Effect of welding on microstructure and mechanical response of X100Q bainitic steel through nanoindentation, tensile, cyclic plasticity and fatigue characterisation

Abstract

This paper presents an experimental characterisation of fatigue at welded connections for the next-generation high-strength low-alloy offshore riser steel, X100Q. An instrumented girth weld is conducted with a parallel programme of physical-thermal simulation (Gleeble) to develop heat affected zone (HAZ) test specimens. X100Q is shown to exhibit superior fatigue performance to the current state of the art offshore riser steel, X80. Significant differences are demonstrated between the parent material and simulated HAZ in terms of hardness, monotonic strength and cyclic plasticity response, which can be related to the observed microstructural transformations: the refined grain and bainitic block size in the fine-grained HAZ are shown to give a harder and stronger response than parent material, whereas the coarsened bainitic lath structure in the intercritical HAZ gives a softer and weaker response. The simulated HAZ materials exhibit superior fatigue performance to the parent material and weld metal. A significant reduction in life is shown for cross-weld specimens, indicating susceptibility to failure due to HAZ softening for matched or over-matched X100Q welds.This publication has emanated from research conducted the financial support of Science Foundation Ireland as part of the MECHANNICS joint project between NUI Galway and University of Limerick under grant number SFI/14/IA/2604 and the I-Form Advanced Manufacturing Research Centre under grant number SFI/16/RC/3872