Microstructure and Toughness of Simulated Heat-Affected Zone of Laser Welded Joint for 960 MPa Grade High Strength Steel

The microstructure and toughness of coarse grain zone (CGZ) and mixed grain zone (MGZ) for laser welded 960 MPa grade high strength steel joints were investigated by thermal simulation with a Gleeble-3500 thermal simulator. The results show that microstructure of the stimulated CGZ mainly consists of uniform interweaved lath martensite, and grain growth is not severe upon increasing the cooling time (t8/5). Microstructure of the stimulated MGZ presents strip-like in low peak temperature, and small block martensite is formed on the grain boundary. However, in high peak temperature, the strip-like microstructure disappears and small block martensite presents net-like structure. The lath character for MGZ and CGZ is very obvious under TEM observation, and the average lath thickness of BM, MGZ, and CGZ is 100, 150 and 200 nm, respectively. The impact energy and microhardness of CGZ are higher than MGZ and reduce with increasing the cooling time. The fracture toughness deteriorating drastically for MGZ may be related with the formation of the mixture microstructure, in which the small block martensite is distributed in the shape of a network.