Microstructural evolution and its influence on toughness in simulated inter-critical heat affected zone of large thickness bainitic steel

Abstract

Simulated inter-critical heat affected zones (ICHAZ) of a large thickness bainitic steel were prepared with three different peak temperatures (Tp) of 750 °C, 780 °C and 800 °C. The microstructure and crystallographic feature were investigated. Toughness of simulated ICHAZ specimens was assessed using instrumental Charpy impact test at − 40 °C. It was found that the volume fraction of newly formed bainite increased and the effective grain size decreased with the increase in inter-critical Tp. The toughness decreased slightly with the Tp of 750 °C compared with the base metal (BM), while the toughness improved sharply when the Tp exceeded 780 °C. The network-like martensite was primarily responsible for the low crack initiation energy with the Tp of 750 °C. In contrast, the fine bainite formed with the Tp above 780 °C was effective in increasing the crack nucleation resistance, resulting in the high initiation energy. The matrix softening as well as the grain refinement played important roles in improving the crack propagation energy. Furthermore, it was confirmed that the presence of a multi-phase microstructure can lead to the scattering of toughness.