furnaces.
®
Ancorsteel is a registered trademark of Hoeganaes Corporation
Ancorsteel 737 SH is one of the most hardenable, prealloyed ferrous PM alloy current available in the
marketplace [3], but its high Mo and Ni content have limited its use to applications were maximum
hardenability is required. A leaner version of this alloy system was introduced in 2008 as
Ancorsteel 721 SH [4], where both Mo and Ni contents were reduced. The addition of chromium to steels
greatly improves hardenability, and chromium-containing PM alloys have also been used for sinterhardening [2, 5, 6]. One such alloy, Ancorsteel 4300, was introduced in 2004, utilizing chromium and
silicon to enhance the hardenability beyond traditional Mo-Ni steels [6]. With a Mo content of 0.8 wt%,
this alloy was subject to similar price pressures as other sinter-hardenable grades. Ancorsteel 4300L was
later introduced with a lower Mo content of 0.3 wt% [7], and while it was found that an increase in carbon
content could apparently offset the loss in hardenability due to the reduced Mo content, the hardenability
of the alloy has not been fully characterized. In this paper, the hardenability of these leaner alloy steels,
Ancorsteel 721 SH and 4300L, are examined within the range of cooling rates typically found in
production sintering furnaces and compared with PM 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 temperature
dilatometry. The method used in this study is similar to that described by Semel and Lindsley [8, 9]. A
brief overview of the procedure follows. The composition of the alloys tested is given in Table I.
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 7.0 or 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. The 721 SH alloys were at temperature for nominally 15
minutes, whereas the 4300L was at temperature for 30 minutes. These times at temperature correspond to
45 and 85 minutes in the hot zone, respectively. Different densities were tested for the different powders
so comparisons with earlier alloys could be made; 4300L was compacted to 7.1 g/cm3 while the 721 SH
alloys were compacted to 7.0 g/cm3. 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.
Table I. Typical chemical compositions of the alloys tested (wt%). Reference alloys are listed below.
Material
Alloy
C
Mn
Si
Ni
Cr
Mo
Cu
Ancorsteel 721 SH
1
0.80
0.4
0.5
0.9
Ancorsteel 721 SH
2
0.60
0.4
0.5
0.9
1.0
Ancorsteel 721 SH
3
0.78
0.4
0.5
0.9
2.0
Ancorsteel 4300L
4
0.64
0.1
0.6
1.0
1.0
0.3
FL-4800
0.4
1.4
1.2
FL-4600
0.1
1.8
0.5
Ancorsteel 4300
0.1
0.6
1.0
1.0
0.8
FLNC-4408
0.75
0.1
2.0
0.8
1.5
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.
Helium is used in lieu of nitrogen owing to its superior thermal properties. 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 1.4 °C/s and a
strong transformation was detected at 446 °C. Above 446 °C, the sample is austenitic. The interpretation
of the temperature-dilation plots becomes more complex as multiple phases are present, Figure 2. At a
faster cooling rate of 2.1 °C/s, one transformation begins at 459°C, appears to end at 340°C, and another
begins at 204 °C. In many cases, the end of transformation is difficult to obtain, so no attempt was made
to include finish temperatures on the CCT diagram.
200
Dilation (µm)
150
100
50
446C
0
-50
0
100
200
300
400
500
600
700
800
900
o
Temperature ( C)
Figure 1. Change in sample length during cooling at 1.4 °C/s of Ancorsteel 721 SH with 0.8% sintered C.
200
Dilation (µm)
150
100
50
459C
0
204C
-50
0
100
200
300
400
500
600
700
800
900
Temperature (oC)
Figure 2. Change in sample length during cooling at 2.1 °C/s of Ancorsteel 721 SH with 0.8% sintered C.
The transformation temperature(s) were correlated with the microstructure of the dilatometer specimens.
The samples were sectioned, 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 ferrite + carbide microstructures are often
difficult to distinguish between bainite and pearlite, in part due to the fineness of the ferrite/carbide
spacing. The microstructural observations were rationalized with the dilatometric results to determine
pearlite and bainite morphologies. Apparent hardness of the samples was measured in the center of the
cross-sectioned face opposite that used for metallography.
