Abstract: The microstructure and properties of stmulated heat affected zone (HAZ) of laser welded ULCB600 steel were investigate by applying the simulation technique with gleeble - 2000 dynamic thermal - me - chanical simulator.The influence of the preheat condition on the microstructure and properties of simu- lated HAZ of laser welded plate was also studied in order to evaluate the feasibility of reducing arisk that the ductility and toedness of HAZ may be poor by using preheat treatment.The results indicate that the grain size of laser - welded HAZ simulated is very small no matter if there is preheat, the toughness of simulated HAZ is therefore improved comparing to the base metal,and there is no obvious brittle - ductile transition in the range from - 80℃ to 20℃. The TEM analyses of sub -microstruc- ture also discover that microstructure constituent of both simulated HAZ is dominative lath martensite. However, the shaf of martensite is relatively coarse,and the dislocation density is relatively high for simulated HAZ with 200℃ preheat because of slower cooling rate. Combination of these tow factors is responsible for farer bardness and better toughness of simulated HAZ with preheat condition.

Key words: physical simulation laser beam welding, ULCB steel, HAZ