Martensite transformation and surface cracking of hydrogen charged and outgassed type 304 stainless steel

The effects of cathodic hydrogen charging and outgassing on martensite transformation, surface cracking and anodic dissolution behavior of type 304 stainless steel have been investigated. Cathodic charging and aging results in the formation of a considerable amount of ε and α′ martensites. Optical observation combined with X-ray diffraction analysis shows that there are critical charging conditions for the martensite transformation. The minimum charging current density to induce the phase transformation is 0.2 mA cm−2. Surface cracks are observed on the specimens, which are hydrogen charged for a longer time than the critical charging time for the matensite transformation. The surface cracks mainly initiate during aging instead of during cathodic charging and propagate along martensite/austenite interfaces, grain boundaries and twin boundaries. The anodic dissolution tests in 0.5 M H2SO4+0.1 M HCl solution reveal that an obvious increase in the weight loss is due to the preferential dissolution of martensite phases and the weight loss decreases with the outgassing time.