Microstructure evolution and properties of reheated coarse grain heat-affected zone of 900 MPa grade HSLA steel

The welding thermal simulation technology was used to simulate secondary thermal cycle of coarse grain heat-affected zone (CGHAZ) of BWELDY960Q steel, and the change of microstructure and toughness in CGHAZ after reheating was studied in detail. The results show that when the peak temperature of the secondary thermal cycle is 1200 and 1000 degrees C, the microstructure of reheated coarse grain heat-affected zone (RCGHAZ) is martensite; when the peak temperature is 800 degrees C, the microstructure of RCGHAZ constitutes of martensite, granular bainite and M-A constituent. The toughness of CGHAZ is improved undergoing secondary thermal cycle, but it is still lower than that of base metal. Compared with the mechanical properties of CGHAZ, the hardness values of inter-critical and sub-critical RCGHAZ are decreased, while the toughness of super-critical and sub-critical RCGHAZ is increased. Impact fracture exhibits evident plastic deformation, the edge of fracture has tearing edges and corners, and microscopic morphology presents the characteristics of dimp fracture.