SINTERING OF CHROMIUM CONTAINING PM STEELS PROCESSED TO HIGH DENSITY

Bruce Lindsley

Hoeganaes Corporation Cinnaminson, NJ 08077, USA

ABSTRACT In recent years there has been a push to develop ferrous powder metallurgy (PM) alloys with chemical compositions that more closely resemble wrought steels. The use of chromium in conjunction with lower amounts of Ni and Mo can result in superior mechanical properties and excellent dimensional stability in PM steels. However, appropriate sintering of such systems is a critical parameter to the development of these properties. In contrast, Fe-Cu-C steels perform reasonably well in almost any atmosphere, time, temperature environment, as long as the Cu melts. As such, a change in sintering philosophy is required when using chromium-containing alloys. In addition, as density increases, the removal of oxygen from the compact becomes more difficult. This can lead to a further increase in time and/or temperature required to sinter these alloys properly. This paper discusses the sintering and physical properties of Ancorsteel 4300 and Ancorsteel 4300L pressed at different densities. INTRODUCTION Popular ferrous PM compositions containing admixed Cu are relatively easy to sinter. These iron-based alloys are not oxygen sensitive and can be sintered in many atmospheres, including endo gas. In addition, a good sinter is achieved once the admixed Cu melts and redistributes in the compact, rounding pores and strengthening sintered necks. PM mixes that contain admixed Ni are also tolerant of many sintering atmospheres, but require higher temperatures and times to gain the full benefit of the Ni by way of solid-state diffusion. Combinations of added Ni and Cu, such as that found in diffusion alloys, fall into the category of easily sintered materials. With a desire for improved properties for new applications and price pressures from traditional alloying elements of Mo, Ni and Cu, powder producers have turned to alternative alloying elements such as chromium. These alloy systems can be more difficult to produce and prealloyed Cr and associated Cr

oxides have a detrimental effect on compressibility. The challenge is to provide a highly compressible powder with low oxygen content. Two recently introduced powders, Ancorsteel 4300 and Ancorsteel 4300L, deliver good compressibility with the enhanced performance of a Cr-containing alloy. Chromium containing alloys must be sintered in atmospheres with low partial pressures of oxygen, such as nitrogen-hydrogen atmospheres. The CO found in endothermic gas will dissociate and both oxidize and carburize Cr-containing alloys, preventing proper sintering. The micro-atmosphere found within the pores of the compact should also contain little oxygen. This becomes difficult to remove as the density of the compact increases, and may require higher sintering temperatures to avoid oxidation of chromium and slower heating rates to allow for removal of the oxygen. Sources of oxygen include air present from the compaction process, lubricant burn off, and reduction of oxygen found on the iron powder. This paper studies the effect of time, temperature and density on the sintering of two chromium-containing alloys along with the diffusion-alloyed material, FD-0405. The benefits of optimum sintering conditions will be discussed. EXPERIMENTAL PROCEDURE The powders used for testing were Ancorsteel 4300, Ancorsteel 4300L and diffusion-alloyed FD-0405, all with 0.6 wt.% graphite. Compositions of these three mixes are shown in Table II. The mixes were made with Asbury 3203H graphite and contained 0.55 wt.% Acrawax C as a lubricant.

Table II. Nominal compositions (in wt.%) of the alloys studied. Balance for all alloys is Fe.

Alloy Designation Cr Ni Si Mo Mn Cu Gr

1 4300+0.6gr 1.0 1.0 0.6 0.8 0.1 - 0.6

2 4300L+0.6gr 1.0 1.0 0.6 0.3 0.1 - 0.6

3 FD-0405 - 4.0 - 0.5 0.1 1.5 0.6 Transverse rupture strength and dogbone tensile bars were compacted at room temperature to densities of 7.0 and 7.2 g/cm3. Sintering was conducted in an Abbott belt furnace using an atmosphere of 90% N2-10% H2 (vol.%). The time at sintering temperature, defined as the time that the samples were within 5 °C (10 °F) of the set point, was varied to achieve sintering times of 8, 15 and 30 minutes. This corresponded to 25, 45 and 85 minutes in the hot zone of the furnace. The average cooling rate over the range of 650 to 315 °C (1200 to 600 °F) was 0.7 °C/sec (1.3 °F/sec). All samples were tempered at 205 °C (400 °F) for 1 hour. Percent dimensional change, sintered density, and apparent hardness were measured from the transverse rupture samples using standard MPIF procedures. Tensile testing was performed using a crosshead speed of 0.065 cm/min (0.025 in/min). The machine is equipped with a 25 mm (1 in) extensometer, which was left on until failure.

