Creep deformation and rupture of butt-welded tubes of cold-worked AISI 316 steel

Abstract

Tubes of AISI316 steel 20% cold-worked and TIG welded without filler metal were creep tested under internal pressure. The creep deformation was concentrated mainly in the weld metal and the heat-affected zone (HAZ). The failure was a result of the formation of axial cracks in the HAZ, a few millimetres from the fusion boundary. The rupture stresses (hoop stresses) of the tubes were about 5% lower than those of uniaxially tested simulated HAZs and about 25% lower than those of the parent metal. The creep ductility was only about 1·5%, which was much less than the unaxial elongation of the cold-worked parent metal. To explain the creep-deformation and rupture behaviour a model for thin-walled tubes was used with uniaxial creep data on simulated HAZs and parent metal. The model gave a good description of the observed creep rates, rupture times, position and orientation of cracks, and a rupture ductility of the right order of magnitude. For the rupture properties a failure criterion based on the maximum principal stress (hoop stress) was applied which was also consistent with metallographic observations.