Role of Dislocations and Deformation-Induced Martensite on Salt Bath Nitriding Response of Austenitic Stainless Steel

The slow kinetics of low-temperature nitriding of stainless steels has motivated the researchers to apply severe plastic deformation techniques to enhance the nitriding kinetics. However, during deformation of austenitic stainless steels, in addition to the production of defects, such as dislocations, deformation-induced martensite (DIM) also develops. The purpose of this study was to investigate the distinct roles of deformation-induced defects and martensite on the nitriding kinetics of AISI 316 and AISI 301 austenitic stainless steels. Cryo-rolling and Ultrasonic Shot Peening were employed for deformation, and low-temperature salt bath nitriding was employed. The results indicated that it is the martensite, which develops during the deformation step, responsible for enhanced kinetics, but not the dislocations. Thermodynamic reasoning for the effect of martensite on enhanced nitriding kinetics has been proposed, and a discussion on varying relative stabilities of austenite and martensite with the N-dissolution on the nitriding kinetics is also presented.