Microstructure evolution induced by cathodic charging in low temperature carburized 304 austenitic stainless steel

This work investigated the effect of hydrogen uptake on the microstructure of low-temperature carburized 304 austenitic stainless steel. Low-temperature carburizing (LTC) treatment introduced a ∼22 μm expanded austenite layer with high carbon content and high hardness on the steel surface. Hydrogen was introduced into the steel by cathodic electrochemical charging in an acidic environment. For untreated 304, it was found that hydrogen uptake led to martensitic transformation and orientation-dependent strain. LTC treatment stabilized austenite and suppressed hydrogen-induced martensitic transformation. Surface cracking evolved due to hydrogen uptake. The underlying factors contributing to cracking were discussed. This study revealed that austenite stability and internal stress associated with carbon concentration in expanded austenite were two competing factors affecting cracking resistance. For cold worked 304, LTC treatment significantly reduces hydrogen-induced cracking because the high stability of austenite outweighed the adverse effect of internal stress. However, for solution annealed 304, the high surface carbon concentration introduced by LTC treatment was more dominant, thereby enhancing surface cracking. • Quasi-in-situ EBSD-SEM was applied to study the effect of hydrogen uptake on the microstructure. • Hydrogen-induced martensitic transformation and strain were orientation-dependent. • Hydrogen-induced cracks occurred at slip bands, grain boundaries and martensite. • Low-temperature carburizing (LTC) stabilized austenite and suppressed martensitic transformation. • LTC resisted hydrogen-induced cracking by stabilizing austenite, but excess carbon increased stress cracking.