Significant influence of heat input on microstructure evolution and mechanical properties of the simulated CGHAZ in a 1000 MPa grade ultra-high strength steel

The microstructure, impact properties, and crystallographic characteristics of the coarse-grained heat-affected zone (CGHAZ) in a new 1000 MPa grade ultra-high strength steel with varying heat inputs through were investigated by single-pass welding thermal simulations. As the heat input increased from 10 to 50 kJ/cm, the main microstructural constituents of the CGHAZ transitioned from lath martensite (LM) to lath bainite (LB) and subsequently to a combination of LB and granular bainite (GB). The CGHAZ exhibited a brittle fracture behaviour, and the impact energy initially increased and then declined with higher heat input. The lowest microhardness and the highest impact toughness were observed at a heat input of 20 kJ/cm. According to the results of EBSD, the proportions of high angle grain boundaries (HAGBs) and lower kernel average misorientation (KAM) values were the highest at a heat input of 20 kJ/cm. These factors collectively contributed to the enhancement of impact toughness.