Effects of Various Heat Inputs and Reheating Processes on the Microstructure and Properties of Low-Carbon Bainite Weld Metals Containing 4% Ni

The effects of welding heat inputs and reheating processes with various cooling rates were investigated by thermally simulated experiments on the microstructure, tensile strength and impact toughness of high-strength weld metals containing 4% Ni. The microstructure was characterized by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). As a result, an extended continuous cooling transformation (e-CCT) diagram was established. The change in the microstructures is affected from three perspectives. The coarser grains and the generated martensite-austenite (M-A) constituents lead to the deterioration of impact toughness and ultimate tensile strength with the increase in heat inputs. The reheating processes with various cooling rates can change the microstructure, hardness, impact toughness and large angle boundaries. When the heat input increases from 15.8 to 17.9 kJ/cm, the content of block ferrite decreases from the initial 18.9 to 8.5%, and the content of lath bainite increases accordingly. When the heat input is 20.6 kJ/cm, the content of block ferrite increases is 17.3% and the rest is lath bainite. The hardness first decreases in the lower cooling rate range (0.05~1 °C/s) and then increases at higher cooling rates. The minimum hardness at a cooling rate of 1 °C/s may be related to the decrease in the coarse block M-A constituents. The reheating process decreases the impact toughness at room temperature from 83 to 37.45 J for the specimen with a cooling rate of 30 °C/s and increases the impact toughness from 83 to 99.71 J for the specimen with a cooling rate of 0.5 °C/s. The impact toughness at −50 °C after the reheating processes decreases from 74 to 32 J, and the lowest impact toughness after the reheating processes reaches only 32 J. The proportion of high-angle grain boundaries (HAGBs) first increases from 12.13 to 26. 44% and then decreases to 16.34% with increasing cooling rate.