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The demand for heat-resistant steel has increased owing to its utility in numerous devices that must withstand high steam pressures and high temperatures, such as turbine rotors and blades in ultra-supercritical power plants. It is inevitable to join heat-resistance steel part by welding method, so it is important to maintain the toughness of the weld metals. In this study, the microstructure, low-temperature impact toughness, and fracture surface of as-welded and post-weld heat treatment (PWHT) of 2.25Cr-1Mo-0.25V weld metal were investigated. The microstructures of the as-welded and PWHT specimens are granular bainite and ferrite, respectively. This work revealed the relationship between effective microstructure nearby crack initiation origin and low temperature impact toughness for both the as-welded and PWHT specimens. The evolution of the microstructure and prior austenite was then investigated using confocal laser scanning microscopy (CLSM) to observe the formation of coarse ferrite grain structures. A suggestion for enhancing the low-temperature toughness was provided based on the effect of adjusting Mn content and forming acicular ferrite. [ABSTRACT FROM AUTHOR]

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. • PWHT of 750 °C+8 h can increase the impact toughness of the weld metals from 6 J to the range from 16 to 177 J. • The pre-tempering with 650 °C+40 h before main tempering increases the impact toughness to 126, 135, 136, 136, 167 J. • The granular bainite microstructure of as-welded metals determined the lower impact toughness. • The improvement of impact toughness after PWHT is attributed to the decomposed degree from M-A constituent into carbides. [ABSTRACT FROM AUTHOR]