Structural characteristics of low temperature plasma carburised austenitic stainless steel

A low temperature plasma carburising process has recently been developed to engineer the surfaces of austenitic stainless steels lo achieve combined improvements in wear and corrosion resistance. The present paper discusses the structural characteristics of the carburised layers produced on AISI type 316 steel at temperatures between 400 and 600 degrees C. It was found that at low temperatures (<520 degrees C), the carburised layers produced were precipitation free and comprised a single phase, which had a face centred cubic structure and was identified as expanded austenite owing to the supersaturation of carbon in austenite. The carburised layer was in a deformed and distorted state. High densities of twins, stacking faults, and dislocations were found in the expanded austenite. The degree of lattice expansion was estimated and was found to vary with processing temperature and depth in the layer. Precipitation of carbides (mainly Cr7C3) occurred when the carburising temperature was relatively high (for example 550 and 600 degrees C). In addition, stress induced martensite was found, particularly in the carburised layers produced at relatively high temperatures.