During the heating and cooling process of the
cold drawn seamless steel pipe, due to the inconsistency between the cooling rate and the time of the surface layer and the core, a temperature difference is formed, which causes volume expansion and uneven shrinkage to generate stress, that is, thermal stress. Under the action of thermal stress, since the surface layer starts to be lower than the core, the contraction is also larger than the core and the core is pulled.
When the cooling is finished, the core is compressed due to the final cooling volume shrinkage of the core pulled. That is, under the action of thermal stress, the surface of the workpiece is finally pressed and the core is pulled. This is now affected by factors such as cooling rate, material composition and heat treatment process.
The faster the cooling rate, the higher the carbon content and the cold-drawn seamless steel pipe material composition, the greater the uneven plastic deformation caused by thermal stress during the cooling process, and the greater the residual stress formed. On the other hand, in the process of heat treatment, when steel changes to austenite to martensite, the increase of specific volume is accompanied by the expansion of the volume of the workpiece, and the parts of the workpiece are successively transformed, resulting in inconsistent volume growth and tissue formation stress.
The final result of the change in tissue stress is the tensile stress on the surface layer and the compressive stress on the core, just opposite to the thermal stress. The magnitude of the tissue stress is related to the cooling rate, shape, and chemical composition of the cold drawn seamless steel tube in the martensitic transformation zone.
The cold drawn seamless steel pipe after heat treatment will have the following improvements:
1. Improve the strength, hardness, surface wear resistance and corrosion resistance of steel pipes.
2. Eliminate internal stress caused by welding and prevent deformation.
3. Improve the internal organization and performance of seamless steel pipes to meet technical requirements.
