Residual stress generation:
After the workpiece is machined, there are residual stresses on the surface layer. The residual compressive stress can improve the wear resistance of the surface of the workpiece and the fatigue strength under tensile stress, and the residual tensile stress acts exactly the opposite. If the tensile stress exceeds the fatigue strength limit of the workpiece material, cracks are generated on the surface of the workpiece to accelerate the damage of the workpiece. There are three reasons for causing residual stress:
(1) Residual stress caused by cold plastic deformation
Under the action of cutting force, the machined surface is subjected to strong cold plastic deformation. Among them, the plastic deformation caused by the extrusion and friction of the tool flank facing the machined surface is most prominent. At this time, the base metal is affected and is in an elastic deformation state. After the cutting force is removed, the base metal tends to recover, but is limited by the surface layer which has been plastically deformed, and is not restored to the original state, so that residual compressive stress is generated in the surface layer.
(2) Residual stress caused by thermoplastic deformation
The surface of the workpiece is thermally expanded under the action of cutting heat. At this time, the temperature of the base metal is low, so the surface metal generates hot compressive stress. When the cutting process is finished, the surface temperature drops faster, so the shrinkage deformation is larger than the inner layer, and the residual tensile stress is generated because the surface deformation is limited by the base metal. The higher the cutting temperature, the greater the thermoplastic deformation, the greater the residual tensile stress, and sometimes even cracks. The thermoplastic deformation produced during grinding is more obvious.
(3) Residual stress caused by changes in metallographic structure
The high temperatures generated during cutting can cause changes in the metallographic structure of the surface layer. Different metallographic structures have different densities, and the change in the metallographic structure of the surface layer results in a change in volume. When the surface layer is expanded in volume, compressive stress is generated because it is restricted by the substrate; otherwise, tensile stress is generated.
In short, the residual stress is the self-phase-balanced internal stress that remains in the object after the external force or uneven temperature field is removed. Both machining and strengthening processes can cause residual stresses. Such as cold drawing, bending, cutting, rolling, shot peening, casting, forging, welding and metal heat treatment, etc., uneven plastic deformation or phase change may cause residual stress. Residual stress is generally harmful. For example, after improper heat treatment, welding or cutting, residual stress may cause warpage or distortion of the part, or even cracking. Cracks may appear on the surface after quenching or grinding. The presence of residual stress sometimes does not immediately manifest as a defect. Cracks and fractures occur when the part is in operation due to the superposition of working stress and residual stress, and the total stress exceeds the strength limit. Most of the residual stress of the part can be eliminated by appropriate heat treatment. Residual stress also has beneficial aspects that can be controlled to improve the fatigue strength and wear resistance of the part.Elimination method:
(a) Natural aging Self-aging is a method of achieving dimensional stability by exposing parts to the outside and after several months or even years. This method of aging has long been widely adopted. A large number of experimental studies and production practices have proved that natural aging has a good effect of stabilizing the dimensional accuracy of castings.
(2) Thermal aging 1. The process used is to perform stress-relieving annealing. This method is characterized by a large reduction in the residual stress of the member and a stable dimensional accuracy.
When the component is heated to 400-700 ° C, the technical components have a fairly single row with significant plasticity. This temperature range is called the elastic transition temperature.
2. Factors affecting the thermal aging effect a. In thermal aging, the annealing temperature is the most important factor affecting the effect of eliminating residual stress.
b. Thermal aging holding time c. Thermal aging heating rate d. Thermal aging cooling rate e. Thermal aging furnace temperature difference f. Placement and support of workpiece in the furnace (3) Vibration aging method The essence of vibration aging is in the form of resonance Applying additional dynamic stress to the workpiece. When the additional dynamic stress and residual stress are superimposed, the workpiece undergoes microscopic or macroscopic plastic deformation when it reaches or exceeds the yield limit of the material, thereby reducing and homogenizing the residual stress inside the workpiece and making it dimensionally accurate. Stable.
Its characteristics are:
1. Low investment 2. Short production cycle 3. Easy to use 4. Adaptable 5. Save energy and reduce cost 6. Significantly improve mechanical performance 7. Meet environmental requirements 8. Simple operation and easy mechanical automation.
It can avoid defects such as warpage, oxidation, decarburization and hardness reduction of metal parts during thermal aging.
(iv) Static overload method
It is temporarily loaded on the member in the form of static or static moment and held under this load for a certain period of time, so that the dimensional accuracy of the part can obtain a stable aging method.
When used in welded parts, the load needs to be increased so that the sum of the original stress and the additional stress is close to the yield limit of the material to eliminate the residual stress.
The accuracy and stability effect of the static overload method depends on the magnitude of the additional stress and the holding time under stress.
In particular, it is noted that the residual stress is still maintained in the component after the static overload method.
(V) Thermal shock aging method A novel aging method for stabilizing the dimensional accuracy of workpieces appeared around 1970.
The essence is to rapidly heat the workpiece, so that the thermal stress caused by the heating process is superimposed on the residual stress, and the plastic deformation is caused by exceeding the yield limit of the material, so that the original residual stress is quickly relaxed and stabilized.
(6) Acoustic aging method Ultrasonic aging method was first born in the former Soviet Union and promoted in developed countries. This method was mainly used in ships, nuclear submarines, aerospace and other military fields where stress relief is very strict. However, since the ultrasonic method can only solve the stress problem in a certain depth of the surface layer of the component, the relative application environment is narrow and the cost is high.
(7) Other methods Explosive method, pressure method, hammering, shot peening, rolling, etc. Shot peening is an effective and widely used method for strengthening parts. Shot peening also changes the residual stress state and distribution of the surface, and the residual compressive stress generated by shot peening is an important factor in the strengthening mechanism.
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