In Situ Observation of Microstructure Transformation in Q1100 High-Strength Steel during Welding Thermal Simulation

The welding thermal simulation experiment of Q1100 low alloy high-strength steel was conducted using high-temperature laser scanning confocal microscope(HT-LSCM) and Gleeble-3500 thermal simulation tester with different welding heat inputs. In situ observation showed that prior austenite grains merged and grew instantly when the temperature reached the peak temperature, and then continued to migrate and grow during the subsequent cooling process. The prior austenite grain sizes and the CGHAZ microstructure were similar for the different welding heat inputs between high-temperature laser scanning confocal microscope and Gleeble-3500 thermal simulation. The microstructure consisted of granular bainite and proeutectoid ferrite, and a small amount of martensite/austenite (M-A) constituents when at t8/5 of 300 s. With increasing cooling rate, the microstructure transformed into lath bainite and lath martensite. When t8/5 was reduced to 15 s, the microstructure became lath martensite. Microhardness increased gradually as the welding heat inputs decreased, and impact toughness first increased and then decreased. By in situ observation of the microstructure and impact toughness testing, it is feasible to rapidly determine appropriate welding parameters and evaluate the weldability of materials.