Maximizing the Value and Performance of Chromium, Manganese, and Silicon Containing PM Steels

Transcription

1 Maximizing the Value and Performance of Chromium, Manganese, and Silicon Containing PM Steels Michael L. Marucci - Director, Research & Development - Hoeganaes Corporation USA Shashi S. Shukla - Managing Director - Hoeganaes Corporation Europe Dr. Bruce Lindsley Manager, Product Development - Hoeganaes Corporation USA Recent improvements in health standards for respirable dust along with continued increase in the cost of common PM alloying elements has prompted the need to reduce the use of copper, molybdenum, and nickel in PM steels. These traditional alloying elements can be replaced with chromium, manganese, and silicon that are attractive because of their lower cost combined with excellent strengthening and hardenability effects. However, special care must be taken during the preparation of the powder mixtures, during compaction, during sintering, and during heat-treatment to ensure optimized apparent hardness, strength, and performance. This work reviews processing conditions required to realize the maximum value of chromium, manganese, and silicon containing PM alloys. Introduction: The global awareness of the risks associated with exposure to fine metallic particles, nickel powders in particular, has increased the need to limit the use of un-bonded additives such as carbonyl nickel powder. Recent proposed regulations from Arbeitsplatzgrenzwert (AGW) list the exposure limits inhalable nickel to be moved from the value used prior to 0.5 mgni/m 3 to a level in the range <0.05 mgni/m 3. This proposed order of magnitude reduction poses challenges for the use of carbonyl nickel in the powder metallurgy industry because very small emissions will exceed the exposure limit. In order to meet the improved health standards powder metal parts producers will need to improve dust control, use wet cleaning procedures, employ bonded powders, or eliminate or reduce the amount of carbonyl nickel powders used. A second trend driving the need for reducing the amount of nickel and copper used is price volatility. Higher value alloying additions increases powder metal material price risk. Figure 1 shows the spot price data for copper and nickel from September 2002 to June of The nickel price has seen a very large variation that has caused nickel containing powder metal alloy prices to spike during certain periods. These market forces have led to a renewed emphasis on using lower cost alloying elements such as chromium and manganese. Chromium, manganese, and silicon are used widely in the wrought steel industry because of their effectiveness in improving strength, toughness, and hardenability and they also have a lower cost than other alloying elements. These elements are less wildly used in powder metallurgy because they are difficult to process due to their affinity for oxygen. Use of prealloyed chromium or manganese is a potential solution but results in a steel powder that has a higher oxygen content that is difficult to remove during sintering and also has a reduced level of compressibility.

2 Figure 1: Spot price of copper and nickel from September 2002 to July 2012 (source: LME) Over the past few years innovative methods of adding chromium, manganese, and silicon to PM alloys have been developed that minimize compaction and sintering issues. In particular two families of alloys have been commercialized including the chromium containing Ancorsteel and the manganese containing ANCORBOND FLM-4400 series of alloys.[1,2] These alloys combine precision bonding technology with proprietary master alloy additions resulting in stable alloys that produce properties that meet or exceed more highly alloyed diffusion alloys. However, to maximize the performance benefits of the chromium, manganese, and silicon these alloys need to be processed somewhat differently than traditional diffusion alloys or iron-copper PM alloys. The following sections outline the effect of varying key process variables and give insight on how best to design compaction, sintering, and heat-treating cycles to optimize performance. Experimental Binder-treated, ANCORBOND, premixes were used for all test materials using Ancorsteel 50 HP or Ancorsteel 85 HP as the base alloy powder. The chemical composition of the test materials are outlined in Table I. All alloys contained natural flake graphite as the carbon source. Were applicable, the chromium, silicon, and manganese were added as proprietary master alloy additives. Laboratory transverse rupture, dog-bone tensile, and un-notched Charpy impact were compacted to the listed density according to MPIF Standards 41, 10, and 40. The laboratory specimens were sintered in a belt furnace at the listed temperature in an atmosphere of 90N2-10H2 (vol.%). Time at sintering temperature is defined as the time that the test samples were within 5 C of hot zone set point. Where quoted, average cooling rates were measured between 650 and 315 C. A tempering temperature of 205 C for 1 hour was used for all test samples. Dimensional change was measured from die size. Tensile yield strength was measured at 0.2% offset (plastic strain). 1 ANCORBOND and Ancorsteel are registered trademarks of Hoeganaes Corporation.