RESULTS
The chemical compositions of the alloys tested are shown in Table I. The carbon content of each sample
was measured after dilatometric testing. The average sintered carbon content is given in the table.
Carbon contents were within ±0.03 wt% of that listed. The thinnest samples (3.8 mm) contained the
lowest carbon contents in this range due to a small amount of decarburization.
The development of the continuous sinter cooling transformation diagram has been described by Semel
[8]. 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 721 SH Hardenability
The hardenability of 721 SH was evaluated at three different Cu, C levels. Figure 3 shows the CSCT
diagram for Alloy 1, which contains 0.8% sintered C. Two transformations are evident in the diagram.
Above 480 °C, the alloy is austenitic (A). As the temperature decreases, a transformation occurred with
start temperatures ranging from approximately 450 to 480 °C. Transformations that occur in this
temperature range are typically bainitic. Another transformation occurred at temperatures of 250 °C and
below with average cooling rates greater than 1.4°C/s. These temperatures are typically associated with
martensite.
An inserted table in the diagram provides the microstructural and apparent hardness results for the
different cooling rates tested. As predicted from the dilatometric results, the microstructure of the alloy
consists of varying levels of bainite and martensite. The lack of martensite in the microstructure at
cooling rates of 1.4 °C/s and below corresponds with the lack of a measured transformation in the
dilatometer at temperatures below 250 °C. The formation of martensite at higher cooling rates causes a
measurable increase in the apparent hardness of the samples. The hardness of the 100% bainite samples
is at or below 55 HRA (or 90 HRB), while at cooling rates of 1.7 °C/s and higher, the presence of
martensite results in hardnesses greater than 60 HRA (20 HRC). The nearly 100% martensite sample has
a hardness of 73 HRA (45 HRC).
The martensite start temperature was found to increase with increasing cooling rate. This result is
consistent with earlier findings [8, 9]. The formation of bainite influences the martensite start
temperature. As less bainite forms, the Ms temperature increases until it reaches a maximum temperature
associated with 100% martensite formation. The grey line in the diagram that connects the bainite start
(Bs) temperature line with the martensite start (Ms) temperature line at 1.7 °C/s separates two regions. To
3
Temperature (C)
721 SH + 0.80 wt% 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
A
C/s
3.1
2.6
2.3
2.1
1.7
1.4
1.0
0.6
Bs
A+B
B
o
F/s
5.6
4.6
4.2
3.7
3.1
2.6
1.8
1.1
M
B
Hardness
%
98
89
86
79
25
0
0
0
%
2
11
14
21
75
100
100
100
HRA
73
70
70
68
61
55
50
52
Ms
B
M+B
o
3.1 C/s
0
5
10
o
0.6 C/s
o
o
2.1 C/s
15
1.4 C/s
20
25
Time (min)
30
35
40
45
Figure 3. CSCT Diagram for Alloy 1 (Ancorsteel 721 SH + 0.80 wt% sintered carbon).
the right of the line, the alloy forms 100% bainite, while to the left is a combination of austenite and
bainite, labeled (A+B). Upon further cooling from this region, the austenite transforms to martensite
resulting in a martensite and bainite structure (M+B). Although the grey line was placed in the diagram at
1.7 °C/s, technically it should be somewhere between the 1.4 °C/s and 1.7 °C/s cooling profiles, as
martensite is present at 1.7 °C/s, but not present when the alloy is cooled at 1.4 °C/s.
Figure 4. Alloy 1 (Ancorsteel 721 SH + 0.8% sintered C) cooled at (a) 1.4 °C/s and (b) 2.1 °C/s.
Martensite (M) and bainite (B) morphologies are labeled.
The microstructure of the samples cooled at 1.4 °C/s and 2.1 °C/s is shown in Figure 4. These
microstructures correspond to the cooling curves presented in Figures 1 and 2. The microstructural
constituents include martensite (M - light etching) and bainite (B – gray/brown). The ferrite and carbide
phases are too fine to resolve in the bainite at either cooling rate.
