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Metal Injection Molding of Co-28Cr-6Mo
J.L. Johnson and L.K. Tan, “Processing of MIM Co-28Cr-6Mo,” Advances in Powder Metallurgy and Particulate Materials, C. Ruas and T.A Tomlin (eds.), MPIF, Princeton, NJ, 2005, pp. 4.13-4.21
A prealloyed Co-28Cr-6Mo powder is injection molded. Debinding and sintering are conducted in different atmospheres to evaluate their effects on the sintering response and carbon, nitrogen, and oxygen contents. Hardness, tensile strengths, and ductility are measured after hot isostatic pressing and solution heat treating. The results are correlated to the interstitial content and microstructure. Nitrogen in the sintering atmosphere increases the yield strength and hardness. The mechanical properties exceed the requirements for cast material and are comparable to those of wrought material.
Water-atomized Co-28Cr-6Mo can be injection molded and sintered to a closed pore condition in vacuum and in hydrogen, nitrogen, and hydrogen/nitrogen atmospheres. Carbon levels are typically 0.02 wt.% or less. Nitrogen in the sintering atmosphere results in nitrogenization, which compensates for low carbon levels and increases the yield strength and hardness. Solution heat treating is needed to keep the nitrogen from segregating to the grain boundaries. The static mechanical properties of HIPed and heat treated Co-28Cr-6Mo with 0.23-0.25 wt.% nitrogen exceed all ASTM F75 requirements for cast Co-28Cr-6Mo. Impurities, such as Mn and Si, in the powder can lead to inclusions in the final microstructure, which may hinder fatigue properties.