Peel crack under stress

Peel crack under stress
When the die is in service, under the effect of stress, the hardened layer is peeled off from the steel matrix piece by piece. Due to the different specific volumes of the surface and central structures of the die, axial and tangential quenching stresses are formed in the surface layer during quenching, tensile stresses are generated in the radial direction, and sudden changes occur internally. Peeling cracks are generated in the narrow range of sharp changes in stress, which often occur during the cooling process of the die after chemical heat treatment of the surface layer. Because the chemical modification of the surface layer is different from the transformation of the steel matrix, the expansion of the quenched martensite in the inner and outer layers is different, resulting in large transformation stress, This causes the chemical treatment layer to peel off from the matrix. Such as flame surface hardening layer, high-frequency surface hardening layer, carburizing layer, carbonitriding layer, nitriding layer, boronizing layer, metallizing layer, etc. It is not suitable to temper the chemical layer rapidly after quenching, especially when tempering at a low temperature below 300 ℃ and heating rapidly, which will cause tensile stress to form on the surface layer, and compressive stress to form in the center of the steel matrix and the transition layer. When the tensile stress is greater than the compressive stress, the chemical layer will be pulled and stripped.

Preventive measures:
(1) The concentration and hardness of the chemical infiltration layer of the die steel should be reduced slowly from the surface to the inside, and the bonding force between the infiltration layer and the matrix should be enhanced. Diffusion treatment after infiltration can make the chemical infiltration layer and the matrix transition uniform;
(2) Before chemical treatment of die steel, diffusion annealing, spheroidizing annealing and quenching and tempering treatment shall be carried out to fully refine the original structure, which can effectively prevent and avoid peeling cracks and ensure product quality.