Residual stress due to heating and cooling

Residual stress due to heating and cooling - Heat treatment defects

Figure 1: Residual stress due to heating and cooling.

Defect name: Residual stress
Record No.: 3223
Type of defect (Internal/Surface): Internal
Defect classification: Induction heating defects, heat treatment defects
Steel name: Steel
Steel composition in weight %: No data.
Note: Thermal stresses are produced first during the heating and then during the cooling of a part.

During heating, the surface remains in compression until the temperature reaches 1000 C (for carbon steel) when the surface become plastic. During cooling, the surface stresses are in tension until the surface is in the martensitic formation range. The level od stress produced depends on the modulus of elasticity and the thermal coefficient of expansion of the material. The very basic nature of induction heating involves the conduction of heat from the surface toward the core. The rate of heating and the amount if heat transfer determine the thermal stresses during heating, as well as any stress relief of pre-existing stresses in the core. Residual stresses on heating have a little effect, except when tensile stresses are high on the inside of a part, and the outside compressive stresses are relaxed through subsequent plastic deformation so that the inside remaining tensile stresses are large enough to cause teh part to crack. Induction heating can cause cracks when heating cold-drawn bar stock, when the bar is not stress relieved uniformly and there are still large tensile stresses inside the core.

During the cooling of induction-hardened parts, different stress conditions are set up. First, due to thermal contraction, the outside surface is increasing in tesnile strength (the ductility is decreasing), while trying to decrease in volume. The resultant net residual stresses in the part after cooling depend on the contraction of the core and its effect on contraction of the surface. If the parts are case hardened fast enough so that the core has little heat, the core may have little effect on the surface stresses. If the part is through heated so that the core has heat and can contract when cooled, there may be substantial effect. The final residual stress distribution with induction heating is more complex when the quenched parts undergo phase transformations to martensite.
Reference: Not shown in this demonstration version.