Effects of pre-tempering on the microstructure, hardness and impact toughness of the 2.25Cr–1Mo-0.25V heat-resistant steel weld metal

In this study, the effects of pre-tempering on microstructure, hardness, impact toughness and fracture modes of weld metals in 2.25Cr–1Mo-0.25 V heat-resistant steel were investigated. Four weld metals with as-welded, three different post welding heat treatments were chosen as the investigated materials. Optical Microscopy (OM), Scanning Electron Microscope (SEM) were used to evaluate the microstructures of the weld metals. The mechanical properties of the weld metals were examined by hardness, and impact toughness experiments. The fracture surfaces of the weld metals were observed and analyzed by SEM. Results indicated that the impact toughness of as-welded metal is only 6 J, Post welding heat treatment (PWHT) of 705 °C + 8 h can increase the impact toughness of the weld metals to 16,18 and 119 J; 44, 80, 100, 169 and 177 J, respectively. The pre-tempering process with 650 °C with the heating rate of 55 °C/h and holding 40 h before 705 °C+8 h tempering process can increase the impact toughness to 126, 135, 136, 136, 167 J. The change of the impact toughness is directly related to the change of the microstructure. The microstructure of as-welded weld metal is mainly composed of granular bainite, which determined the lower impact toughness. The improvement of impact toughness for the weld metals after post welding heat treatment is attributed to the decomposed degree from Martensite-Austenite constituent into carbides.