Effects of flow forming parameters on the development of residual stresses in Cr-Mo-V steel tubes

Residual stress generation during flow forming tubular components of Cr-Mo-V ferritic steel has been studied in detail using laboratory X-ray and neutron diffraction. In the present work, flow formed tubes were available for a detailed parameter study including level of wall thickness reduction, starting hardness of the preform material, feed rate, and roller geometry. The high sensitivity of the residual stress distributions to the process parameters is showcased by selecting values within relatively short ranges. In general, low wall thickness reduction, low preform hardness, high feed rate, and rollers with a low contact angle were preferable for keeping stresses at a low level. Mohr׳s circle analysis was employed in order to determine the magnitude and direction of the principal stress, which were found to change depending on the combination of parameters. Finally, the level of stress mitigation by post-processing heat treatments was also characterised for selected samples.