Thermal stress and phase change stress of forgings

Heat treatment distortion may occur after annealing, normalizing, hardening, tempering and surface modification heat treatment. The root cause of distortion lies in the internal stress produced by the forging during heat treatment, that is, because of the difference in temperature difference between inside and outside and the structure transformation, the forging after heat treatment remains internal stress.
When this stress exceeds the yield point of steel in a moment during heat treatment, it will cause distortion of forging. There are thermal stress and phase change stress in the process of heat treatment, their causes and functions are different.
Forgings in heating and cooling accompanied by thermal expansion and cold contraction phenomenon, when the forging surface and the heart due to heating or cooling speed is different, resulting in temperature difference, the volume expansion or contraction on the surface and the heart is not the same, this temperature difference and volume change caused by the different internal stress, called thermal stress.
Forgings in the process of heat treatment, the change of thermal stress is mainly manifested as: when the forgings are heated, the surface temperature rises faster than the core, the surface temperature is high and expands, the core temperature is low and does not expand, at this time the surface compressive stress, the core tensile stress. When the forgings are dithermic, the core temperature increases and expands, at this time the forgings show volume expansion; Workpiece cooling, the surface cooling faster than the core, surface shrinkage, high temperature of the heart to prevent shrinkage, tensile stress on the surface, the heart produces compressive stress, when cooled to a certain temperature, the surface have chilled no longer contract, and the core cooling to occur due to the continued contraction, the surface is compressive stress, and the heart of tensile stress, This stress still exists in the forging after cooling, which is called residual stress.
During the heat treatment of forgings, the mass volume of different structures is different, so the mass volume of forgings is bound to change. Because there is a temperature difference between the surface and the heart of the forging, the surface and the heart of the organization transformation is not timely, so the internal and external mass volume changes will produce internal stress. This internal stress caused by the heterogeneity of organizational transformation is called phase transition stress.

The mass volume of the basic structure of the steel increases in the order of austenite, pearlite, sortenite, troosite, lower bainite, tempered martensite and martensite. For example, forgings quenching fast cooling, due to the surface of the first cold to his point, so the surface from austenite into martensite, volume swell, but the heart is still in austenite state, prevent surface swell, so the forging heart by tensile stress, surface by compressive stress; When the cooling continues, the surface temperature is reduced and will no longer swell, while the core due to the transformation into martensite, the volume will continue to swell, so it will be prevented by the surface, so the heart is subjected to compressive stress, and the surface is subjected to tensile stress. This stress remains in the forging after cooling as a residual stress.
Therefore, in the quenching cooling process, the change of thermal stress and phase change stress is opposite, and the final residual stress in the forging is also opposite. The combination of thermal stress and phase change stress is called quenching internal stress. When the residual internal stress in the forging exceeds the yield point of steel, the workpiece will produce plastic deformation, resulting in distortion of the forging.