Effect of heat input on microstructure, variant pairing, and impact toughness of CGHAZ in 1000 MPa-grade marine engineering steel

This study investigates the evolution of the coarse-grain heat-affected zone (CGHAZ) in 1000 MPa-grade marine engineering steel under varying heat inputs through welding thermal simulation, establishing the relationships between heat input, microstructure, variant pairing, and impact toughness. The results show that under different heat-input conditions, the microstructure is primarily composed of lath martensite and lath bainite, with various tendencies of clustering in the form of close-packed parallel plane groups, thus improving the impact toughness of the CGHAZ. As the heat input increases, the toughness first increases and then decreases. At 20 kJ/cm, the prior austenite grain size is smaller and many block boundaries formed within the same packet, with the boundary-length fraction of the V1/V2 variant pair being the highest, thus resulting in a higher density of highangle grain boundaries and better toughness. At 30 kJ/cm, a single Bain group dominates the phase transformation within a single austenite grain and the boundary-length fraction of the V1/V4 variant pair exceeds 50 %, thus resulting in a relatively high density of low-angle grain boundaries, which is detrimental to impact toughness. At 10 kJ/cm, a stronger variant-selection tendency reduces the boundary-length fraction of the V1/V2 variant pair, which is unfavorable for toughness.