Sintering densification and microstructural evolution of injection molding grade 17-4 PH stainless steel powder
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THE alloy 17-4 PH is a precipitation-hardenable martensitic stainless steel. Due to its high strength and good corrosion resistance, 17-4 PH has widespread applications, especially in medical instruments. Many 17-4 PH components can be manufactured cost-effectively by powder-injection molding, a net-shape forming process with an advantage of shape complexity, material utilization, and high final density.[1,2] Previous investigations on powder-injection-molded (PIM) 17-4 PH[3–12] have focused on the effects of different debinding, sintering, and heat-treating cycles on the final density, microstructure, and mechanical properties. It was found that PIM 17-4 PH stainless steel could be sintered to near-full density only in pure hydrogen or vacuum.[3,4,6] In addition, the residual carbon content had a strong influence on the sintered density, corrosion resistance, and microstructure.[6,8] Among different researchers, the optimum sintering temperature varied largely in the range from 1250 ⬚C[12] to 1390 ⬚C,[5] depending on composition and sample size. Although it is still difficult to derive an optimal sintering cycle for a given PIM 17-4 PH component on the basis of previous experimental results, there is evidence that sintering densification is closely related to microstructure. Zhang and German[3] found that sintering in 30 pct nitrogen and 70 pct hydrogen gave incomplete densification, with a much lower final density compared to sintering in pure hydrogen. A highly densified PIM 17-4 PH stainless steel is always associated with the presence of uniformly distributed ␦ ferrite in a martensite-based microstructure.[3,4,7–9,12] However, few efforts have been devoted to understanding the densification behavior and shrinkage mechanism as related to in-situ microstructural evolution during sintering. Moreover, there exists an increasing demand for accurate prediction of the sintering shrinkage by computer simulation, which also requires a detailed description of the densification behavior and its relation to microstructure.[13,14] YUNXIN WU, Research Associate, DEBBY BLAINE and CONNIE SCHLAEFER, Graduate Assistants, BRIAN MARX, Undergraduate Assistant, and RANDALL M. GERMAN, Brush Chair Professor in Materials, are with the P/M Lab., Pennsylvania State University, University Park, PA 16802-6809. Contact e-mail: [email protected] Manuscript submitted October 18, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
This work aims to establish the relation between the sintering densification and microstructural evolution of PIM 17-4 PH. As part of the basic examination of sintering kinetics, some die-compacted (DC) samples with no binder were used to separate debinding and polymer contamination from the basic sintering behavior. In addition, a partial nitrogen atmosphere was introduced into hydrogen as an austenitic stabilizer, so as to isolate the effect of the ␥ → ␦ phase transformation. Dilatometry was used to investigate the shrinkage behavior at various stages of sintering, while a vertical quench furnace was employed to captu
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