Under the conditions of quenching and tempering, annealing, and normalizing of the workpiece, the hardness is lower than 45HRC, and the quality of the workpiece including surface finish, residual stress, machining allowance, surface decarburization, and carbon-poor layer removal, etc. are not significantly affected by cutting processing. As for the change in the latent performance of the forging.
The processing of hardened steel or workpieces of forging manufacturers is also called hard processing. The hardness of the workpieces is as high as 50~65HRC. The materials mainly include ordinary quenched steel, quenched die steel, bearing steel, rolling steel and high-speed steel, etc., cutting The influence of processing is more obvious. Factors such as the generation and conduction of cutting heat, high-speed friction and wear during the cutting process will cause a certain degree of damage to the processed surface. The integrity of the processed surface in hard cutting mainly includes the surface structure and hardness, surface roughness, dimensional accuracy, residual stress distribution and white layer formation.
The hardness of the processed surface increases with the increase of cutting speed, and decreases with the cutting amount of feed. And the higher the hardness of the processed surface, the greater the depth of the hard layer. The results show that the uniform residual compressive stress on the surface of the workpiece after hard cutting, and the large compressive stress of the workpiece after grinding should be closely concentrated on the surface of the workpiece.
The larger the tool obtuse angle radius, the greater the residual compressive stress value; the higher the workpiece hardness, the greater the residual compressive stress value. The influence of workpiece hardness on the integrity of the workpiece surface, the greater the hardness value of the workpiece, the more conducive to the formation of residual compressive stress.
Another important factor affecting the quality of the machined surface in hard cutting is the formation of white layer. The white layer is a structural form formed with the hard cutting process. It has unique wear characteristics: on the one hand, it has high hardness and good corrosion resistance; , Even causing the workpiece to crack after being placed for a period of time after processing. When cutting hardened AISIE52100 bearing steel with ceramic and PCBN tools on a high-rigidity CNC lathe, it was found that the structural conditions of the surface and subsurface of the forging changed, and its microstructure was composed of a white untempered layer and a black over-tempered layer.
The current concept of treating the white layer as a martensitic structure has been unanimously recognized, but the main controversy lies in the fine layout of the white layer. One view is that the white layer is the result of phase transformation, which is composed of fine-grained fine-grained martensite formed by the rapid heating and sudden cooling of the forging material during the cutting process. Another concept thinks that the formation of the white layer is only a deformation mechanism, which is the normal martensite obtained by plastic deformation.