A large number of micro-cracks were found after the finishing of 300M steel components.The cracking reasons and mechanisms of this batch of components were analyzed from the aspects of appearance,material chemical composition,microstructure and crack characteristics and hardness.The results show that these are grinding micro-cracks,which initiate in the near-surface layer,mainly exhibiting intergranular and quasi cleavage cracking characteristics.The reason for the cracking is that after chrome plating,the surface material undergoes plastic flow during the grinding process and residual tensile stress is generated under rapid cooling conditions.The surface and near-surface material are burned and the phase transition stress is formed.Meanwhile,the hardness of burned material decreases.After a long time,hydrogen atoms from chrome plating diffuse to near-surface and gather at the dislocations and grain boundaries,that induces the grinding crack to have delayed forming in near-surface under the action of residual tensile stress and microstructure tensile stress.
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