Effect of thermo-mechanical cycling on the microstructure and toughness in the weld CGHAZ of a novel high strength low carbon steel

Abstract We have studied the microstructural evolution in the simulated coarse grain heat affected zone (CGHAZ) of novel low carbon microalloyed steel with yield strength of 1000 MPa using electron microscopy, while the crystallographic characteristics were studied by electron backscatter diffraction (EBSD). The deterioration in low temperature toughness after the simulated welding cycle was attributed to the formation of coarse blocky M–A constituent. However, the lower bainite microstructure of the matrix was beneficial for low temperature impact toughness because of similarity in the crystal structure of variants of lath martensite, which were present in high proportion at the high angle grain boundaries. A high fraction of small M–A constituent also influenced impact toughness.