3 Table I: Nominal test alloy compositions Alloy Fe Mo* Ni Cr Si Mn Ancorsteel 4300 Bal ANCORBOND FLM-4000 Bal ANCORBOND FLM-4400 Bal *prealloyed Results: Gr The compressibility of the Cr and Mn containing PM steels is compared to the common diffusion alloy FD-0405 grade in figure 2. The compressibility of the diffusion alloy grade is g/cm 3 higher than the Cr and Mn containing materials. This is expected because of the addition of the master alloy. However, using a master alloy in combination with bonding results in compressibility that is g/cm 3 than a fully prealloyed Fe-Ni-Mo material. At normal compacted densities up to g/cm 3 the addition of the master alloy should require minimal changes to the press and tooling set up compared to a more common PM powder mixtures. However, at higher compacted densities care must be taken when choosing tooling material and lubricant systems because the lower compressibility translates into a higher green spring back that can lead to a faster rate of tool wear. At compacted densities above 7.10 g/cm 3 it is advised that tools be made of more wear resistant materials in combination with powders containing a high density lubricant/binder system. Figure 2: Compressibility of chromium and manganese containing steels compared to a common diffusion alloyed and a prealloyed material.

4 Effect of Sintering Conditions on Chromium Containing PM Steels Sintering conditions are very important when using chromium containing PM alloys. Chromium has and oxide that is very difficult to reduce. In general it is advised that sintering atmospheres have low moisture content with a dew point below -40 C. The preferred sintering atmosphere is N2-H2 with a minimum of 5-10 vol% hydrogen to ensure good reduction. Endothermic gas atmospheres are not recommended for sintering or post sintering heat-treatments.[1] In addition to sintering atmosphere, sintering temperature and time has been determined to have a large impact on the properties of chromium containing PM steels. A study was conducted to quantify the impact of sintering time and temperature on the commercially available chromium containing Ancorsteel 4300 material.[2] Figure 3 shows the dimensional size change behavior of this alloy as a function of sintering time and temperature. These data show that for a given sintering time the dimensional change is largely unaffected by sintering temperature especially for the 8 and 15 minute samples. There is some additional shrinkage as the sintering temperature is increased for the 30 minute samples, but this is still a relatively small effect compared to a more typical diffusion alloy material. Sintering time, however, has a much more pronounced effect on the dimensional size change. Changing the sintering time at temperature from 8 to 30 minutes can result in a size change difference of as much as 0.30%. Understanding this behavior is important when designing a PM part with a chromium containing material so that the sintering furnace is sized correctly. Typically PM part production facilities will use sintering temperature as a primary method to modify size change to deal with process variability. Since sintering temperature has only a small impact of the size change of this material system sintering time adjustments must be made in the form of belt speed adjustments. This has to be considered with care because increasing the belt speed impact mechanical performance (see following figures) and slowing the belt speed reduces the production output. Figure 3: Dimensional change from die for a chromium containing PM steel as a function of sintering temperature and time.

5 Figure 4 shows the apparent hardness, ultimate tensile strength, and total elongation as a function of sintering temperature and time. The sintering time has a larger effect on the apparent hardness at lower sintering temperatures but at higher sintering temperatures the apparent hardness values begin to converge. This is explained by two competing mechanisms; at lower sintering temperatures longer sintering times are needed to ensure complete diffusion of the chromium from the master alloy into the steel matrix. At higher sintering temperatures chromium diffusion is more complete but test sample decarburization is more pronounced leading to a slightly lower final carbon content and lower apparent hardness. This can be countered by an additional small addition of graphite. Figure 4: Apparent hardness of a chromium containing PM steel as a function of sintering temperature and time. The ultimate tensile strength and total elongation as a function of sintering temperature and time are shown in figures 5 and 6. The strength increases as a function of both sintering time and temperature. The total elongation also increases with sintering temperature and time. These performance increases are caused by the diffusion of the master alloy into the steel matrix and the sintering cycle must be considered when specifying this type of material for an actual application as the properties vary widely with changes to the cycle. It is recommended that this alloy be sintered at a minimum temperature of 1150 C. Higher sintering times and temperatures should be considered for parts with thicker cross sections.

