Abstract: In this paper, both as-received tool steel and heat-treated tool steel were used for underwater explosive welding, which had two objectives:to investigate the possibility of welding a super-high hardness material with a thin copper foil using underwater explosive welding, and to investigate the effect of initial hardness on the bonding interface of identical compositional materials (with only hardness varying) under identical welding conditions. The microstructure and hardness at interface of the welded metals were evaluated. The hardness of as-received tool steel and copper increases near the interface and approaches a peak value followed by a gradual decrease away from the interface. This indicates that the microstructure near the interface was modified by the severe plastic deformation and cold working of the surfaces of the materials during the collision of the plates. It is interestingly found that a decrease of the hardness was seen in the case of quenched martensitic tool steel, which was absolutely different with the previous research on explosive welding. It was proved that it is the transformation from martensite phase s to ferrite phase in quenched tool steel that caused the decrease of hardness.