"Stress Relief Annealing" of seamless precision steel pipe

The seamless precision steel pipe is heated to a certain temperature (usually below the phase change temperature or recrystallization temperature), held for a period of time, and then slowly cooled to eliminate various internal stresses in the annealing process.

In the process of pressure processing, casting, welding, heat treatment, cutting and other processes,  precision steel pipe may produce internal stress. In most cases, after the end of the process, the steel pipe metal will retain a part of the residual stress. Residual stress can lead to workpiece rupture, deformation or dimensional change, residual stress also improves metal chemical activity, and is particularly easy to cause intergranular corrosion rupture under the action of residual tensile stress. Therefore, the residual stress will affect the performance of the material or lead to premature failure of the steel pipe workpiece.

In the process of stress relief annealing, the residual stress of the metal is relaxed at a certain temperature through internal local plastic deformation (when the stress exceeds the yield strength of the material at the temperature) or local relaxation process (when the stress is less than the yield strength of the material at the temperature). In the stress removal annealing, the workpiece is generally slowly heated to a lower temperature (gray cast iron is 500 ~ 550 ° C, steel is 500 ~ 650 ° C, non-ferrous metal alloy stamping parts are below the recrystallization starting temperature), maintained for a period of time, slow cooling to prevent the generation of new residual stress.

Stress relief annealing can not completely eliminate the residual stress inside the steel pipe, but only most of the elimination. To completely eliminate the residual stress, the workpiece needs to be heated to a higher temperature. Under such conditions, other organizational changes may be brought about, jeopardizing the performance of the material.