The CSCT diagrams for Alloys 2 and 3 follow a trend similar to Alloy 1 in Figure 3. Bainite formed at a
slightly higher temperature (470 °C to 480 °C) for Alloy 2 as compared with Alloy 1, Figure 5. The
martensite start temperature is also higher due to the lower carbon content in Alloy 2. The addition of
admixed Cu results in some martensite formation at all but the lowest cooling rate, thereby shifting the
boundary given by the gray line between the bainite and the austenite + bainite fields toward the right.
The amount of martensite and bainite is similar for Alloys 1 and 2 for corresponding cooling rates,
indicating that the two alloys have similar hardenability. The lower carbon content is counterbalanced by
the increased Cu content. Nevertheless, the apparent hardness is significantly lower for Alloy 2 due to the
reduced carbon content.
3
Temperature (C)
721 SH + 1 wt% Cu + 0.60 wt% 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
A
C/s
3.4
2.7
1.9
1.7
1.4
1.0
0.6
Bs
A+B
o
F/s
6.1
4.9
3.4
3.1
2.5
1.8
1.1
M
B
Hardness
%
97
92
61
29
5
1
0
%
3
8
39
72
95
99
100
HRA
71
68
67
59
53
50
48
B
Ms
B
M+B
o
3.4 C/s
0
5
10
o
0.6 C/s
o
o
1.9 C/s
15
1.4 C/s
20
25
Time (min)
30
35
40
45
Figure 5. CSCT Diagram for Alloy 2 (Ancorsteel 721 SH + 1% Cu + 0.60 wt% sintered carbon).
Alloy 3 contains 2% Cu and 0.78% sintered C, and the addition of the Cu results in a significant
improvement in alloy hardenability, Figure 6, compared with Alloy 1. Some martensite was found at all
cooling rates tested, and cooling rates of 1.6 °C/s or higher resulted in a nearly 100% martensitic
microstructure. This cooling rate can be easily obtained in a sintering furnace equipped with a convective
cooling unit. At 2.3 °C/s, a trace amount of bainite was found in the structure and is indicated in the
microstructure table as “t” under the bainite column. The higher copper-carbon levels also shift the
bainite and martensite start temperatures down. The 100% bainite region present in Alloys 1 and 2 shifts
to cooling rates below 0.6 °C/s and therefore off the diagram in Figure 6. The grey line on the diagram
now separates the austenite and the austenite + bainite fields.
3
Temperature (C)
721 SH + 2 wt% Cu + 0.78 wt% 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
3.2
2.3
2.0
1.6
1.0
0.6
A
Bs
o
F/s
5.8
4.1
3.6
2.9
1.9
1.1
M
B
Hardness
%
100
100*
98
96
69
12
%
0
t
2
4
31
88
HRA
74
74
74
73
71
58
A+B
Ms
M
3.2 C/s
0
5
o
M+B
o
10
o
2.0 C/s
15
0.6 C/s
o
1.6 C/s
20
25
Time (min)
30
35
40
45
Figure 6. CSCT Diagram for Alloy 3 (Ancorsteel 721 SH + 2% Cu + 0.78 wt% sintered carbon).
The martensite morphology changes with the different Cu/C compositions of Alloys 1-3, Figure 7. The
martensite of Alloy 1 is plate or acicular, while Alloy 2 contains lath martensite due to its lower carbon
content. Alloy 3 contains well defined plate martensite, with a small amount of retained austenite visible
between the plates (white). The different martensite morphologies correspond to the maximum hardness
measured in the test bars.
Alloy 1
Alloy 2
Alloy 3
Figure 7. Martensite comparison of Alloys 1-3. All samples contain a small amount of bainite.
