Development of a High Performance Nickel-Free P/M Steel
Bruce Lindsley
Senior Materials Engineer
Hoeganaes Corporation
Cinnaminson, NJ 08077, USA
ABSTRACT
Chromium and manganese are used frequently in the wrought steel industry to improve mechanical
properties and hardenability. However, these elements have been difficult to incorporate into P/M steels
due to their high affinity for oxygen. A new nickel-free P/M steel containing Cr, Mn, Si and Mo is being
developed for high performance applications. Sintering studies performed over a range of temperatures
have shown that reasonable oxygen contents along with superior mechanical properties can be achieved
with this new steel. The effect of sintering temperature and density on mechanical properties will be
discussed.
INTRODUCTION
The use of chromium and manganese in wrought steels is a result of the relatively low cost of these
elements and their effectiveness as alloying elements. Cr and Mn improve both the mechanical properties
and hardenability of low-alloy steels. It would be desirable to incorporate these elements in P/M steels,
especially with the adoption of sinter hardening practices. However, oxidation concerns have limited the
use of these elements (1,2). High carbon ferro-alloys of chromium and manganese have been
successfully used as sources for Cr and Mn in P/M steels (3-6), and, while lowering the oxygen content
compared with prealloys, these additives require very high temperatures and long times to be effective.
Alloy 2, described below, is an example of this type of alloying system, which uses additions of high
carbon FeCr, FeMn and nickel to an iron-molybdenum base alloy (4). The use of admixed Ni has also
become a concern, both from a health and a cost standpoint. Copper additions also have become a
problem with respect to scrap recycling. Silicon containing steels had been developed earlier that utilized
the strengthening contribution of the silicon (7,8). A Cr, Mn, Si, Mo containing nickel-free steel is being
developed that utilizes the advantages of Cr and Mn (and Si) while limiting the amount of oxygen in a
sintered part to acceptable levels. The properties of this developmental steel are compared with two
nickel containing steels (a Ni-Si-Mo and a Mn-Cr-Ni-Mo steel) over a range of sintering temperatures and
densities.
EXPERIMENTAL PROCEDURE
Three iron based materials with similar total alloy content were chosen for study. All contained
nominally 0.8 wt% Mo and 0.6 wt% added carbon and have different amounts of Ni, Cr, Mn and Si. A
Cr-Mn-Ni-Mo (Alloy 2 : Ancorsteel® 41AB), a Ni-Si-Mo (Alloy 3 : Ancorloy® MDB) and a
developmental nickel free CrMnSiMo alloy were tested, Table I. All mixes were binder treated and five
compaction conditions were used to produce 5 different densities: 415 MPa (30 tsi), 550 MPa (40 tsi) and
690 MPa (50 tsi) at room temperature with EBS wax lubricant and 620 MPa (45 tsi) and 690 MPa with a
die heated to 63 °C (145 °F) and the AncorMax D® lubricant system. Standard transverse rupture bars,
dogbone tensile bars and impact bars were pressed.
Table I. Composition of the alloys tested (wt%).
Alloy
CrMnSiMo
Alloy 2
Alloy 3
C
0.55
0.55
0.55
Mn
1
0.9
0.1
Si
1
0.7
Ni
1
2
Cr
0.9
0.75
-
Mo
0.8
0.8
0.8
Samples were sintered in a high temperature belt furnace with a 90 vol% nitrogen – 10 vol%
hydrogen atmosphere and sintering temperatures of 1120 °C (2050 °F), 1150 °C (2100 °F), 118 0°C
(2150 °F) and 1260 °C (2300 °F) were used. Two different sintering times (13 and 38 minutes) were
investigated with samples compacted at 690 MPa with the EBS wax. Sintering time was defined as the
time at or above the set point temperature. The maximum temperature was typically 17 °C (30 °F) above
the set point. The cooling rate between 650 °C (1200 °F) and 315 °C (600 °F) was 94 °C/min
(2.8 °F/sec). Samples were tempered at 200 °C (400 °F) for 1 hour under a nitrogen atmosphere.
Green and sintered density was measured on the transverse rupture (TRS) bars and impact bars.
Immersion density methods were used to measure sintered tensile bars. Dimensional change was
measured on TRS bars. Sintered carbon and oxygen was measured on 12.5 mm thick TRS bars pressed at
690 MPa. Samples were cut from the center of the bar to avoid surface effects such as surface
decarburization and surface de-oxidation. The measured sintered carbon for all samples was found to be
0.55 ±0.03 wt% C.
In order to compare the properties of the three materials at a given density, each mechanical
property was plotted vs. sintered density for the range of compaction conditions. A linear best fit line
(least squares method) was used to describe the data, and from the equation, mechanical properties were
obtained for a 7.1 g/cm3 density. These values were then plotted vs. sintering temperature.
RESULTS
The study outlined above generated a large volume of data which will be presented in two ways:
comparisons at a given compaction pressure and at a given density. The effects of alloying on sample
density, dimensional change and oxygen content are presented at the two sintering times with a
compaction condition of 690 MPa with a room temperature die. The mechanical properties are presented
graphically at a density of 7.1 g/cm3.