RESULTS Microstructures The microstructures of the 3 alloys are presented in Figures 1, 2 and 3. In all cases, alloy-rich regions are apparent and limited alloying is observed at 1120 °C for the shortest sintering time. As time and temperature increase, diffusion of alloying elements is enhanced and the area fraction martensite (tan colored) increases. The conventional cooling used in this study (0.7 °C/s) accentuates the differences in alloy content and hardenability. Only those regions with sufficient alloy content transform to martensite. Tempered martensite is responsible for the high strength found in these alloy systems. In Figure 1, a large fraction of martensite is apparent at 1120 °C and 30 minutes at temperature for Ancorsteel 4300 (Alloy 1). While it is possible to sinter this alloy at 1120 °C and obtain a desirable microstructure, the time required at temperature is impractical under typical production conditions. An improvement in microstructure is apparent with higher sintering temperatures. The structure at 1260 °C is predominantly martensitic. The lean Cr alloy (4300L) shows a similar trend, Figure 2. The microstructure at 1120 °C and 8 minutes at temperature shows very little martensite. This alloy is not recommended under these conditions. The lower Mo content in the alloy results in lower hardenability and lower strength, and additional time is required for diffusion. Again, with 30 minutes at temperature, the microstructure improves but sintering is more expensive. It is recommended that this alloy should not be sintered at a temperature less than 1150 °C. The diffusion-alloyed FD-0405 also exhibits an increase in the amount of martensite with increasing time and temperature. In this alloy, the amount of non-etching alloy rich regions (white) also increases at intermediate times and temperatures. With longer times and higher temperatures, the alloy in the Ni-Mo-Cu rich regions diffuses into the matrix. Given enough diffusion, the FD-0405 alloy is sinter-hardenable as seen by the large amount of martensite at 1180 °C and 30 minutes at temperature. The larger base iron particles contain little alloy and remain pearlitic. A rim about these particles is observed where the carbon content is apparently low, as evidenced by a lack of carbides. This is an effect of Ni diffusion into the base iron as reported by Wu [1]. Mechanical Properties Apparent Hardness The chromium-containing alloys have good hardenability, resulting in high apparent hardness at conventional cooling rates. It should be noted that significantly higher hardness values have been achieved with accelerated cooling [2]. Cr-containing Alloy 1 sintered for only 8 minutes has a higher apparent hardness than FD-0405 sintered for 30 minutes for all temperatures (Figure 4a). The efficient alloying in the Cr alloys is more effective than the more heavily alloyed FD-0405. The unalloyed iron base of FD-0405 has very low hardenability prior to diffusion of Mo, Ni and Cu. Two behaviors were found with respect to time and temperature. The diffusion-alloyed FD-0405 showed an increase in hardness with increasing time and temperature. Again, this is expected as the Mo, Ni and Cu diffuse into the iron and increase hardenability and solid solution strengthening. The hardness of the Cr alloys also increase with temperature at short sintering times (8 minutes) as the amount of martensite increases. With sintering times longer than 8 minutes, the hardness of the Cr alloys is relatively insensitive to sintering temperature. The hardness of the 4300 alloy is nominally 64 HRA (28 HRC) at 7.0 g/cm3. However, at long sintering times at high temperature, the hardness values decrease for the Cr alloys. This is a result of decarburization of the samples. No carbon was added to the sintering atmosphere, and at long sintering times and high temperature, significant carbon was lost. In all but the 1260 °C and 30 minute condition, carbon contents ranged from 0.55% to 0.51% for the Cr alloys and

8 15 30

726 (105) 853 (121) 964 (140)

817 (119) 905 (131) 1040 (151)

1019 (129) 1047 (142) 1078 (156)

Figure 1. Ancorsteel 4300 microstructure sintered at 8, 15 and 30 minutes (columns left to right) and 1120, 1150 and 1180 °C (rows top to bottom). Ultimate tensile strength in MPa (103 psi) is listed below each micrograph.

