Microstructure evolution and impact toughness variation for high strength steel multi-pass weld metals with various cooling rates

Abstract

Thermally simulated experiments on high-strength weld metals were performed. Based on the results, an extended continuous cooling transformation (CCT) curve with microstructure, hardness, and impact toughness can be proposed. The microstructures, hardness, high-angle grain boundaries, impact toughness, and fracture surfaces of the weld metals treated with different cooling rates were systematically compared and analyzed. The microstructure of the original weld metal corresponds to a cooling rate of 5 °C/s and is composed of uniform granular bainite and a small amount of acicular ferrite, which is similar to that of the specimens with cooling rates of 3−30 °C/s. The increase in the cooling rates leads to the evolution of the microstructure from quasi-polygon ferrite to granular bainite. The proportion of large angle grain boundaries increases with the increasing cooling rate, which can increase the resistance of crack propagation and induce a higher impact toughness. The impact toughness at room temperature is in the range of 80–131.81 J. Various cooling rates lead to various impact toughness. Except for the cooling rate of 5 °C/s, the other cooling rates decrease the impact toughness. Moreover, at the lowest cooling rate, the decrease in the impact toughness reaches 63 %. The impact toughness at -50 °C is in the range of 34–109.86 J. The impact toughness of the weld metals without thermally simulated processes at -50 °C is 88.6 J. The reheating process decreases the impact toughness from 88.6 J to 34 J, and the decrease rate of the impact toughness at -50 °C reaches 38 %.