ECONOMICS AND BENEFITS OF HIGH TEMPERATURE SINTERING OF HIGH PERFORMANCE ALLOYS - PDF

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ECONOMICS AND BENEFITS OF HIGH TEMPERATURE SINTERING OF HIGH PERFORMANCE ALLOYS John W. Schaberl Hawk Precision Components Allegheny Clearfield Division, Falls Creek, PA Michael C. Baran and Michael L.
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ECONOMICS AND BENEFITS OF HIGH TEMPERATURE SINTERING OF HIGH PERFORMANCE ALLOYS John W. Schaberl Hawk Precision Components Allegheny Clearfield Division, Falls Creek, PA Michael C. Baran and Michael L. Marucci Hoeganaes Corporation, Cinnaminson, NJ Ronald A. Posteraro Gasbarre Products, Inc. - Sinterite Furnace Division, St. Marys, PA Presented at PM 2 TEC 2002 International Conference on Powder Metallurgy & Particulate Materials JUNE 16 21, 2002 ORLANDO, FLORIDA USA ABSTRACT Today s global economy has produced an extremely competitive marketplace. Design engineers constantly compare the economics and benefits of powder metallurgy (P/M) with those of stamping, casting and wrought machining. While parts manufacturers have traditionally exploited the near net shape cost savings of conventional P/M compositions, many have turned to higher performance alloys and elevated temperature sintering in an effort to optimize the metallurgical and mechanical properties of their products. Powder producers and furnace manufacturers have continued to support these endeavors through new product and process development. This study will examine the metallurgical and mechanical enhancements achieved through the combination of high performance alloy systems and high temperature sintering. In addition, it will show how these benefits can be exploited to produce superior parts economically. High performance silicon-containing P/M steels sintered at 1150 to 1343 o C (2100 to 2450 o F) will be used to explore these property enhancements. These silicon-containing P/M steels yield excellent properties when sintered above 1260 C (2300 F) and are capable of attaining high apparent hardnesses. INTRODUCTION High temperature sintering (HTS) is certainly not a new concept. The idea of sintering at temperatures above 1150 C (2100 F) has been common in industry for some time. For instance, the typical metal injection molding (MIM) process employs sintering temperatures of greater than 1250 C (2280 F) for ferrous materials [1]. By properly implementing these higher sintering temperatures, the P/M parts manufacturer can realize measurable benefits over their conventional process [2]. However, there are several obstacles to the realization of these process improvements. The advantages of this technology as well as key parts manufacturer concerns are shown in Table I. 1 TABLE I: Advantages and Concerns Associated with Choosing High Temperature Sintering over Conventional Sintering Temperatures for the Production of P/M Parts ADVANTAGES KEY CONCERNS Increased homogeneity Elimination of some reducible oxides Increased diffusion rates Ability to properly process new high performance materials / stainless More pore rounding Larger mean pore spacing CAN TRANSLATE TO INCREASE IN: Equipment / consumable expenditures Higher energy requirements Furnace maintenance costs Lower throughput* Part distortion / concentricity issues Slower cooling rates Other processing issues Density Mechanical properties Axial / rotating bending fatigue strength Rolling contact fatigue performance Corrosion resistance Physical properties HIGHER PERFORMANCE! *depends heavily on type of furnace chosen It is illogical to expect a parts manufacturer to turn up the sintering temperature without incurring some additional costs. Thus, prospective applications are carefully screened for processing at higher sintering temperatures. Invariably, those selected for HTS fall into one or more of the following categories (with examples of each): Material being used requires HTS New silicon-containing materials Higher performance stainless steel HTS is most efficient / only way to achieve required properties