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J.L. Johnson and D.F Heaney, “Metal Injection Molding of Co-28Cr-6Mo,” Medical Device Materials III, R. Venugopalan and M. Wu (eds.), ASM International, Materials Park, OH, 2006, p. 99-103.
Metal injection molding of gas- and water-atomized Co-28Cr-6Mo powders is evaluated. Sintering is conducted in different atmospheres to evaluate their effects on sintering response and carbon, nitrogen, and oxygen contents. The effects of hot isostatic pressing and heat treat on the mechanical properties are investigated. Properties are correlated to the interstitial content and microstructure. Optimized processing gives mechanical properties that exceed ASTM requirements for cast and wrought Co-28Cr-6Mo.
Both water-atomized and gas-atomized Co-28Cr-6Mo powders can be injection molded and sintered to a closed pore condition. Sintering in a nitrogen or a hydrogen/nitrogen atmosphere results in nitrogenization, which can compensate for low carbon levels and increase the strength. Static mechanical properties of as-sintered Co-28Cr-6Mo exceed ASTM F75 requirements for cast material. Mechanical properties exceeding ASTM F1537 requirements for wrought material can be achieved after HIP of the water-atomized powder, and after HIP and heat treat of the gas-atomized powder. Solution heat treating keeps the nitrogen from segregating to the grain boundaries, but it has a mixed effect on mechanical properties depending on the powder source. Impurities, such as manganese and silicon, in the water-atomized powder produce inclusions in the final microstructure, which may hinder fatigue properties.