Role of Interface between Ferrite and Martensite in Hydrogen Embrittlement Behavior of Ultra-high Strength Dual-phase Steel Sheets

The effects of interfacial conditions between the ferrite and martensite phases of an ultra-high strength dual phase (DP) steel sheet on its hydrogen embrittlement behavior has been investigated by a sustained tensile-loading test using as-quenched DP steel and tempered DP steel specimens. The yield ratio (yield stress/tensile strength) of the as-quenched DP steel is lower than that of the tempered DP steel. In the hydrogen thermal desorption analysis, the second desorption peak disappeared and the amount of absorbed hydrogen is decreased by tempering. Under the same applied stress in the sustained tensile loading test, the time to fracture shows no significant difference between the two steels, but the critical applied stress for fracture is increased by tempering. A quasi-cleavage fracture occurs at the fracture initiation site of both steels. On the cross section near the fracture surface, many cracks nucleate in blocks or packets in martensite and the interface between prior austenite grains, but no cracks is observed in ferrite grains. Under applied stress higher than the yield stress of the as-quenched DP steel, fracture occurs in a short time. A unique intergranular-like morphology is observed at the fracture initiation area, and the crack propagates in blocks or packets in martensite or along the interface between the ferrite and martensite phases while avoiding ferrite grains. Early fracture is inhibited by tempering. When excessive plastic deformation is applied before the sustained tensile-loading test, the time to fracture and critical applied stress of the as-quenched DP steel decreased slightly. The results of the present study indicate that the interfacial conditions between ferrite and martensite play important roles in crack propagation associated with hydrogen embrittlement of the DP steel.