Effect of nitrogen content on precipitation during isothermal annealing of P/M high nitrogen austenitic stainless steels

The aim of the study is to determine the effect of nitrogen content on precipitation reactions in nitrogen alloyed austenitic stainless steels during isothermal annealing up to 500 h at 450 degrees C and up to 72 h at 750 degrees C. The studied steels were PIM austenitic stainless steels with the nominal composition of 24Cr-22Ni-7.3Mo-3.5Mn-0,3...0,9N. The studied steels were manufactured by HIPing (Hot Isostatic Pressing) from gas atomized powders. Nitrogen content of the steels was varied by nitrogen alloying of the atomized melt and by further solid state nitridihg of the gas atomized powders in a fluidized bed furnace when nitrogen contents exceeding nitrogen solubility in the liquid state were needed. The effect of nitrogen content on precipitation was studied with TEM as well as by tensile testing, Charpy-V impact testing and localized corrosion testing (pitting and crevice corrosion). Precipitating phases in the studied steels were either intermetallic phases (sigma and Laves) and/or chromium nitrides (Cr2N) depending on the nitrogen content of the steel.