Surface nanocrystallization, austenization and hardening of medium carbon steel by an explosive impact technique

• An explosive impact technique was introduced to harden medium carbon steel.
• Austenite nanocrystallines with 10 nm in size and 5 μm in depth were formed in impact surface.
• The hardness and wear-resistance of the impact surface were greatly improved.
• The hardening depth on both sides was approximately 500 μm.
• The cementite flakes appeared slip, segmentation and fracture during the severe deformation.

An explosive impact technique was used to harden a medium carbon steel plate with a thickness of 6 mm. The explosive impact produced relatively uniform microstructure and mechanical properties in the impact surface layer. The austenite equiaxed nanocrystallines were formed in this layer; the grain size and the depth respectively are 10 nm and 5 μm approximately. The microhardness of the impact surface was increased to 475 HV from 130 HV and the wear resistance was doubled. Moreover, the impact surface has a greater hardness than the explosive surface, and the hardness gradually decreased with the depth increases from both sides. The plastic deformation penetrated through the entire plate depth, the depth of highly hardening layers was arrived at 500 μm and a sandwich structure was produced. By the server plastic deformation, the cementite flakes appeared slip, segmentation and the fracture. Comparing with the surface severe plastic deformation (SPD) processes, the explosion impact has a stronger ability for hardening the entire plate depth and a little weaker surface strengthening effect. The nanocrystallization, austenization and hardening mechanisms were discussed.