Work hardening
(1) Causes of work hardening Different degrees of plastic deformation will occur in the machined surface layer during cutting
Severe deformation will change the processing properties of the material to be cut. The cutting edge of any tool can not be sharpened absolutely, when the cut, extrusion and friction of the blunt arc cutting edge and its adjacent flank face, the metal particles on the machined surface are twisted, extruded and broken, as shown in Figure 2-6-11, this phenomenon of increasing the hardness of the surface layer due to severe plastic deformation is called work hardening, also known as cold work hardening. After hardening, the yield strength of the metal material is increased, and microcracks and residual stresses appear on the machined surface, reducing the fatigue strength of the material.
(2) Factors affecting work hardening:
1) Increase the rake angle of the tool, reduce the radius of the obtuse circle of the tool, and reduce the plastic deformation of the metal of the cutting layer, thereby reducing the degree of work hardening of the workpiece.
2) The greater the plasticity of the workpiece material and the greater the strengthening index, the more serious the hardening. For general carbon structural steel, the less carbon content, the greater the plasticity, and the more serious the hardening. The strengthening index of high manganese steel Mn12 is very large, and the hardness of the machined surface is increased by more than 2 times after cutting; Non-ferrous alloy metals have a low melting point and are easy to weaken, and the work hardening is much lighter than structural steel, copper parts are 30% smaller than steel parts, and aluminum parts are about 75% smaller than steel parts.
3) When the feed is relatively large, the cutting force increases, the plastic deformation of the surface layer metal intensifies, and the degree of hardening increases.
4) When the cutting speed increases, the plastic deformation decreases, the plastic deformation zone also decreases, and therefore, the depth of the hardened layer decreases. On the other hand, when the cutting speed increases, the cutting temperature increases and the chemical process accelerates. However, the increase in cutting speed will shorten the heat conduction time, so it is too late to weaken. When the cutting temperature exceeds Ac, the surface layer structure will produce a phase transformation and form a quenching structure. As a result, the depth and degree of hardening will increase. The depth of the hardened layer first decreases with the increase of the cutting speed, and then increases with the increase of the cutting speed.
Effective cooling and lubrication measures can be used to reduce the depth of the work hardening layer.