1120

1150

1180

8 15 30

594 (86) 680 (99) 791 (115)

664 (96) 740 (107) 852 (124)

740 (107) 811 (118) 875 (127)

Figure 2. Ancorsteel 4300L microstructure sintered at 8, 15 and 30 minutes (columns left to right) and 1120, 1150 and 1180 °C (rows top to bottom). Ultimate tensile strength in MPa (103 psi) is listed below each micrograph.

1120

1150

1180

8 15 30

635 (92) 695 (101) 789 (114)

672 (98) 729 (106) 830 (120)

709 (103) 766 (111) 849 (123)

Figure 3. FD-0405 microstructure sintered at 8, 15 and 30 minutes (columns left to right) and 1120, 1150 and 1180 °C (rows top to bottom). Ultimate tensile strength in MPa (103 psi) is listed below each micrograph.

1120

1150

1180

a b

Figure 4. Apparent hardness of alloys 4300 (red), 4300 Lean (blue) and FD-0405 (green) at (a) 7.0 g/cm3 and (b) 7.2 g/cm3 for different sintering temperatures and times. 0.57% to 0.53% for the FD-0405, decreasing as sintering time and temperature increased. In the case of 1260 °C and 30 minutes, the sintered carbon content dropped to 0.46% for the Cr alloys and 0.49% for FD-0405. The carbon loss is responsible for the lower hardness in the Cr alloys at 7.0 g/cm3. Interestingly, even though the FD-0405 samples lost a similar amount of carbon, the improvement in alloy distribution offset the carbon loss and led to higher hardness values. The higher density samples did not lose nearly as much carbon, and therefore little drop off in hardness was noted. Tensile Properties As expected, both yield and ultimate tensile strength increase with sintering temperature for all alloys, Figures 5 and 6. The improvement in strength with sintering temperature is nearly linear. The effect of time is quite pronounced for all three alloys. The increase in UTS for alloy FD-0405 is consistently 140 MPa (20 x 103 psi) for all temperatures as time is increased from 8 to 30 minutes. The Cr alloys exhibit a 210 (30) to 280 MPa (40 x 103 psi) increase with sintering time at conventional temperatures. This differential decreases as sintering temperature is increased, in part due to the carbon loss at 1260 °C, as discussed above. The ultimate tensile strength of the samples compacted to 7.0 g/cm3 is given in Figures 1-3 under the respective microstructures. Diffusion of alloying elements plays a critical role on the strength of these alloy systems and as the temperature increases, shorter times are required to achieve good mechanical properties. With respect to the yield strength, all three alloys increase approximately 140 MPa (20 x 103 psi) with longer times at temperature. The elongation of the Cr alloys greatly improves with sintering time and temperature. The elongation reported is total elongation at failure and includes both the elastic and plastic deformation. A 1% increase in plastic elongation was found using high temperature sintering compared with conventional temperatures. The elongation of FD-0405 samples did not change with sintering time or temperature. The expected improvement in elongation due to better sintered necks and pore rounding was offset by diffusion of the alloy elements, resulting in higher hardness and stiffening of the microstructure. This is a rather unique behavior of FD-0405.

a b

Figure 5. 0.2% yield strength of alloys 4300 (red), 4300 Lean (blue) and FD-0405 (green) at (a) 7.0 g/cm3 and (b) 7.2 g/cm3 for different sintering temperatures and times.

a b

Figure 6. Ultimate tensile strength of alloys 4300 (red), 4300 Lean (blue) and FD-0405 (green) at (a) 7.0 g/cm3 and (b) 7.2 g/cm3 for different sintering temperatures and times. The leaner Cr alloy (4300L) compares favorably to FD-0405 with respect to strength properties at all sintering times and temperature. The hardness and yield strength values are slightly better with the 4300L and the tensile strength is similar. The ductility is greater with the FD-0405, but this limitation can be overcome with high temperature sintering. With lower Mo and Ni contents, the 4300L is a more cost effective alloy compared with FD-0405. If higher strengths and hardenability are required, the 4300 should be used. Effects of Time and Temperature on Dimensional Change The sensitivity of dimensional change in the Cr-containing alloys is relatively flat with sintering temperature, especially Alloy 1 (4300). For sintering times of 8 and 15 minutes, which are considered typical sintering times in the industry, little change (less than 0.05%) in dimensions for Alloy 1 was found

a b

Figure 7. Total elongation of alloys 4300 (red), 4300 Lean (blue) and FD-0405 (green) at (a) 7.0 g/cm3 and (b) 7.2 g/cm3 for different sintering temperatures and times.

