Thermal decomposition of N-expanded austenite in 304L and 904L steels

In this study, expanded austenite was prepared by an industrial low-temperature plasma nitriding process in 304L and 904L austenitic stainless steels. The current investigation focuses on the assessment of the thermal stability and related phase evolution of the expanded austenite layer during isothermal annealing in protective argon atmosphere at temperatures ranging from 450 to 600 degrees C for 24 and 168 h. Characterisation of the original expanded austenite and the decomposed surface layers was performed. Denitriding, inward N-diffusion and Cr-compounds precipitation occurred at different extent, depending on annealing conditions and alloy composition. Expanded austenite in 304L exhibited a near complete eutectoid decomposition after a short annealing time, while 904L showed significantly better thermal stability. A fine dispersion of small CrN precipitates resulting from expanded austenite decomposition at relatively low annealing temperature or short duration could further positively affect the surface hardness of both materials. Precipitate growth reduces hardness at higher annealing temperatures/times.