It is not surprising that the green and sintered density of the CrMnSiMo alloy is lower than that
for Alloys 2 and 3 that contain admixed nickel. The green density of the CrMnSiMo alloy (7.08 g/cm3) is
0.10 g/cm3 and 0.07 g/cm3 lower than Alloy 2 (7.18 g/cm3) and Alloy 3 (7.15 g/cm3), respectively, at
690 MPa. Upon sintering, the higher nickel alloy (Alloy 3) shrinks considerably, resulting in the highest
density at a given compaction pressure. The sintered density of the CrMnSiMo alloy is 0.11 to
®
Ancorsteel, Ancorloy and AncorMax D are trademarks of Hoeganaes Corporation
0.17 g/cm3 lower than Alloy 3 and 0.04 to 0.08 g/cm3 lower than Alloy 2, depending on temperature and
time at temperature.
Dimensional change is an important variable in P/M part manufacturing. There is an overall
desire to minimize the magnitude of dimensional change and to ensure that it is relatively stable with
small changes in processing. The effect of time and temperature on dimensional change is illustrated in
Figure 1. Little effect of time or temperature was found for Alloy 2. This material had a dimensional
change of nominally +0.4%. At short sintering times, the CrMnSiMo alloy had a similar response to
Alloy 2. However, at longer times, a reduction in dimensional change, to between +0.2 and +0.3%,
occurred. Both time and temperature had the largest effect on Alloy 3. The slope of the dimensional
change vs. temperature line is large, indicating this material is very sensitive to sintering temperature.
This behavior is similar to other Ni-Mo alloys, such as FLN2-4405. The admixed nickel causes
significant shrinkage during sintering. It is also important to note that much of the microstructure in the
three alloys studied is martensitic. Martensite is the lowest density phase in steel, and this results in
growth of the samples. Tempering reduces this effect, but a fast cooled, martensite containing sample
will have a larger dimensional change than a slow cooled, pearlite containing sample. The large, positive
magnitude of the dimensional change in the Cr-Mn containing alloys is accentuated by martensite
formation in these sinter-hardenable alloys.
Figure 1. Effect of Sintering Temperature and Time (13 and 38 minutes) on dimensional change.
Oxygen content is an important variable in the performance of P/M steels. The oxygen content of
the three alloys was measured in samples pressed at 690 MPa, and sintered for two times and four
temperatures, Figure 2. Alloy 2 had relatively low oxygen contents over the range of sintering conditions.
The oxygen content in both of the Si-containing materials, Alloy 3 and the CrMnSiMo alloy, has a strong
relationship with time and temperature; decreasing with increasing time and temperature. The
CrMnSiMo alloy showed an interesting behavior at the shorter sintering time with temperatures of
1120 °C through 1180 °C; there was little effect of temperature on oxygen content. At the longer sinter
time, the CrMnSiMo alloy showed a large reduction in sintered oxygen content. The oxygen content also
was a strong function of sintering temperature, and when sintered at 1260 °C, the CrMnSiMo alloy had
the lowest oxygen content of the three alloys tested (< 0.02 wt%). As a reference, a 0.85% Mo-Fe alloy
with 0.6% graphite would have an oxygen content ≤ 0.03 wt% at this density and temperature range.
Nitrogen content also was measured in the three alloys, and the presence of Cr had a large effect on
nitrogen content. Alloy 3 had nitrogen levels ≤ 0.02% for all sintering conditions, while both of the CrMn alloys had levels between 0.03 and 0.04%.
Figure 2. Effect of Sintering Time (13 and 38 minutes) and Temperature on Oxygen Content.
Mechanical Properties
The apparent hardness of the CrMnSiMo alloy is significantly higher than for the two nickelcontaining alloys. The hardness of the three alloys sintered at 1120 °C (2050 °F) is shown as a function
of sintered density (Figure 3). The apparent hardness increases with density for all three alloys. Using
the equation of the best linear fit linear, the hardness was calculated at a density of 7.1 g/cm3. This
process was repeated for the other three temperatures. Apparent hardness at 7.1 g/cm3 was then plotted
vs. sintering temperature, Figure 4. At this density, the hardness range of the CrMnSiMo alloy is 68 to 70
HRA (35 to 39 HRC) over the temperatures tested. A hardness of 66.5 HRA (32 HRC) and 64 HRA
Figures 3 & 4. The apparent hardness of the three alloys plotted as a function of density at 1120 °C
(Figure 3) and as a function of sintering temperature at a density of 7.1 g/cm3 (Figure 4).
(25 HRC) was obtained for Alloys 2 and 3, respectively at 1260 °C (2300 °F). The largest difference in
hardness was found at 1120 °C, where the hardness of the CrMnSiMo alloy is 68 HRA (35 HRC) and the
hardness of the other alloys is approximately 62 HRA (23 HRC). It is important to note that the high
hardness of the CrMnSiMo alloy is relatively insensitive to sintering temperature. The hardness of
Alloy 2 only begins to approach the CrMnSiMo alloy at high temperature. The difference between the
three alloys is somewhat reduced at longer sintering times. Recall that all samples were tempered at
204 °C (400 °F) for 1 hour after sintering prior to hardness testing.
