Forgings are workpieces or blanks obtained by forging and deforming metal blanks. The mechanical properties of the metal blank can be changed by using the pressure on it to make it deformed. Forgings can be divided into cold forging temperature forgings and hot forgings according to the processing temperature of the blank. Cold forgings are generally processed at room temperature, while hot forgings are processed at a higher recrystallization temperature than the metal blank.
Under the condition of workpiece quality adjustment, annealing and normalizing, the hardness is lower than 45HRC. The impact of cutting on the quality of the workpiece is not obvious, including the surface finish, residual stress, machining allowance, decarburization and carbon-poor layer on the surface must be removed, so as not to cause changes in the potential performance of the workpiece.
For workpiece hardened steel or workpiece processing, also known as hard processing, the workpiece hardness is up to 50~65HRC, and the materials mainly include ordinary hardened steel, quenched mold steel, bearing steel, rolled steel, high-speed steel, etc. The impact of cutting processing is obvious. The generation and transmission of cutting heat, high-speed friction and wear and other factors will cause a certain degree of cutting surface integrity, mainly including surface texture and hardness, surface roughness Dimensional accuracy, residual stress distribution and white layer generation.
The surface hardness of machined surface increases with the increase of cutting speed and decreases with the increase of feed rate. The higher the surface hardness after machining, the greater the depth of the hardened layer. The results show that the residual compressive stress on the workpiece surface is uniform, and the compressive stress after grinding is mainly concentrated on the workpiece surface.
The larger the obtuse angle radius is, the greater the residual compressive stress value is, the higher the workpiece hardness is, and the greater the residual compressive stress value is. The hardness of the workpiece has a great influence on the integrity of the workpiece surface. The greater the hardness of the workpiece, the more conducive to the formation of residual compressive stress.
Another important factor affecting the surface quality of hard cutting is the formation of white layer. The white layer is the organizational form formed during hard cutting, and has unique wear characteristics: on the one hand, it has high hardness and good corrosion resistance; On the other hand, the brittleness is high, which is easy to lead to early peeling failure, and even to crack after the workpiece is processed and placed. When cutting and quenching AISIE52100 bearing steel with ceramic and PCBN cutters on a high rigidity CNC lathe, it was found that the microstructure of the surface and sub-surface of the workpiece changed, and its microstructure was composed of white untempered layer and black overtempered layer.
At present, the view that the white layer is regarded as martensite has been unanimously recognized. The main controversy is the fine structure of the white layer. One view is that the white layer is the result of phase transformation and consists of small crystal martensite formed by rapid heating and sudden cooling of the material during cutting. Another view is that the formation of white layer is only a term deformation mechanism, and is only a kind of unconventional plastic deformation martensite.