The hydrogen embrittlement behavior of an ultra-high strength (1180 MPa grade) dual phase steel sheet composed of ferrite and tempered martensite, as compared with that of a single phase steel sheet composed of tempered martensite, has been investigated by a sustained tensile-loading test. No fracture of the dual phase steel occurs under the low hydrogen-charging current density of 5 A/m 2 except under high applied stress substantially larger than the yield stress. With the high current density of 50 A/m 2 , the time to fracture of the dual phase steel varies widely, but is almost the same as that of the single phase steel. The critical applied stress for fracture of the dual phase steel is higher than that of the single phase steel. Under the high applied stress, however, the time to fracture of the dual phase steel is shorter than that of the single phase steel, and a unique intergranular-like morphology is observed at the crack initiation area on the fracture surface. Upon plastic deformation before the sustained tensile-loading test under the high applied stress, the time to fracture of the dual phase steel increases and the initiation area on the fracture surface exhibits typical quasi-cleavage features. The results of the present study indicate that the hydrogen embrittlement of the dual phase steel displays some anomalous behavior.KEY WORDS: hydrogen embrittlement; delayed fracture; high strength steel; dual phase steel; martensite; ferrite.

The role of the dynamic interactions between hydrogen and the strain-induced martensite transformation in the hydrogen embrittlement of type 304 stainless steel has been investigated by fractographic observations after a modified hydrogen charging. The modified charging is cathodically conducted in 3.5% NaCl solution at 80°C under aerated conditions while preventing the dissolution of chlorine and oxygen gases evolving on the platinum counter electrode, thus increasing the amount of hydrogen thermally desorbed at low temperatures. Upon tensile testing at 160°C, plastic deformation of the austenite phase in the stainless steel occurs, but no strain-induced martensite transformation occurs. The fracture surface of the hydrogen-charged specimen exhibits the double-cup mode and consists of microscopic shallow dimples. Upon tensile testing at 25°C, the martensite transformation and plastic deformation both occur and are intricately related; a brittle area is observed on the outer part of the fracture surface that exhibits both transgranular and intergranular fracture. At −196°C, the martensite transformation increases, but the amount of plastic deformation decreases and the amount of intergranular fracture increases. It is found that, when the martensite transformation occurs before hydrogen charging, the amount of intergranular fracture decreases. Moreover, when charged hydrogen is trapped in defects in the austenite phase, the amount of intergranular fracture also decreases. The present study indicates that the dynamic interactions between hydrogen and the martensite transformation play an important role in the hydrogen embrittlement of type 304 stainless steel.

No abstract