Effect of aging on microstructures and mechanical properties of high nitrogen austenitic stainless steel

The effect of aging on microstructures and mechanical properties of 1%N-23%Cr-4%Ni-2%Mo austenitic stainless steel aged at 600 similar to 800 degrees C for up to 1000 h were investigated with electron microscope, analysis of extracted residues, ferrite scope, and mechanical tests such as hardness, tensile, and Charpy impact. Chromium nitrides precipitated on grain boundary at the aging temperature below 650 degrees C, while lamellar precipitates, which were composed of plate like Cr2N and martensite, were formed above 700 degrees C. sigma phase precipitated at 700 degrees C for 1000 h and above 750 degrees C for longer than 100 h. Martensite was also observed around a phase, and the total amount of martensite was 40-50% after aging at 700-800 degrees C for 1000 h. Age hardening was observed above 700 degrees C and hardness, which increased with aging time and temperature, reached about Hv 500 after aging at 800 degrees C for 1000 h. Increase in tensile strength by aging was higher than that in proof strength, because of pronounced increase in work hardening rate by aging. Strengthening by aging depended mainly on the amount of lamellar precipitates and additionally on a phase and its surrounding martensite at higher temperature. Tensile ductility and impact toughness were decreased by aging due to lamellar precipitates and a phase.