Meet specifications for an previously impossible application by adding HTS to a double press / double sinter operation that also employs a heat treating step HTS reduces processing steps / other equipment requirements Change from double press to single press operation More efficient use of prealloyed / admixed alloying additions Reduce some oxides so that elements can fully contribute to hardenability and increasing mechanical properties More homogenization of admixed additions such as nickel, ferro-manganese, ferrochromium, ferro-vanadium, ferro-boron, and copper 2 Nearly all P/M parts manufacturers currently have the ability to produce simple, low tech, and/or low density parts. The competition for these parts is enormous, especially from overseas markets. As higher densities and more complex part geometries become the norm, several key technologies will help the P/M parts manufacturer thrive. Two of these are high temperature sintering and lubricant/binder technology. In fact, these technologies are intimately connected. Segregation within a premix has been identified as a major cause of variation in the HTS of P/M parts [2]. Bonding can reduce or eliminate such segregation and allow for more consistent sintering behavior [3,4]. Additionally, warm compaction and some room temperature binder/lubricant systems have been shown to reduce green density variations throughout a part, resulting in a more consistent density distribution that minimizes part distortion and concentricity issues [2,5]. BACKGROUND The use of silicon in P/M materials can be highly beneficial. In addition to its relatively low cost and tendency to increase hardenability, it can accelerate sintering in hybrid alloys and substantially impact mechanical properties [6]. However, despite the inherent metallurgical advantages of silicon, its use in P/M has been largely unsuccessful in the past. The difficulties associated with conventional methods of silicon addition are numerous. For example, the addition of ferro-silicon almost invariably leads to a high degree of shrinkage and distortion in a P/M compact. Oxidation of silicon during sintering frequently results in an undesirable decline in mechanical properties. Similar problems are experienced in prealloyed powders. Silicon oxide pickup during atomization and/or annealing leads to lower compressibility and diminished mechanical performance. Through advancements in atomizing, annealing, and binder-treatment technology, new siliconbearing materials have been developed to provide exceptional combinations of properties without the use of copper. Additionally, production experience has indicated that these materials exhibit higher mechanical properties than diffusion-alloyed materials without the production costs associated with diffusion alloying. EXPERIMENTAL PROCEDURE Two press ready, binder-treated materials, Ancorloy MDB and Ancorloy MDC (hereafter referred to as MDB and MDC), were tested in a production environment in order to evaluate mechanical properties and economics achieved through a single press / single sinter process. The chemical compositions for these materials are listed in Table II. TABLE II: Chemical Compositions, Apparent Density, and Hall Flow for Material Grades Tested Grade AD (g/cm 3 ) Flow (s/50g) Fe Si Cr Mn Ni Mo Gr Ancorloy MDB Bal Ancorloy MDC Bal All test pieces were sintered at Hawk Precision Components Falls Creek Plant in a 12 C.I. Hayes / Sinterite pusher furnace. The sintering / tempering cycles used for specimens are 3 shown below. Additionally, the actual zone temperatures setting, gas flow rates, and measured dew points are listed in Table III. Sintering Temperature: Various (from C or F) Atmosphere: Various (from 100 v/o H 2 to 85 v/o N 2 / 15 v/o H 2 ) Time in Hot Zone: 33 minutes Cooling: Standard Water-Jacketed Section Tempering: 205 C (400 F) for 1 hour in Air TABLE III: Pusher Furnace Temperature Settings, Gas Flow Rates, and Measured Dew Points Test # Atmosphere 100H2 25N2/75H2 85N2/15H2 85N2/15H2 25N2/75H2 100H2 100H2 25N2/75H2 85N2/15H2 Test Temp Speed (IPM) PreHeat N PreHeat H Shell N Shell H High Heat N High Heat H Coolers N Coolers H Curtain N Zone 1 Temp Zone 2 Temp Zone 3 Temp Zone 4 Temp Zone 5 Temp Zone 6 Temp Zone 5 Dew Point N/A N/A Apparent hardness measurements were performed on the surface of the specimens using a Rockwell hardness tester. All measurements were conducted on the Rockwell C scale (HRC). Transverse rupture strength was measured according to ASTM B 528. Impact energies were determined according to ASTM E 23. Tensile testing was performed on a 267 kn Tinius Olsen universal testing machine at a crosshead speed of millimeters/minute. Elongation values were determined by utilizing an extensometer with a range of 0 to 20%. The extensometer was left on until failure. All specimens were tested in the as-tempered condition. Dimensional change was determined per ASTM B 610. Tensile tests were conducted using standard dogbone tensile specimens. RESULTS AND DISCUSSIONS General Points of Interest The green density of specimens was relatively consistent at g/cm 3 for compaction at 550 MPa (40 tsi) and 7.00 g/cm 3 at 620 MPa (45 tsi). Hence, green density is not included in the presented data. Some incipient melting was noted on specimens sintered at 2450 F. For this reason, this data was used sparingly and should be viewed as suspect. Tensile Properties It is often the combination of ultimate strength, yield strength, ductility, impact energy, and hardness that differentiate good materials from workhorses in the P/M industry. However, the 4 strength and ductility of a material are often the initial considerations during the design process. As can be seen in Tables IV and V, both materials show promise in these categories. TABLE IV: Tensile Properties and Sintered Densities for Dog-bone Tensile Specimens Sintered at 1150 C (2100 F) in Different Sintering Atmospheres Comp Sintered UTS Yield Atmosphere Elongation Material Pressure Density (MPa / (MPa / (v/o) (MPa / tsi) (g/cm 3 ) 10 3 psi) 10 3 (%) psi) 550 / / / / / / MDB 550 / / / / / / / / / / / / / / / / / / MDC 550 / / / / / / / / / / / / TABLE V: Tensile Properties and Sintered Densities for Dog-bone Tensile Specimens Sintered at 1260 C (2300 F) in Different Sintering Atmospheres Comp Sintered UTS Yield Atmosphere Elongation Material Pressure Density (MPa / (MPa / (v/o) (MPa / tsi) (g/cm 3 ) 10 3 psi) 10 3 (%) psi) 550 / / / / / / MDB 550 / / / / / / / / / / / / / / / / / / MDC 550 / / / / / / / / / / / / As evidenced by the combination of strength and ductility at both 1150 C (2100 F) and 1260 C (2300 F), the silicon did not impart its maximum benefit at the standard sintering temperature. In fact, previous unpublished work suggested that a minimum sintering temperature of 1230 C (2250 F) was necessary to facilitate proper sintering of these siliconcontaining materials. Additionally, it should be noted that although the more alloyed MDC sintered at 1150 C (2100 F) was capable of meeting / exceeding the strength of less alloyed MDB sintered at 1260 C (2300 F), the lower sintering temperature could not provide the same ductility. The hydrogen / nitrogen composition of the sintering atmosphere was found to have a 5 marked effect on densification at the higher sintering temperature. As hydrogen content increased, sintered density was generally seen to increase. This trend was thought to arise from enhanced reduction with higher hydrogen gas mixtures. Recall that all specimens (at like compaction pressures) started at the same green density. TABLE VI: Apparent Hardnesses and Impact Energies for Specimens Sintered at 1150 C (2100 F) in Different Sintering Atmospheres Comp Apparent Impact Atmosphere Material Pressure Hardness Energy (v/o) (MPa / tsi) (HRC) (J / ft.lbf) 550 / / / / 11 MDB 550 / / / / / / / / / / / / 16 MDC 550 / / / / / / / / 16 TABLE VII: Apparent Hardnesses and Impact Energies for Specimens Sintered at 1260 C (2300 F) in Different Sintering Atmospheres Comp Apparent Impact Atmosphere Material Pressure Hardness Energy (v/o) (MPa / tsi) (HRC) (J / ft.lbf) 550 / / / / 16 MDB 550 / / / / / / / / / / / / 