After steel parts are induction quenched by high-frequency induction heating power supply, different internal stresses will be generated during cooling.

1. Thermal stress

During the rapid cooling process of the workpiece, the surface layer is cooled first and the center is cooled later. There is always a temperature difference between the surface and the core. In the early stage of cooling, the temperature of the surface layer drops faster than that of the core. The larger contraction of the surface layer is restrained by the core. Tensile stress occurs on the surface layer and compressive stress occurs on the core. As the cooling continues, the surface core temperature continues to increase, causing the surface tensile stress and core compressive stress to continue to increase.

When the stress increases to the yield strength at a certain temperature, it will cause plastic deformation of surface elongation and core compression, causing the stress on the section to relax to a certain extent. During the further cooling process, the surface temperature decreases at a slower rate than The core is fast, and the core shrinks more than the surface, which causes the tensile stress of the surface layer and the compressive stress of the core to tend to decrease, and the thermal stress reverses in the final stage of cooling, that is, the surface layer has compressive stress and the core has tensile stress. stress. At this time, because the temperature is already very low and the yield strength has increased significantly, the internal stress of the steel part can no longer cause plastic deformation, so this stress state is retained and becomes residual stress.

2. Tissue stress

The structural stress of steel parts when quenched by high-frequency induction heating power is mainly caused by the difference in martensite transformation time caused by temperature differences. The figure above shows the evolution of the structural stress of a cylinder assuming there is no thermal stress in the case of full hardening. Between t1 and t2, the surface temperature drops below the Ms point, and the core temperature is still above the Ms point. Martensite is formed on the surface and undergoes volume expansion, which is restrained by the untransformed core. Compressive stress is generated on the surface, and the core is Tensile stress. The curve on the right in the figure above represents the change in tissue stress in the elastic state. The figure also shows the change in yield strength of steel. At this time, the core is still in the austenite state with lower strength and higher plasticity. When the tensile stress in the core and the compressive stress in the surface exceed the yield strength at this temperature, plastic deformation will occur, causing stress relaxation (see the figure above) . After continuing to cool to t2, the core temperature drops below Ms, and martensitic transformation occurs in the core, causing volume expansion. Since the surface layer has been transformed into martensite with high strength and low plasticity, plastic deformation cannot occur, which ultimately results in residual tensile stress on the surface and residual compressive stress in the core.

3. Residual stress in quenched steel

The residual stress field of quenched steel parts is the result of the superposition of residual thermal stress and residual structural stress. Several situations may occur as shown in the figure below. The first set of curves in the figure shows that there is no phase change during the cooling process, and a thermal stress-type residual stress distribution appears. The second is that the tissue stress offsets part of the thermal stress and becomes a residual stress distribution dominated by thermal stress. The third one is the transitional state. The tissue stress in the fourth one exceeds the thermal stress to form tissue stress-type residual stress. The fifth one is the situation where the effect of organizational stress far exceeds that of thermal stress. The last type of residual stress state is often the cause of longitudinal cracks in quenched steel.

When the steel parts are not hardened by high-frequency induction heating power, there are also stresses caused by the specific tolerance difference between the hardened layer and the unhardened core. The specific tolerance between the two usually makes the hardened layer tend to be in a stress state, while the unhardened core is in a tensile stress state. In the transition zone between hardened and unhardened, the stress changes suddenly. When the hardened layer is thicker, the surface compressive stress decreases and the core intercept stress increases. When some high-carbon steels are quenched using high-frequency induction annealing power, the transverse arc-shaped cracks formed in the transition zone between hardened and unhardened are caused by the maximum tensile stress in the transition zone.