Stainless steel head in the process of heating and cooling, the cooling speed of the stainless steel head due to the surface and center and time inconsistency, formation temperature, will lead to uneven volume expansion and contraction stress, thermal stress. Under the action of thermal stress, as surface temperature is lower than the core, and make the heart of contraction is greater than the core department in tension, when at the end of the cooling, due to the heart of the final cooling volume shrinkage can’t free and make the surface tension of compression of the core. That is, under the action of thermal stress, the surface of the workpiece is subjected to pressure and the heart is pulled. This phenomenon is affected by factors such as cooling speed, material composition and heat treatment process. When the cooling rate is faster, the carbon content and the alloy composition are higher. The greater the non-uniform plastic deformation under thermal stress in the cooling process, the more the residual stress will be formed.
Steel in the process of heat treatment on the other hand due to the change of organization of austenite to martensite transformation, due to the increase of the volume will be accompanied by the expansion of the size of workpiece, workpiece parts successively phase transition, grew up volume caused by inconsistent and organization stress generation. The ultimate result of stress change of stainless steel seal head is tensile stress at the surface and stress in the core, which is the opposite of thermal stress. The size of tissue stress is related to the cooling speed, shape and chemical composition of the material in martensite phase transition. Only the thermal stress in the organization transformation had produced stainless steel head, and organizational stress is produced in the process of organizational change, in the cooling process, a result of the combined action of thermal stress and organization stress, is the real stress in the workpiece. The results of these two kinds of stress synthesis are very complicated and are influenced by many factors, such as composition, shape, heat treatment, etc. In terms of the development process, there are only two types, namely thermal stress and organizational stress, which cancel out when the direction of action is opposite. Whether it is offsetting or overlapping, the two stresses should have a dominant factor. The result of the thermal stress is the tension of the part of the workpiece and the surface under pressure. The result of the dominant stress is the tension surface of the workpiece core.
Source: China Stainless Steel Head Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)