Quantitative study on the effect of hydrogen on fracture toughness and plastic zone size of X52 steel

Fracture toughness, reflecting a material's resistance to crack initiation and propagation, is a crucial mechanical parameter for evaluating hydrogen compatibility of pipeline steels. In this study, single-edge notched bending (SENB) specimens, combined with microstructural characterization and microhardness measurements, were employed to systematically investigate the effects of various hydrogen charging durations on the fracture toughness and failure mechanisms of X52 pipeline steel. It is found that fracture toughness sharply decreases at first and then gradually stabilizes as hydrogen charging duration increases. Specifically, fracture toughness drastically declines from 650.88 kJ m −2 under hydrogen-uncharged condition to 92.02 kJ m −2 after charging for 0.75 h, further decreases to 62.05 kJ m −2 at 24 h, and eventually stabilizes at 53.39 kJ m −2 at 45 h. This behavior results from the gradual saturation of hydrogen concentration within the alloy over prolonged hydrogen charging times. Additionally, hydrogen significantly reduces the plastic component of fracture toughness, whereas its impact on the elastic component is relatively minor. This conclusion is further rationalized by the significant reduction of the crack-tip plastic zone size upon hydrogen charging, comparing experimentally measured crack-tip plastic zone sizes with theoretical predictions based on the Irwin model for plane-stress and plane-strain conditions. Moreover, fracture surface examinations reveal a micro-mechanism transition from ductile dimple fracture to brittle quasi-cleavage and intergranular fracture with increased hydrogen charging duration. • Hydrogen primarily reduces the plastic component of fracture toughness. • Hydrogen significantly reduces the crack-tip plastic zone size. • Hydrogen alters fracture mode from ductile dimple to brittle quasi-cleavage or IG. • Microhardness method can be used to determine the crack-tip plastic zone size.