6 Figure 5: Ultimate tensile strength of a chromium containing PM steel as a function of sintering temperature and time. Figure 6: Ultimate tensile strength of a chromium containing PM steel as a function of sintering temperature and time. A selection of the test microstructures are shown in figure 7. The etched microstructures reveal the hardening and strengthening mechanisms that are taking place as both time and temperature are increased. At the 1120 C and 8 minutes at temperature the microstructure consists largely of pearlite and bainite; under these conditions the chromium has not diffused into the Fe-Mo steel matrix. The 1150 C and 15 minutes at temperature condition produces a mixed microstructure of divorced pearlite/bainite with a substantial area fraction of martensite. This hybrid structure provides a very good balance of strength, hardness, and toughness suitable for many applications. The final structure shown was sintered at 1180 C for 30 minutes at temperature. This high temperature and very long sintering time produces a structure that is mostly martensite with remaining areas of bainite. In this case the master alloy has diffused into the structure to a high degree. This set of sintering conditions gives the highest strength of the test matrix.

7 1120 C 8 min 1150 C 15 min 1180 C 30 min Figure 7: Microstructure of a chromium containing PM steel as a function of sintering temperature and time. Samples were etched with 2%picral 4% nital. The cooling rate from sintering temperature also has a significant impact on final mechanical properties as this alloy system will sinter-harden. Table II shows the effect of increasing the cooling rate by using an accelerated cooling zone that uses convective gas flow.[3] The data show that increasing the cooling rate from 0.7 C/s to 2.2 C/s results in substantial increases in apparent hardness and tensile properties, conversely, the higher cooling rate results in reduced total elongation and impact energy. This is expected as more martensite is produced creating the harder and less ductile microstructure. One caution to the reader is that these tests were conducted on small test samples that had a mass of about 20 g. The actual cooling rate observed in a larger PM part may be lower with thicker cross sections. Additionally, tempering the sinter-hardened microstructure is recommended to ensure a good balance between strength and hardness while limiting the possibility of brittleness due to un-tempered martensite. Cooling Rate C/s Table II: Effect of cooling rate on the properties of Cr containing PM steel (Fe - 1Cr + 1Ni + 0.6Si + 0.8Mo C, Tempered at 205 C) Dimensional Change % Apparent Hardness HRC Ultimate Tensile Strength MPa Yield Strength MPa Total Elongation % Impact Energy Summary for processing of Chromium Containing Steels: Sintering in dry N2-H2 atmospheres with a minimum of 5-10 vol% hydrogen is recommended to ensure chromium oxide reduction and to avoid oxidation of the chromium. Endothermic gas atmospheres should be avoided for both sintering and post sintering heat treatments. J

