Microstructural characterization and layer stability of low-temperature carburized AISI 304L and AISI 904L austenitic stainless steel

Carbon stabilized expanded austenite (S-phase) is prepared by commercial low-temperature carburizing process (LTC) on austenitic stainless steels AISI 304L and AISI 904L. The current paper deals with the material response to LTC and thermal stability of the formed metastable structure. The aim is to investigate the influence of the alloy composition and surface finishing as well as thermal annealing on the microstructure, phase constituents and hardness of the modified layer by means of combined analysis techniques. It has been found that the formation of expanded austenite is accompanied by Hagg carbides on as-carburized 304L. However the highly alloyed 904L exhibits mainly S-phase with larger degree of lattice expansion after carburizing. S-phase has proven to be more stable in 9041, whereas residual and/or induced ferrite/martensite in 3041 makes the formation of S-phase less favourable. LTC induces significant enhancement of surface hardness, more effectively on 904L. The hardening mechanism is discussed. In order to evaluate the phase evolution and stability of the expanded austenite at elevated temperatures, annealing has been performed in vacuum at temperature of 600 degrees C for 150 hours. The decomposition of S-phase and the related microstructure evolution give rise to reduced hardening effect and declined corrosion resistance in both S-phase layer and the region below.