Structural steel workpiece in the heating and cooling process, due to inconsistencies cooling rate of the surface layer and the core section and time form a temperature difference, it will lead to volume expansion and contraction of the uneven stress that the heat stress. In the thermal stress due to a temperature below the surface began to core section, the contraction is also greater than leaving the center core section section of tension, when the end of the cooling, since the center portion of the final cooling volume contraction can not freely leaving the center portion of the surface pressure tension. That is under the influence of thermal stress ultimately the workpiece surface pressure and heart Ministry tension. This phenomenon is cooled velocity, material composition and heat treatment and other factors. When cooled, the faster, the higher the carbon content and alloy composition, uneven cooling process of plastic deformation under thermal stress generated by the greater, the final form of the residual stress it gets. 






On the other hand the steel during heat treatment due to the change in tissue i.e. austenite to martensite, accompanied by the increase of the specific volume of the volume expansion of the workpiece, the various parts of the workpiece has a phase change, resulting in inconsistency of the volume of tissue grown stress. The end result is a surface tissue stress tensile stress, the heart portion compressive stress, and thermal stress in exactly the opposite. The stress in the workpiece size and cooling rate of martensite transformation zone factor, shape, chemical composition of the material.