After machining, the hardness of the surface metal of the machined surface is often higher than the hardness of the base material, this phenomenon is called work hardening.
In machining, the factors that influence the surface hardening are:
1. Tool. The larger the rake angle of the tool, the smaller the plastic deformation of the cutting layer metal, the smaller the depth of the hardened layer; The bigger the blunt circle radius of the blade, the greater the degree of extrusion in the forming process of the machined surface, and the greater the work hardening; With the increase of the tool flank wear, the friction between the flank and the machined surface increases, the depth of the hardened layer increased until it reaches a certain level.
2. Workpiece. The greater the plasticity of the workpiece material, the greater the strengthening index, the more serious the hardening. For general carbon structural steels, the less carbon content, the greater the plasticity, the more serious the hardening.
3. Cutting condition. When the feed rate is large, increase the feed rate, the cutting force increased. The plastic deformation of the surface layer metal is increased, and the degree of chill is increased. But for the case of small cutting thickness, the degree of metal chill of the surface layer will not only decrease, but increase on the contrary. This is because the cutting thickness decreases, the specific cutting pressure increases. When the cutting speed increases, the plastic deformation decreases and the plastic deformation zone decreases. Therefore, the depth of the hardened layer decreases. On the other hand, when the cutting speed increases, the cutting temperature increases and the weakening process speeds up. But the cutting speed increases, will make the heat conduction time shortened, and thus no time to weaken. When the cutting temperature exceeds Ac3, the surface layer structure will produce phase change, the formation of quenched tissue. Therefore, the hardening layer depth and degree of hardening will increase again. The depth of hardened layer first decreases with the increase of cutting speed, and then increases with the increase of cutting speed; The use of effective cooling and lubrication measures, can reduce the depth of processing hardened layer.