A comparison of the 721 SH alloy was made with FL-4600 and FL-4800 base alloys. The hardenability
of FL-4800 measured in the dilatometer was reported in [8] for different Cu-C combinations, while the
hardenability of FLC-4608 was reported in [9]. Plots of average cooling rate versus percent martensite
were made to allow for direct comparison, Figure 8 and 9. All materials were tested at a density of
7.0 g/cm3. It can be seen that the FL-4800 alloy has better hardenability than the 721 SH at both coppercarbon compositions, which is expected as the 721 SH has a lower alloy content. With no admixed Cu,
the FL-4800 alloy has considerably higher martensite fractions between 1.5 and 2 °C/s, which is within
the range of cooling rates found in sinter-hardening furnaces. Recall that FL-4800 was designed to have
superior hardenability by utilizing high Ni and Mo contents at a time prior to the recent raw material price
volatility, and the data shows the ability to sinter-harden this alloy without the addition of copper. With
the addition of 2% Cu (Figure 9), both alloys exhibit a significant increase in hardenability, and the
difference between FLC2-4808 and Alloy 3 is greatly reduced. With accelerated cooling, Alloy 3 can be
used in lieu of FLC2-4808 for small to medium sized parts, where cooling rates of 1.5 °C/s can be
achieved. The hardenability of Alloy 3 is greater relative to FLC-4608, resulting in higher martensite
fractions over a wide range of cooling rates, including 1 °C/s to 2 °C/s. There is a clear advantage of
Alloy 3 compared with FLC-4608 for producing sinter-hardened parts. This result is consistent with the
findings in [10].
0.8% C
2%Cu,
0.8% C
Figure 8
Figure 9
Figures 8 and 9. Plot of martensite content versus average cooling rate measured between 850 °C to
315 °C for base alloys FL-4800 (blue), FL-4600 (green) and Ancorsteel 721 SH (red).
Ancorsteel 4300L Hardenability
The hardenability of Alloy 4 was evaluated at a sintered carbon content of 0.64%. The CSCT diagram is
shown in Figure 10. The initial transformation in this alloy occurred in the temperature range between
535 and 560 °C. Transformations above 500 °C are often pearlitic. A second transformation was
observed at temperatures below 270 °C and was martensitic. It can be seen in the metallographic results
(Figures 10 and 11) that three phases are present, martensite, bainite (lower) and pearlite, yet the bainitic
transformation is not present on the diagram. The amount of bainite present in the microstructure is quite
low, nominally 10% and below, which resulted in a weak signal during dilatometric testing. A slight
indication of a transformation occurring at 410 °C was present at select cooling rates, but was far from
definitive and it was elected not to add a bainite transformation start line. The bainite present in Figure 11
3
Temperature (C)
4300L + 0.62 wt% 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
3.7
2.8
2.3
2.1
1.9
1.5
1.0
0.6
Ps
A+P
o
F/s
6.6
5.0
4.1
3.7
3.4
2.7
1.8
1.1
M
B
P
Hardness
%
86
75
75
72
80
71
41
39
%
3
3
6
6
7
9
12
10
%
10
22
19
22
13
20
47
51
HRA
74
72
73
71
70
68
66
64
A+B+P
Ms
M+B+P
oo
3.1 C/s
186
3.7
C/sec
C/min
0
5
10
oo
128 C/s
2.1
C/sec
C/min
15
oo
35 C/min
0.6
C/sec
C/s
oo
88 C/min
1.5
C/sec
C/s
20
25
Time (min)
30
35
40
45
Figure 10. CSCT Diagram for Alloy 4 (Ancorsteel 4300L + 0.64 wt% sintered carbon).
Figure 11. Microstructures of Alloy 4 at cooling rates of 1 °C/s and 1.9 °C/s.
is acicular lower bainite, which corresponds well with a transformation temperature of 410 °C. The
pearlite in this alloy system is not lamellar, but rather a divorced pearlite morphology.
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 > 1.0 °C/s, and the primarily martensite/bainite
microstructure produced as-sintered hardnesses of 68 HRA (35 HRC) and higher. At cooling rates of
1 °C/s and below, 40% of the microstructure remains martensitic, resulting in good hardness even at slow
cooling rates. It should be noted that although this alloy was sintered at only 1120 °C, it was sintered for
an extended sintering time that would likely be considered impractical in production environments. It is
therefore recommended that this lean alloy system be sintered at higher temperatures to achieve the
desired microstructures. Reference 11 provides a guide to sintering times, temperatures and
microstructures for this alloy system.