The yield strength of the three alloys follows a trend similar to the hardness. The 0.2% offset
yield strength of the CrMnSiMo alloy is considerably higher than for the other two alloys (Figure 5). At a
density of 7.1 g/cm3, the yield strength is > 825 MPa (120,000 psi) for all sintering temperatures and
times. The other two alloys only exceed the 825 MPa mark at 1260 °C sintering. The clear advantage of
the CrMnSiMo alloy in both hardness and yield strength is reduced with respect to the ultimate tensile
strength (Figure 6). This is in part due to the lower ductility of the alloy. However, at the density of
7.1 g/cm3, the UTS of the CrMnSiMo alloy is still higher than the other alloys. The ultimate tensile
strength of all three alloys is greater than 1170 MPa (170,000 psi) at 1260 °C.
The effect of time at temperature on strength is shown in Figure 7 for the CrMnSiMo alloy. The
increased time had a larger effect at lower temperatures than at high temperature. The largest change was
found at 1120 °C, where a 170 MPa (25,000 psi) and 240 MPa (35,000 psi) increase in yield and tensile
strength, respectively, was obtained. These represent 22% and 28% increases in yield and tensile
strength. The increase in strength at 1260 °C is only 12% and 8%, respectively. These changes in
strength are related in part to the amount of sintering that occurs in the furnace. The other two alloys
showed similar trends. Alloy 3 showed more benefit in ultimate tensile strength from the additional time
at temperature than Alloy 2.
Figures 5 & 6. Yield and ultimate tensile strength as a function of sintering temperature at 7.1 g/cm3.
Figures 7. Effect of time at temperature on yield and tensile strength for the CrMnSiMo alloy compacted
at 690 MPa.
The high strength CrMnSiMo alloy had the lowest elongation of the three alloys, Figure 8. The
elongation improved with increased sintering temperature, from 1% to 1.5%. Alloy 3 showed a similar
trend, with elongations from 1.5 to 1.9%. Alloy 2 had a rather atypical response with increasing
temperature. This alloy exhibited decreasing elongation with increasing temperature. It is thought that,
due to the way this alloy is made through the use of ferrochromium and ferromanganese additions, the
increase in sintering temperature increased the diffusion of Cr and Mn in the alloy, thereby hardening the
alloy and reducing ductility.
The impact resistance of the three alloys is shown in Figure 9 for the shorter sintering time.
Alloy 2 has the highest impact resistance, especially at lower sintering temperatures. The CrMnSiMo
alloy and Alloy 3 have similar impact responses, and both approach that of Alloy 2 at high temperature.
Although not tested, it is expected that all of the alloys would have considerably higher impact energies if
the longer sintering time was used.
Figures 8 & 9. Elongation and impact response as a function of sintering temperature at 7.1 g/cm3.
Microstructure
The primary microstructural constituent of the three alloys is martensite. Figure 10 shows the
microstructures of the alloys sintered at 1260 °C for 38 minutes. Both of the Cr-Mn containing alloys
have similar structures, which are predominantly martensitic with a small percentage of bainite. An
occasional region of pearlite/upper bainite was found in Alloy 2 (Figure 10b). Alloy 3 contained a
significant amount of pearlite/upper bainite (Figure 10c). The effect of sintering time on microstructure
can clearly be seen in Alloy 3. The amount of pearlite increases considerably at the shorter sintering time
(Figure 11). This is related to the diffusion of admixed nickel in the alloy. This effect was seen to a
lesser extent with Alloy 2.
A
B
C
Figure 10. Microstructures of the CrMnSiMo alloy (A), Alloy 2 (B) and Alloy 3 (C) sintered at 1260 °C
for 38 minutes.
A
B
Figure 11. Effect of time at temperature on the microstructure of Alloy 3. (A) 13 minutes,
(B) 38 minutes at 1260 °C.
CONCLUSIONS
A new nickel-free P/M steel containing Cr, Mn, Si and Mo is being developed for high performance
applications. The alloy can be sintered over the entire range of temperatures typically used for iron-based
P/M alloys. The addition of chromium and manganese improves the alloy hardenability, producing a
virtually fully martensitic microstructure in the as-sintered condition. The high martensite content results
in the alloy having high hardness and yield strength compared with other high performance alloys.
Oxygen content, which can often be a problem in Cr-Mn containing P/M steels, could be reduced below
0.02 wt% with high temperature sintering. Reasonable oxygen contents (<0.08%) at conventional
sintering temperatures could also be achieved at sintering times closer to 30 minutes.
ACKNOWLEDGEMENTS
The author would like to thank Jerry Golin and Ron Fitzpatrick of the Hoeganaes R&D department for
their assistance in acquiring the microstructures and data in this manuscript.
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