Work hardening is the increase in the strength and hardness of metal materials with the increase of cold deformation, but the plasticity and toughness are reduced.
It is a very important reinforcement that can be used to increase the strength and hardness of forging materials. This is especially important for alloy forgings that cannot be strengthened by heat treatment. For example, cold rolled steel sheets have higher strength and hardness than heat-capacitor-rolled steel sheets.
The factors affecting the surface layer work hardening are as follows:
1. Cutting force: The greater the cutting force, the greater the plastic deformation, the greater the degree of hardening, and the greater the depth of the hardened layer. Therefore, increasing the feed depth and reducing the rake angle will increase the cutting force and make the work hardening severe.
2. Cutting temperature: The heat generated during cutting will most soften the surface layer of the workpiece, so the higher the cutting temperature, the greater the degree of work hardening recovery of the surface layer.
3. Deformation speed (cutting speed): When the deformation speed is fast, the contact time of the workpiece is short, and the plastic deformation is insufficient, so the degree of hardening is lowered.
4. When the hardness of the workpiece material is low and the plasticity is large, the surface layer of the machined surface is hardened.
Work hardening is beneficial to the deformation uniformity of plastic deformation processing of forgings. When the deformed part of the forging is strengthened, the continuous deformation will mainly develop in the final deformation part, so that the material can be uniformly deformed, such as drawing of the wire, cylindrical shape. Deep drawing of forgings, etc.
It can ensure the safety of metal parts and components. For example, if the parts are overloaded during work, etc., if the stress of a part of the part is greater than the yield point, a small amount of plastic deformation occurs, and the yield point of the part is improved due to work hardening. It is possible to stop further deformation and breakage of the place.
Although work hardening can improve the strength, it reduces the plasticity, which will undoubtedly cause trouble for the deformation processing of large deformation. For example, if the diameter of the steel wire is pulled, if the diameter is large, the work hardening phenomenon cannot be used once. Pulling out, it needs to be pulled several times, and it is necessary to arrange annealing treatment to eliminate work hardening between the two drawing, so that the plastic recovery of the steel wire can carry out the next drawing and reducing diameter, for example, the material sensitive to work hardening. (High manganese steel), it is difficult to perform machining due to the existence of work hardening.
The work hardening of the surface layer is caused by the cutting force of the surface metal of the workpiece during the machining process, resulting in strong plastic deformation, severely distorting the metal character, grain breakage, elongation and fiberization, thereby hindering further deformation of the metal. The surface hardness of the workpiece is increased and the plasticity is lowered. However, the cutting heat will cause the chilling of the surface of the workpiece to return to a certain condition (the hardened metal returns to the normal state), which is also called softening; higher temperatures will also cause phase transition. Therefore, the final work hardening of the metal processed table is the combined result of this hardening, softening and phase transformation.