(1) in the forging tempering, annealing, normalizing state, the hardness is less than 45HRC, cutting the quality of forgings, including surface finish, residual stress, processing allowance, surface decarburization carbon layer removal, etc., the impact is not obvious, do not cause the potential performance of the workpiece changes.
(2) for forgings hardened steel or forgings processing, also known as hard state processing, workpiece hardness up to 50~65HRC, materials mainly include ordinary quenching steel, quenching state mold steel, bearing steel, rolling 锟 steel and high-speed steel, the impact of the cutting process is more obvious, cutting processing in the production and conduction of heat, High-speed friction and wear factors will cause a certain degree of damage to the machined surface. The integrity of machined surface is mainly composed of surface microstructure, hardness, surface roughness, dimension precision, residual stress distribution and white layer formation.
The machined surface hardness increases with the increase of cutting speed, and decreases with the increase of the cutting volume. And the higher the hardness of machined surface, the greater the hardening layer depth. The results show that the surface of the workpiece is uniformly residual compressive stress after hard cutting, and the maximum compressive stress of the forgings is mainly concentrated on the workpiece surface.
The greater the obtuse angle radius, the greater the residual compressive stress value, the higher the hardness of forgings, the greater the residual compressive stress value. The hardness of workpiece has great influence on the surface integrity of workpiece, the greater the hardness value of workpiece, the more favorable to the formation of residual compressive stress.
Another important factor affecting the machined surface quality of hard machining is the formation of white layers. White layer is accompanied by a hard cutting process formed an organization, it has a unique wear characteristics: On the one hand, high hardness, good corrosion resistance, on the other hand, also showed high brittleness, easy to cause early spalling failure, and even the formation of forging processing after a stage after the cracking. When using ceramic and PCBN tool to cut hardened AISIE52100 bearing steel on high rigidity NC lathe, it is found that the microstructure of forging surface and subsurface is changed, and its microstructure is composed of white tempering layer and black over tempering layer.
The idea of White as a martensitic organization is unanimously recognized, with the main controversy being the fine structure of the white layer. One view is that the white layer is the result of phase transformation and is composed of fine grained martensite formed by rapid heating and sudden cooling of the material during cutting. Another view is that the formation of the white layer is only the term deformation mechanism, only by plastic deformation of the unconventional martensite.