Cutting the real causes of residual stress and stress type (B)
Machining residual stress generated in the metal surface by three main reasons: cold plastic deformation, thermal deformation, the local microstructure changes. Here’s a closer look at the substance of these three reasons and what the type of stress, and overall performance have any impact on the workpiece.
Cold plastic deformation caused by residual stress in the cutting process, the machined surface by the blunt part of the cutting edge and flank compression and friction, resulting in plastic deformation. As the plastic deformation produced only in the surface layer, the surface layer of metal specific volume increases, volume expansion, but connected to it by the inner layer of metal to contain, so the metal surface residual compressive stress in the layer of residual tensile stress.
Plastic deformation caused by thermal residual stress in machining, the cutting area will be a lot of cutting heat generation, the surface temperature is often high, then the metal substrate temperature is lower. Therefore, the surface stress generated hot. After the cutting process, the surface temperature and the substrate temperature down to the same, because the surface has produced thermoplastic deformation, the matrix shrinkage is subject to the constraints arising from the residual tensile stress in the residual compressive stress layer. Grinding the higher the temperature the greater the thermoplastic deformation, the greater the residual tensile stress, and sometimes even cracks.
Changes in microstructure caused by the residual stress of cutting the metal surface layer temperature will change in the microstructure. Different microstructure have different densities. Surface layer of metal microstructure changes in the volume change, will be subject to the limitations of metal matrix residual stress. When the surface layer of the metal volume expansion, the metal surface residual compressive stress in the metal layer residual tensile stress; smaller in size when the surface layer of metal, the metal surface residual tensile stress, resulting in residual compressive stress layer of metal.
Grinding hardened steel, if the surface layer have tempered their microstructure into martensite sorbite by the trust’s or body surface metal density increases smaller in size. Surface layer will have a residual tensile stress in the layer will have a residual compressive stress. When the surface layer of residual compressive stress exists, can delay fatigue cracks, expand and improve the fatigue strength of the workpiece; when the surface layer of the existence of residual tensile stress, the workpiece surface is prone to cracks and reduce fatigue. So try to avoid tensile stress in the process of production, and to try to change the methods and processes to be stress. The next section will briefly explain Haoke metal surface processing technology pre-good in the application of compressive stress.