Surface Hardening of Powder Injection Molded 316L Stainless Steels Through Low-Temperature Carburization
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THE powder injection molding (PIM) process is widely used in fabricating structural parts with complicated shapes. Among the materials used, 316L stainless steel is one of the most widely employed. With a high sintered density, above 7.60 g/cm3 or 96.0 pct of theoretic density, and a low carbon content of less than 0.03 wt pct, excellent corrosion resistance can be obtained for the as-sintered 316L. However, the strength and hardness of PIM 316L are relatively low due to the austenitic structure and the coarsened grains induced during high-temperature sintering. According to the Metal Powder Industries Federation (MPIF) standards, the typical properties of sintered PIM 316L are 7.60 g/cm3 density, 520 MPa tensile strength, 50 pct elongation, and 67 HRB hardness.[1] With this low hardness, PIM 316L stainless steels are prone to damage such as scratches, wear, and deformation. Several processes, such as work hardening, have been used to increase the strength and hardness of wrought 316L. But, this hardening method cannot be applied to most PIM 316L parts because of the complicated shapes. Hardening by quenching and martensitic transformation is not possible either because there is no phase LI-HUI CHENG, formerly Graduate Student with the Department of Materials Science and Engineering, National Taiwan University, 1 Roosevelt Road, Sec. 4, Taipei, 106, Taiwan, ROC, is now Principal Engineer with the TSV Packaging Module Department, Integrated Interconnect & Packaging Division, Taiwan Semiconductor Manufacturing Company, Ltd, 6, Creation Rd. 2, Hsinchu Science Park, Hsinchu, Taiwan, ROC. KUEN-SHYANG HWANG, Adjunct Professor, is with the Department of Materials Science and Engineering, National Taiwan University. Contact e-mail: [email protected] Manuscript submitted December 11, 2011. Article published online October 9, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
transformation during cooling of the austenitic 316L. As a result, the development of the hardening process for 316L has been focused on surface treatments, including sand blasting, physical vapor deposition, shot peening, or nitriding.[2–4] Rhouma et al. reported that the surface hardness of 316L can be increased to 340 HV after sand blasting.[2] However, for PIM 316L with complicated shapes, it would be difficult to insure the uniformity of the sand blasting. Carbide or nitride surface coatings can also increase the surface hardness of stainless steels, but the bonding between the hard layers and 316L matrix can be problematic, shortening the service life of 316L parts. It has also been reported that the surface hardness of PIM 316L can be increased to 450 HV after sintering in dissociated ammonia and hot-isostatic pressing in N2. However, the corrosion resistance decreases due to Cr2N precipitation.[5] One plausible approach to increasing the surface hardness and maintaining the corrosion resistance at the same time is to carburize 316L at low temperatures.[6–10] Michal et al. have demonstrated that substitutional solutes, such as Cr and Ni, are
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