Hydrogen embrittlement and fracture mechanism of friction stir welded quenching and partitioning 980 steel

Abstract

In this study, the hydrogen embrittlement and fracture mechanism of friction stir welded Q&P980 steel were investigated systematically. It was found that the intercritical heat affected zone was most susceptible and the stir zone was resistant to hydrogen embrittlement. The hydrogen-induced cracks initiated and propagated along the grain boundaries of ferrite and martensite or inside the martensite. This was attributed to the strain and stress concentration caused by the martensite and ferrite and the large-angle grain boundary formed during thermal cycle of welding process. With the charging time increased from 10 min to 16 h, the strength decreased by ~5% whereas the elongation decreased from 10.2% to 3.1% dramatically, which indicated that the hydrogen diffused into the joints had serious effect on elongation. At lower strain rates (1 × 10−5 s−1), hydrogen moved with dislocations and accumulated which promoted the initiation of microcracks and occurrence of fracture ultimately. The fracture surfaces changed from ductile mode for uncharged weld joint to brittle mode for charged weld joint. In addition, the TRIP effect ofretained austenite in SCHAZ during tensile test process aggravated the susceptibility to hydrogen embrittlement which accelerated the nucleation and propagation of hydrogen-induced cracks, and thus intergranular fracture occurred in charged weld joints.