The magnitude of the stress is related to the cooling rate, shape and chemical composition of the workpiece in the martensitic transformation zone. The faster the cooling rate is, the higher the carbon content and alloy composition are. The larger the non-uniform plastic deformation caused by thermal stress is, the larger the residual stress is. On the other hand, in the process of heat treatment, when the steel becomes austenite to martensite, the increase of specific volume is accompanied by the expansion of the workpiece volume, and the part of the workpiece is continuously deformed, resulting in inconsistent volume growth and structure formation. Emphasize. The ultimate result of the change of tissue stress is the tensile stress on the surface layer and the compressive stress on the core, which is opposite to the thermal stress.


When the large diameter direct welded pipe is finished at the end of cooling, the core of the surface will not be pulled freely due to the shrinkage of the final cooling volume of the core. That is to say, under the action of thermal stress, the workpiece surface is eventually pressed and the core is pulled. This phenomenon is influenced by factors such as cooling rate, material composition and heat treatment process.