Effect of bainite fraction on hydrogen embrittlement of bainite/martensite steel

Abstract

The paper quantifies how the fraction of bainite in dual-phase (bainite/martensite) steel affects its resistance to hydrogen embrittlement (HE). For this purpose, three steels that had different amounts (0, 0.9, and 1.8 wt%) of nickel were used. Heat treatment was conducted to vary the volume fraction of bainite while maintaining same strength level. Hydrogen charging was achieved using an electrochemical method, and the notch fracture strength was measured using a slow strain-rate test. The steel that had 1.8% Ni had the highest bainite fraction and showed excellent resistance to HE. This resistance was attributed to hydrogen trapping at irreversible trapping sites, which were regarded to be the cementite/ferrite interfaces with interfacial dislocations. They were more pronounced in bainite than in martensite because of higher cementite fraction in bainite than in martensite. To increase HE resistance of high-strength steels, the number of fine cementite particles in the bainite should be increased to provide irreversible trapping sites for migrating hydrogen during plastic deformation.