Influence of sintering atmosphere and temperature on the properties of a superaustenitic stainless steel

A superaustenitic stainless steel containing 20 w/o Cr, 19 w/o Ni, and 6.3 w/o Mo (balance iron) was sintered in a vacuum furnace varying the sintering atmosphere and temperature. Sintering was carried out with and without nitrogen backfilling at temperatures in the range 1,250°C-1,350°C. The two conditions were compared in order to assess the effect of microstructural changes induced by nitrogen pickup on density, microstructure, mechanical properties, and corrosion resistance. Sintering in nitrogen suppresses the formation of sigma phase, whereas sintering in vacuum leads to an enhanced densification due to the formation of delta ferrite at high temperature. A post-sinter solution anneal was required to eliminate the sigma phase in specimens sintered in vacuum. Mechanical properties are significantly affected by nitrogen pickup in specimens with a low level of porosity and hence a high fraction of the cross-sectional area carrying the load. Ductility is affected negatively by sigma phase, and a solution anneal is needed to recover tensile elongation. Corrosion behavior in sulphuric acid depends primarily on sintered density and porosity, and on sigma phase in NaCl solution.