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∼800°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°C, while lamellar precipitates, which were composed of plate like Cr2N and martensite, were formed above 700°C. σ phase precipitated at 700°C for 1000 h and above 750°C for longer than 100 h. Martensite was also observed around σ phase, and the total amount of martensite was 40-50% after aging at 700-800°C for 1000 h. Age hardening was observed above 700°C and hardness, which increased with aging time and temperature, reached about Hv 500 after aging at 800°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 σ phase and its surrounding martensite at higher temperature. Tensile ductility and impact toughness were decreased by aging due to lamellar precipitates and σ phase.