Effect of austenite transformation degree on microstructure and fracture toughness of high-strain pipeline steel

In order to clarify the relationships among austenite transformation degree, microstructure and fracture toughness, the simulated inter-critical heat-affected zones (ICHAZ) of high-strain pipeline steel were prepared with five different austenitizing degrees (0%, 20%, 50%, 80% and 100%). The microstructure and the fracture toughness of simulated ICHAZ specimens were investigated. It was found that the microstructure evolution and fracture toughness of ICHAZ were closely related to the austenite transformation degree. In partially austenitized ICHAZ, the fresh bainite and ferrite could break the original structure and decreased effective grain size. The fine grains with disorderly arranged M–A constituents could improve fracture toughness of partially austenitized ICHAZ. In contrast, the linearly aligned M–A constituents in 0%-austenitized region and the chain-distributed ones in 100%-austenitized region could lead to deterioration of fracture toughness. Furthermore, the excellent fracture toughness of partially austenitized ICHAZ was related to the high density of high-angle grain boundaries (HAGBs), homogeneous distribution of local strain as well as the high percentage of small-deformed grains (SDGs).