27 MDC 550 / / / / / / / / 26 Apparent Hardness / Impact Energy The impact energies and apparent hardness values for test specimens are listed in Table VI and VII. Again, it can be seen that the higher sintering temperature was imperative for achieving the unique combination of hardness and impact properties. Material MDC was most impressive with impact energies as high as 37 J (27 ft.lbf) coupled with sinter-hardening capabilities. It should be noted that impact energies in excess of 27 J (20 ft.lbf) are all but unheard of in the P/M industry (and can even be difficult to obtain in wrought steels) at the 6 hardness levels listed. The conventional relationship between impact energy and apparent hardness in common P/M grades is illustrated in Figure 1 [7]. Apparent Hardness (HRC) FN-0208HT FN-0405HT FLC-4608 MDC 2300 F MDC (2100 F) MDB MDB (2300 F) MDB (2100 F) FN-0208 FD-0205 FL-4605 FN Impact Energy (ft.lbf) FIGURE 1: The Relationship between Impact Energies and Apparent Hardness Values for Some Common P/M Grades (as listed in MPIF Standard 35) Compared to MDB and MDC Sintered at 1260 C (2300 F) Metallography In the interest of brevity, the only photomicrographs shown in this effort are those used to illustrate the microstructural constituents of each material and/or any microstructural differences between sintering temperatures. Firstly, the effect of sintering temperature on final microstructure is evident in Figures 2 and 3 for MDB and Figure 4 and 5 for MDC. At the higher sintering temperature, the nickel distribution and overall microstructure in the field of view were more homogenous. This improved nickel uniformity caused an increase in hardenability, apparent hardness, and strength. Furthermore, densification and pore rounding assisted in increasing apparent hardness, strength, and ductility. 7 2% Nital / 4% FIGURE 2: Photomicrograph of MDB Sintered at 1150 C (2100 F) in 25 v/o N 2 75 v/o H 2 Containing Pearlitic, Bainitic, Nickel Rich, and Martensitic Regions (Originally 200X) 2% Nital / 4% Picral FIGURE 3: Photomicrograph of MDB Sintered at 1260 C (2300 F) in 25 v/o N 2 75 v/o H 2 Containing Bainitic, Pearlitic, and Martensitic Regions (Originally 200X) 8 2% Nital / 4% Picral FIGURE 4: Photomicrograph of MDC Sintered at 1150 C (2100 F) in 25 v/o N 2 75 v/o H 2 Containing Martensitic, Bainitic, Nickel Rich, and some Pearlitic Regions (Originally 200X) 2% Nital / 4% Picral FIGURE 5: Photomicrograph of MDC Sintered at 1260 C (2300 F) in 25 v/o N 2 75 v/o H 2 Containing Martensitic and Bainitic Regions (Originally 200X) 9 Economics High temperature sintering was required to properly process both materials utilized in this study. At lower sintering temperatures, these materials offer good mechanical and physical properties, but do not exhibit their full potential. By fully exploiting these materials, high apparent hardnesses coupled with exceptional impact energies and tensile properties were possible. These properties came at a price. This price was largely due to the increased costs associated with the high temperature sintering process. At the time of this effort, very little work existed to characterize the relative cost of high temperature sintering versus conventional continuous belt sintering. Cost categories analyzed for both a 24 continuous belt furnace and a 12 high temperature pusher were capital, utilities (electricity and atmosphere), and labor. Based upon conditions observed during operation, the following items were concluded (Appendix A lists the assumed costs and method of calculation of the economic data.): Labor was deemed the same for loading / unloading of each furnace Atmosphere for pusher was ~20% less costly due to difference in gas flows Electricity for pusher was ~35% less costly due to differences in size and heat retention The pusher furnace was approximately two and a quarter times the cost of the continuous belt, depending on size / options / special requests for each furnace When all of the above items were considered, the use of a high temperature pusher furnace cost 