Alleviation of Hydrogen Embrittlement to Delay Effective Hydrogen Diffusion Against MA Constituent Formation Due to Mo Solutes for 420 MPa Grade Offshore Steels

Abstract

Hydrogen embrittlement (HE) is a significant issue in high-strength offshore steel. Mo is added for high strength, and several studies on the HE resistance by Mo solutes and Mo segregation have been reported. However, the effect of Mo addition on HE in bainitic steels has not been studied yet. This study investigated the effect of increasing Mo content (up to 0.2 wt%) on HE resistance in terms of crack initiation and hydrogen diffusion behaviors in bainitic steels. With the addition of Mo, the strength of 0.2 Mo steel increased, but it’s HE resistance was nearly identical to that of 0.02 Mo steel. As the Mo content increased, the martensite-austenite (MA) constituents sensitive to HE increased; however, the Mo solutes, which serve as trap sites for hydrogen, also increased. The delay in hydrogen diffusion by the Mo solutes played a role in alleviating the degradation in the HE resistance of the MA constituents of 0.2 Mo steel.