Effect of heat treatment on the microstructure and mechanical properties of structural steel–mild steel composite plates fabricated by explosion welding

Two dissimilar steel plates, structural steel and mild steel, were joined by explosion welding to form a composite. The composite was then heat-treated by quenching at 840°C for 30 min followed by tempering at 200°C for 3 h. The microstructure was investigated under an optical microscope and a scanning electron microscope. The mechanical properties were measured using Vickers microhardness and Charpy impact tests. The results show a deformed interface with typical wave features at the welding zone, but no defects were observed. Moreover, the ferrite in the parent plate in the weld zone was elongated due to the strong plastic deformation from the explosion. After heat treatment, the hardness of the flyer plate (structural steel) was over HV_0.2 800, while that of the parent plate (mild steel) was HV_0.2 200. The increase in hardness was due to the presence of martensite. Moreover, the average impact energy was increased from 18.5 to 44.0 J following heat treatment; this is because of the formation of recrystallized grains at the weld interface, which is due to the dynamic recovery and local recrystallization, and the strong elemental diffusion that occurred between the two plates.