10-15% more per pound than a continuous belt furnace when material throughput was identical. As furnace loading and belt / pusher speeds vary throughout the industry, the cost differential can fluctuate likewise. P/M parts manufacturers are urged to discuss their specific needs with a sintering furnace manufacturer. Pusher furnace processing generally improves mechanical properties. Therefore, parts produced in a pusher furnace should command a higher price in the marketplace. Thus, part manufacturers must selectively process materials in the pusher to maximize profitability and provide the highest return on the investment. Additional Considerations When this work and its data are reviewed, several important items must be considered. Firstly, as is the case in most high temperature furnaces, the cooling rate seen by the materials was slow. At faster cooling rates, one would expect strength / apparent hardness to increase slightly and impact energy / ductility to decrease slightly. Secondly, despite incipient melting of specimens at 1343 C (2450 F) and the omission of said data, some promise was seen in sintering at temperatures higher than 1260 C (2300 F). Finally, dimensional change was largely overlooked in this effort, but previous work suggested that the dimensional consistency of these materials was superior to a standard FLN composition [9]. The reader is referred to this previous work for detailed dimensional change information. 10 CONCLUSIONS High temperature sintering of two silicon-containing materials, Ancorloy MDB and Ancorloy MDC, was explored as an avenue to higher P/M performance. While sintering at 1150 C (2100 F) was capable of achieving good mechanical and physical properties, raising the temperature to 1260 C (2300 F) yielded exceptional combinations of strength, ductility, apparent hardness, and impact energies. In fact, Ancorloy MDC exhibited impact energies superior to any other P/M grade at apparent hardness values of HRC. Since high temperature sintering incurred additional costs, a cursory cost analysis was also presented. ACKNOWLEDGEMENTS The authors would like to thank Gerald Golin of Hoeganaes for his metallographic work and George Fillari and David Southwick of Hoeganaes for their assistance in specimen testing. In addition, John Lougee, Jason Forster, and Jodie Smith of Hawk for their help in running the sintering runs and obtaining furnace cost data. REFERENCES 1. German, Randall M., Powder Injection Molding, in ASM Handbook, Vol. 7, ASM, Metals Park, Ohio, 1998, pp Sanderow, Howard I., High Temperature Sintering in Ferrous Powder Metallurgy Components, in ASM Handbook, Vol. 7, ASM, Metals Park, Ohio, 1998, pp Luk, S.H. and Hamill, J.A., Dust and Segregation-Free Powders For Flexible P/M Processing, Advances in Powder Metallurgy & Particulate Materials , Vol. 1, pp 153, Metal Powder Industries Federation, Princeton, NJ. 4. Semel, F.J., Luk, S.H., Continuing Improvements in Binder Treatment Technology, Advances in Powder Metallurgy & Particulate Materials , Vol. 4, pp 353, Metal Powder Industries Federation, Princeton, NJ. 5. Hanejko, Francis G., Warm Compaction, in ASM Handbook, Vol. 7, ASM, Metals Park, Ohio, 1998, pp Salak, A., Ferrous Powder Metallurgy, Cambridge International Science Publishing, Cambridge, England, 1995, p MPIF Standard 35 Materials Standards for P/M Structural Parts, Metal Powder Industries Federation, Princeton, NJ, 1997, p Shah, Suresh O., Baran, M.C., Patel, S.N., and Causton, R.J., High Performance Materials Ancorloy MD Series, Advances in Powder Metallurgy & Particulate Materials 2001, Vol.1, p.227, Metal Powder Industries Federation, Princeton, NJ. 11 Appendix A: Sintering Furnace Economic Analysis Cost 24 Belt Furnace 12 Pusher Furnace CAPITAL COST Furnace Cost $200,000 $450,000 Life 7 7 Utilization 85% 85% Shifts/Day 3 3 Days/Week 5 5 Weeks/Year Hours/Year 5,100 5,100 Hours/Life 35,700 35,700 Depr Cost/Hour $5.60 $12.61 UTILITIES Electricity $/kw-hr $ $ Full Load kw Electric Cost/Hour $8.68 $5.68 At
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