Stainless steel head in the heating and cooling process, the stainless steel head due to the surface and the heart of the cooling rate and time inconsistency, the formation of temperature difference, it will lead to volume expansion and shrinkage uneven stress, that is, thermal stress. Under the action of thermal stress, because the surface temperature is lower than the heart, the contraction is greater than the heart and the heart of the stretch, when the end of the cooling, due to the final cooling volume shrinkage can not be free to make the surface by the heart Pull up. That is, under the action of thermal stress, the final surface of the workpiece pressure and the heart of the stretch. This phenomenon is affected by cooling rate, material composition and heat treatment process and other factors. When the cooling rate is faster, the higher the carbon content and the alloy composition, the greater the uneven plastic deformation produced by the thermal stress during the cooling process, and the greater the residual stress formed.
On the other hand, the steel in the heat treatment process due to changes in the organization that austenite to martensite transformation, due to the increase in specific volume will be accompanied by the expansion of the workpiece volume, the workpiece has changed the various parts, resulting in volume growth is inconsistent and produce tissue stress. The ultimate result of the stress change of the stainless steel head is the tensile stress of the surface layer, the compressive stress of the heart, and the opposite of the thermal stress. The size of the tissue stress is related to the cooling rate, shape, chemical composition of the workpiece in the martensitic transformation zone. But the thermal stress in the organization before the transformation has been produced before the stainless steel head, and tissue stress is generated in the process of transformation, in the whole cooling process, the thermal stress and the combined effect of tissue stress is the actual existence of the workpiece The stress. The combined effect of these two stresses is very complex and is influenced by many factors such as composition, shape, heat treatment and so on. There are only two types of developmental processes, namely, thermal stress and tissue stress, the opposite direction of action when the two offset, the role of the same direction when the two are superimposed on each other. Whether it is offset each other or overlap each other, the two stresses should have a dominant factor, the role of thermal stress when the dominant position is the workpiece heart tension, the surface pressure. The effect of tissue stress on the dominant position is that the workpiece is pressed against the surface of the workpiece.
Source: China Head Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)