8 Dimensional change is minimally impacted by changes in sintering temperature but part size change can be modified by adjusting the sintering time at temperature. This should be taken into account when sizing a sintering furnace for parts using this type of alloy system. Sintering time and temperature control is critical to ensure good mechanical properties. For this alloy system it is recommended that a minimum sintering temperature of 1150 C be used along with an appropriate sintering time to allow the chromium containing master alloy to diffuse and create the desired microstructure. Mechanical properties can be improved by using convective cooling from sintering temperature to create a harder microstructure. This results in higher apparent hardness and strength with slightly lower total elongation and impact energy values. When accelerated cooling is used to increase strength and hardness, tempering is recommended to avoid brittle un-tempered martensite. Effect of Sintering Conditions on Magnesium Containing PM Steels Like chromium, manganese containing PM steels require special attention when sintering due to manganese s higher affinity for oxygen relative to more traditional alloying elements such as nickel or copper. The recently introduced ANCORBOND FLM-4000 and FLM-4400 manganese contacting steels offer very high performance in a completely nickel and copper free alloy system but also require very good control of sintering furnace atmosphere to ensure that manganese oxide will be reduced and not oxidized. Dry N2-H2 atmospheres should be used with dew points < -40 C with a minimum hydrogen content of 5-10 vol.%. Previous work has shown that endothermic atmospheres cannot be used for sintering or post sintering heat-treatments. While atmosphere heat-treatment and quenching is difficult with this alloy system it has been shown that this material system can be locally induction hardened because the time at the temperatures were oxidation takes place is very short.[4,5] Table III: Effect of sintering temperature and time rate on the properties of manganese containing PM steel (Fe - 1.3Mn + 0.5Mo C, 7.0 g/cm3, Cooled 0.7 C/s, Tempered at 205 C) Sintering Temperature C Dimensional Change % Transverse Rupture Strength MPa Apparent Hardness 5 min 15 min 5 min 15 min 5 min 15 min Table III summarizes a study on the effect of sintering time and temperature on the mechanical properties of manganese containing PM steels.[6] In this case a lower than normal sintering temperature (1065 C) and lower than normal sintering time (5 minutes) was used to determine the sinter-ability of the manganese containing master alloy. These data show that the substantial strengthening and hardening is taking place at these low sintering temperatures. The performance improves by increasing the sintering time at HRB

9 temperature to 15 minutes indicating that the manganese master readily diffuses into the steel matrix. This ease of sintering is an advantage when compared to the chromium containing and traditional nickel PM alloys. This manganese alloy system can be sintered at standard sintering temperatures and times with excellent performance. The dimensional change is also impacted by sintering time indicating that part dimension adjustments can be made by changing the sintering belt speed to counteract PM production process variations. A second study was completed to see if the performance can be further improved by using accelerated cooling and high temperature sintering.[7] This manganese alloy system with either 0.5 or 0.8 Mo will sinter-harden but requires accelerated cooling. Figure 8 shows the dimensional change as a function of cooling rate and sintering temperature. The dimensional change increases as the cooling rate increases at both the standard and higher sintering temperatures. The reason for the dimensional growth is the increased formation of martensite as the cooling rate is increased. This is explained by the volumetric growth observed within steels as the microstructure transforms from austenite to martensite. The magnitude of the dimensional change is also of note. At 1120 C the dimensional change is between %, very similar to the Fe-Cu-C PM materials. High temperature sintering at 1260 C results in about 0.10% less dimensional growth. Figure 8: Dimensional change from die of a manganese containing PM steel as a function of sintering temperature and cooling rate. The apparent hardness is presented in figure 9. These data show that the apparent hardness is not impacted by changing the sintering temperature from 1120 C to 1260 C. This indicates that the higher sintering temperature does not change the material s hardenability because the master alloy is completely diffused into the steel matrix when sintered at 1120 C. Apparent hardness does increase substantially when the cooling rate is increased. The sinter-hardening response of this alloy is important because post sintering heat treatments to harden the parts are limited due to the manganese s affinity for oxygen.

10 Figure 9: Apparent hardness of a manganese containing PM steel as a function of sintering temperature and cooling rate. Cooling rate increases also improve the mechanical properties of this alloy system. The ultimate tensile strength, total elongation, and impact energy are shown in figures 9, 10, and 11. The ultimate tensile strength increases as the cooling rate increases, conversely, the total elongation goes down. This is expected as the faster cooling results in more martensite formation within the microstructure that increases strength but lowers ductility. Previous work also shows that these properties can be optimized by varying the amount of carbon added to the material system.[6] Figure 10: Ultimate tensile strength of a manganese containing PM steel as a function of sintering temperature and cooling rate. High temperature sintering of this type of alloy system is also very beneficial as it creates a set of mechanical properties that rival traditional diffusion alloyed PM steels that contain much higher levels of copper, nickel, and molybdenum. Combining a moderate accelerated

