Effect of Low-Temperature Carbonitriding on the Microstructure and Corrosion Resistance of Severely Deformed 304 Austenitic Stainless Steel

This study examined the effects of low-temperature carbonitriding on the microstructure and corrosion resistance of heavily pre-strained metastable 304 austenitic stainless steel. Electron backscatter diffraction (EBSD) and electrochemical measurements were employed. Plastic deformation produced abundant deformation-induced α′-martensite, dense dislocations and slip bands, increased kernel average misorientation (KAM), and a high fraction of low-angle grain boundaries (LAGBs). The α′-martensite enhanced electrochemical activity and formed micro-galvanic couples with untransformed austenite, accelerating local anodic dissolution. Moreover, defect-rich regions associated with dense dislocations and slip bands impaired corrosion resistance. Although the increased LAGB fraction was beneficial, it could not offset the detrimental effects of α′-martensite and high KAM values. Low-temperature carbonitriding formed an approximately 23 μm thick layer with an N-rich surface zone containing about 6 wt.% N. It also caused nearly complete reversion of α′-martensite to austenite, recovery of deformation structures, and a further increase in the low-angle grain boundary (LAGB) fraction. These changes eliminated micro-galvanic couples and high-energy defect sites, whereas supersaturated nitrogen further improved corrosion resistance. The treatment effectively restrained the initiation of localized corrosion. After electrochemical testing, initial and pre-strained specimens displayed pits of various sizes, whereas carbonitrided specimens showed shallow grooves along slip lines.