Influence of Baushinger effect on yield strength after pipe forming

Abstract

During a pipe forming, the pipe-wall fibers undergo different deformation history depending on the forming process and position inside a pipe-wall. Moreover a cold flattening and subsequent tensile-test operation lead into an additional tension-tension and compression-tension deformation history on the inner and outermost fibers of a pipe wall, respectively. The compressive strain on outer surface reduces the yield strength due to the Bauschinger effect, while an additional cold work in an outer surface increases the strength due to work hardening. The final yield strength after a pipe forming, therefore, becomes much different from that of original materials. Such a phenomenon should thus be considered in designing high strength line-pipe steels. The relation between work hardening and Bauschinger effect has been experimentally investigated in this study in order to evaluate the final yield strength of a pipe, assuming that line-pipe steels go through an ideal deformation procedure during a pipe forming and a subsequent test processes. The evaluated yield strengths from a simple mechanical test and calculation are in a good agreement with the real values obtained after a 3-roll pipe forming.