Cold-bending of linepipe steel plate to pipe, detrimental or beneficial?

Systematic investigation was carried out to understand the effect of plate-to-pipe forming (U-bending) strain on the microstructure and mechanical properties of X80 linepipe steel and to establish a structure-property correlation. Finite element simulation predicted the von Mises stress state and through-thickness residual stress distribution. Transmission electron microscopy and electron back scattered diffraction studies detected the local strain gradients, which were verified by nanoindentation testing along with the estimation of through-thickness residual stress. Both yield strength and tensile strength increased after cold-bending operation due to dislocation generation. Also, post-uniform elongation increased, especially at inner diameter (ID) portion due to an overall more dislocation interaction. In the presence of compressive residual stress, ID sample revealed improved impact toughness than the as-received (AR) one at all tested temperatures in the range of −80 °C to room temperature. Low temperature (-80 °C) impact toughness increased by ~16% even at outer diameter (OD) of the pipe, despite the presence of a tensile residual stress state. This improvement in toughness is contributed by many factors, such as the increase in critical stress for yielding, the decrease in crystallographic unit size, less availability of potential sites for cleavage crack initiation, texture strengthening, and crack-divider type fissure formation.