In the process of working, the mold bears the impact load, and in order to reduce the damage of fracture and blade in the process of use, the mould steel has some toughness.

Die steel chemical composition, grain size, purity, the number of carbide and inclusion, morphology, size and distribution, as well as the system of heat treatment of steel and heat treatment microstructure of factors have great influence on the toughness of steel. In particular, the net degree of steel and the deformation of thermal machining are more obvious to the transverse toughness. The toughness, strength and wear resistance of steel are often contradictory. Therefore, it is necessary to rationally select the chemical composition of steel and adopt reasonable refining, heat processing and heat treatment process to make the wearability, strength and toughness of the die material achieve the best cooperation.

The total energy absorbed by the sample during the whole fracture process during the shock process is the impact toughness system. However, many tools are fatigue fracture under different working conditions, so the conventional impact toughness cannot fully reflect the fracture properties of die steel. The test techniques of small energy multiple impact fracture or multiple fracture life and fatigue life are being used.