The hardenability of Ancorsteel 4300 and 4300L was compared. Data taken from [9] and the current
study were used to plot the martensite content versus cooling rate for the two alloys. The 4300L alloy
contains 0.5% less Mo than the 4300 alloy, so additional graphite was added to compensate for the lower
alloy content. Work by Castro et al has shown that the 4300L system is more tolerant of higher carbon
contents than the 4300 alloy [12]. Figure 12 shows a similar hardenability response for the two alloys,
although the higher carbon 4300L results in a greater amount of martensite at intermediate cooling rates.
With respect to alloy cost, it is therefore advantageous to select the 4300L alloy with additional graphite
as compared with the 4300 alloy. This again assumes the sufficient sintering is applied to achieve these
properties.
Figure 12. Plot of martensite content versus average cooling rate measured between 850 °C to 315 °C for
Ancorsteel 4300 [9] and Alloy 4 (4300L).
A final comparison of the lean alloys was made with another sinter-hardening grade. Using the
dilatometric procedures outlined in the paper, martensite content was obtained at various cooling rates for
MPIF alloy FLNC-4408. Figure 13 shows the hardenability response of the three alloys. Alloy 3 (blue)
has superior hardenability in the cooling rate range of 1 °C/s to 2 °C/s compared with FLNC-4408
(green), yet in referring to Table I, it can be seen that Alloy 3 has less total alloy content than FLNC4408. Ni content is reduced by 1.5%, while Mn, Mo and Cu are increased by 0.3%, 0.1% and 0.5%,
respectively. A fully martensitic structure is more easily obtained in Alloy 3.
The FLNC-4408 alloy contains more martensite than Alloy 4, due in part to the lower carbon content of
Alloy 4. If the carbon contents were similar, it is believed that the hardenability of the two alloys would
be somewhat similar. Alloy 4 contains additions of 1% Cr and 0.6% Si to improve its hardenability,
while it has reduced content of Ni, Mo and Cu by 1%, 0.5% and 1.5%, respectively, compared with
FLNC-4408. The reduction in the amount of cost sensitive alloying elements should make Alloy 4 a cost
effective alternative. This again assumes proper sintering of the Cr-containing alloy.
Figure 13. Plot of martensite content versus average cooling rate measured between 850 °C to 315 °C for
Alloy 3 (721 + 2%Cu + 0.78%C), Alloy 4 (4300L with 0.64%C) and FLNC-4408 (0.75%C).
CONCLUSIONS
Typical sinter-hardenable PM steels are based on prealloys of Mo and Ni (and Mn), to which Cu and
graphite are admixed. Initial sinter-hardenable PM steel alloys were designed with high Mo and Ni
contents when alloying element costs were relatively low and stable. In response to recent price
fluctuations, newer, leaner alloy systems have been introduced. Ancorsteel 721 SH has reduced Ni and
Mo contents relative to FL-4800. The leaner 721 SH alloy was found to maintain good hardenability,
especially at a composition of 2% Cu and 0.8% sintered carbon. It also has a significantly lower Ni
content than FL-4600, allowing for better compressibility, while having improved hardenability through
intelligent alloy design.
Ancorsteel 4300L was also evaluated and found to perform well compared with Ancorsteel 4300. The
lower Mo 4300L is a Cr-containing steel that does not require copper to enhance hardenability. As the
4300L alloy is more tolerant of high carbon contents than the original 4300 alloy, the use of increased
carbon content in the 4300L alloy more than offset the reduced Mo level with respect to hardenability. A
fully martensitic microstructure was not obtained with this alloy system under any of the cooling rates
studied and thus should not be used in applications requiring nearly 100% martensitic structures.
However, a significant amount of martensite (between 40 and 75%) is easily obtained, providing good
mechanical properties over a range of cooling rates.
ACKNOWLEDGEMENTS
The author would like to thank the following people and their contributions in gathering the data found
within this paper: Paul Kremus for the dilatometer sample preparation and sintered carbon analysis, and
Gerard Golin for the metallographic analysis and photomicrographs.
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