Effects of residual stress and plastic strain on hydrogen embrittlement of a stretch-formed TRIP-aided martensitic steel sheet

Hojo, Tomohiko*; Akiyama, Eiji*; Saito, Hiroyuki*; Shiro, Ayumi*; Yasuda, Ryo*; Shobu, Takahisa  ; Kinugasa, Junichiro*; Yuse, Fumio*

Hydrogen assisted cracking on hemispherically-stretch-formed specimens of transformation induced plasticity-aided martensitic steel was investigated. Hydrogen charging induced cracking around the foot of the impression formed on the steel sheet, and the cracks propagated along the radial direction toward the hillside and the plains. Distributions of stress, plastic strain and volume fraction of retained austenite were analyzed employing the energy-dispersive X-ray diffraction method utilizing the synchrotron X-ray radiation at SPring-8. It was notable that the crack initiation took place in the region where the measured tensile stress was the highest. Influences of plastic strain and resulted martensitic transformation were also suggested.