Hydrogen embrittlement and corrosion behavior of low-temperature carburized austenitic stainless steel

Licentiate thesis, 2023

For metallic components used in hydrogen environments, hydrogen embrittlement and corrosion have always been important considering potential failure risks. Among many metallic materials, austenitic stainless steel has found broad application because of its excellent corrosion and hydrogen embrittlement resistance. Type AISI304 austenitic stainless steel is promising due to its low cost compared to high alloyed ones, such as 316 and 904 etc. However, 304 is a metastable austenite stainless steel, which is susceptible to strain-induced martensitic transformation. This may reduce corrosion and hydrogen embrittlement resistance in hydrogen-containing environments. Moreover, low hardness and poor fatigue properties also limit their application. It is therefore of great importance to improve hydrogen embrittlement and corrosion resistance of austenitic stainless steel 304.

In recent years, extensive studies have been done to improve the hydrogen embrittlement and corrosion resistance of austenitic stainless steels, such as composition design, processing technology and surface engineering. Low-temperature carburizing (LTC), a surface engineering approach, has great potential because of its economic benefit and sustainability. This treatment can introduce interstitial carbon into the surface region of the steel and form precipitate-free supersaturated solid solution, greatly improving the surface hardness and fatigue properties without compromising the corrosion resistance. It is of great interest to evaluate the feasibility of LTC on the alleviation of hydrogen embrittlement and corrosion for commercial austenitic stainless steel 304 after hydrogen uptake.

In the present study, industrial low-temperature carburizing was performed on commercial AISI304 stainless steel in two conditions (cold worked and solution annealed). Mechanical properties, corrosion behavior, and microstructure of the S phase after hydrogen uptake have been studied and linked. It was found that low-temperature carburizing introduced ~ 22 μm thick S-phase with ultra-high hardness (775 HV) and high surface carbon concentration (2.2 wt.% in solution condition). Hydrogen uptake caused reduced corrosion resistance and hydrogen embrittlement due to hydrogen-induced cracking and hydrogen-induced martensite. For cold-worked 304, hydrogen-induced cracking and martensitic transformation resulted in high susceptibility to hydrogen embrittlement. Solution-annealed 304 showed low hydrogen embrittlement susceptibility due to the austenitic phase with less defects. Low-temperature carburizing improved the hydrogen embrittlement resistance due to the carbon-stabilized austenite. However, the high carbon concentration on the surface of the solution annealed 304 with LTC treatment led to hydrogen-induced cracking and reduced ductility. Potentiodynamic polarization curves and corrosion morphology/chemical analysis revealed that low-temperature carburizing improved corrosion resistance due to high carbon content and stabilized austenite.