Residual Stress Relaxation and Microstructure Evolution in Cold Radial Forged High-Strength Steel Tube Using Thermal-Vibratory Stress Relief Technique

The residual stress greatly deteriorates the dimensional stability and the mechanical performance of the cold radial forged steel tube. In this work, the effect of thermal-vibratory stress relief (TVSR), thermal stress relief (TSR), and vibratory stress relief (VSR) on the cold radial forged high-strength steel tube was compared. The evolution of grain size, grain boundary, dislocation density, and texture with different stress relief methods was characterized. The results suggest that the reduction rate of the axial residual stress is less than 30% after the VSR. After the TSR, the microhardness of the forged tube is reduced by 8%. And there is still a tensile residual stress (150-200 MPa) near the inner surface of the forged tube, whereas the tensile residual stress is reduced to as low as 50 MPa with the TVSR technique. The microhardness of the forged tube, which is treated with TVSR, only drops by 4%. After the VSR, TSR, and TVSR, the texture components remain unchanged. The dislocation density of the forged tube, which is treated with the TSR, is 16% lower than that of the as-forged tube without stress relief (WSR). However, the dislocation density is only reduced by 4% after the TVSR. The strength of dislocation strengthening treated with the TSR is 9% lower than that of the as-forged tube. The strength of dislocation strengthening is only reduced by 2% after the TVSR. Among the VSR, TSR, and TVSR, the TVSR technique is the best method to relieve the residual stress of the forged tube, as it can maintain the strength to the greatest extent.