a b

Figure 8. Dimensional change of alloys 4300 (red), 4300 Lean (blue) and FD-0405 (green) at (a) 7.0 g/cm3 and (b) 7.2 g/cm3 for different sintering temperatures and times. over the range of sintering temperatures. For long sintering times, the difference in dimensional change increased to approximately 0.10%. The leaner Alloy 2 (4300 L) showed a slightly larger variation in DC with sintering temperature, with a change of 0.10% at shorter times and 0.15% for longer times. The diffusion-alloyed FD-0405, which contains 4% added Ni, exhibits greater changes in DC over the range in sintering temperatures. At shorter times, the change is 0.2% and at long times, the change was found to be 0.35%. The shrinkage at 1260 °C is quite apparent. Sintering time has a strong effect on dimensional change and a similar decrease in DC was found for all three alloys as time increased.

Effect of Sintered Density The deleterious effect of oxygen on mechanical properties of chromium bearing PM steels has been reported earlier [3]. As the density of Cr alloys increase, it may be more difficult to remove oxygen from the compact. Oxygen, in the form of CO and/or H2O, must be removed from the atmosphere within the pores of the compact before chromium-rich oxides can be reduced. One of the goals of this research was to investigate the possible effects of oxygen and density in Cr alloys. It was thought that the diffusion alloyed FD-0405 may show a greater improvement in properties than the Cr-containing alloys as density increased, as FD-0405 is not oxygen sensitive and should not be adversely affected by higher density. However, no detrimental effect was observed as the density was increased from 7.0 to 7.2 g/cm3, as shown in Figure 9 for ultimate tensile strength. All alloys showed improved properties with higher density, and the improvement was similar for all three alloys. A uniform shift in tensile properties can also be observed in Figures 5 and 6 as the density increases. It is possible that 7.2 g/cm3 is not a high enough density in these alloy systems to cause problems with oxidation reduction. The low oxygen content of the Cr-containing powders used in this study was also beneficial.

Figure 9. Effect of sintered density on ultimate tensile strength of the three alloys sintered at 1180 °C for 15 minutes. CONCLUSIONS The Cr alloys presented in this study have an excellent combination of properties. They can be sintered at 1120 °C, but require extended times in the hot zone that most manufactures would consider impractical. A minimum sintering temperature of 1150 °C is therefore recommended for these alloys. However, the best properties can only be achieved at higher temperatures. This statement is also valid for the diffusion alloyed FD-0405. As alloy costs have increased, the relative cost of sintering has decreased. Those part makers with the ability to high temperature sinter will be able to maximize performance from current alloy systems and may be able to utilize leaner alloys, gaining a competitive advantage over other part makers.

• The morphology of the three alloys studied evolves from a predominantly pearlitic/bainitic microstructure to a martensitic microstructure as time and temperature increase. Short sintering times and low temperatures fail to utilize the alloy content in these alloy systems.

• Ancorsteel 4300L has strength and hardness values equivalent or superior to diffusion-alloyed FD-0405 at a significantly lower alloy content. Similar elongation can be achieved with high temperature sintering. Ancorsteel 4300 delivers superior strength and hardenability compared with the other alloys.

• The chromium-containing alloys exhibit a much lower dependence of sintering temperature on dimensional change, allowing for more robust part manufacturing.

ACKNOWLEDGEMENTS The author would like to thank Paul Kremus, Ron Fitzpatrick and Gerard Golin for their assistance in collecting the data found within this paper. REFERENCES 1. M. W. Wu, K. S. Hwang and H. S. Huang, “Identifications of crack initiation sites of Ni-containing

steels and methods for property improvement”, Advances in Powder Metallurgy & Particulate Materials, compiled by W. R. Gasbarre and J. W. von Arx, Metal Powder Industries Federation, Princeton, NJ, 2006, part 5, p. 58-71.

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, compiled by W. R. Gasbarre and J. W. von Arx, Metal Powder Industries Federation, Princeton, NJ, 2006, part 7, p. 1-11.

3. Sigl, L. S., Delarbre, P., “Impact of oxygen on the microstructure and fracture morphology of Fe(Cr,Mo)-PM steels”, Advances in Powder Metallurgy & Particulate Materials, Metal Powder Industries Federation, Princeton, NJ, 2003.