11 cooling rate (1.6 C/s) with sintering at 1260 C produces ultimate tensile strength approaching 1100 MPa, total elongation just under 2% with apparent hardness around 30 HRC. High temperature sintering causes complete homogenation of the master alloy along with rounding of the porosity. These properties rival that of FD-0405 sintered at 1120 C indicating that much alloy cost can be saved if high temperature sintering is available. Figure 11: Total elongation of a manganese containing PM steel as a function of sintering temperature and cooling rate. Figure 12: Un-notched Charpy impact energy of a manganese containing PM steel as a function of sintering temperature and cooling rate. Impact energy is typically a property that requires the use of a high proportion of added carbonyl nickel to increase beyond J. When sintered at 1120 C the manganese containing PM steel behaves like most sinter-hardened PM steels with moderate to low impact energy values see figure 12. High temperature sintering this alloy improves the impact energy values above 22 J. This again is comparable to FD-0405 sintered at 1120 C. The impact energy goes down at the highest cooling rate due to the formation of a higher

12 proportion of martensite. However, this property can be increased by a slight reduction in the carbon content used.[4] Select microstructures of the test samples are shown in figure 13. The sample cooled at 0.7 C/s is only partially hardened with a hybrid microstructure consisting of a mixture of bainite and martensite. However, by accelerating the cooling rate to 1.6 C/s results in a microstructure that is mostly martensite with a small area fraction of bainite. This hardened structure explains the increase in strength and hardness observed. The high temperature sintered samples also have a microstructure that is mostly martensite but the porosity is smaller and more rounded C 0.7 C/s 1120 C 1.6 C/s 1260 C 1.6 C/s Figure 13: Microstructures of a manganese containing PM steel sintered for 15 minutes as a function of cooling rate. Samples were etched with 2%picral 4% nital. Summary for processing of manganese PM steels: Sintering in a N2-H2 atmosphere with a minimum of 5-10 vol.% hydrogen is required to ensure that the manganese does not oxidize. Sintering and post sintering heattreatment cannot be performed in endothermic gas because of oxide formation that reduces mechanical performance. Manganese steels can be locally hardened using induction hardening. This manganese containing alloy system can be easily sintered at 1120 C using standard sintering times as the master alloy diffuses quickly. High sintering temperatures are only needed to further improve mechanical properties. These steels exhibit dimensional growth after sintering. While the dimensional change is stable, the size change of a production part can be changed by making adjustments to the sintering temperature and the furnace belt speed to accommodate PM production process variations. This alloy system can be sinter-hardened and using accelerated cooling increases the apparent hardness and strength. A minimum accelerated cooling rate of 1.6 C/s is suggested to obtain a hybrid microstructure however faster cooling rates may be needed for parts with larger cross section sizes. A post sintering tempering cycle at 205 C should be used on sinter-hardened parts using this alloy system.

13 Using sintering temperatures >1230 C results in apparent hardness, ultimate tensile strength, total elongation and impact energy values that are comparable to FD-0405 that is sintered at 1120 C. This offers the possibility of reducing the need for using higher levels of copper and nickel. Conclusions Chromium, manganese, and silicon containing alloy powders are a viable alternative to copper, nickel, and molybdenum containing diffusion alloys and prealloyed PM steels provided that the alloy design, mix composition, compaction, sintering, and post-sintering heat-treatment cycles are designed as a complete system optimized to deal with the metallurgical requirements associated with alloying elements that have a high affinity for oxygen. References 1. King, P, Chromium Containing Materials for High Strength-High Fatigue Applications, PM World Congress 2004, EPMA Lindsley, B, Sintering of Chromium Containing PM Steels Processed to High Density, PowderMet 2008, MPIF King, P and Lindsley, B, Chromium Steels for High Performance PM Applications, PowderMet 2007, MPIF Lindsley, B and James, W, PM Steels that Contain Manganese, PowderMet 2010, MPIF Lindsley, B, et al, Mn Containing Steels for High Performance PM Applications, PowderMet 2011, MPIF Lindsley, B, et al, High Performance Manganese Containing PM Steels, PM World Congress 2008, MPIF Lindsley, B and James, W, PM Steels that Contain Manganese, PM World Congress 2010, EPMA 2010