Line Pipe HAZ Softening Susceptibility and Response

Abstract Unexpected girth weld failures have occurred in newly constructed pipelines with acceptable line pipe materials and weldments (i.e., API 5L and 1104 compliant) and yet these welds fabricated without significant discontinuities failed in a ductile manner with strain localized in the weldment region. Heat affected zone (HAZ) softening was identified as one of the contributing factors in these failures. This paper presents the result of a research project developing a “simple” HAZ softening susceptibility test (i.e. easier to do right than wrong) that may be used identify materials prone to HAZ softening. The test developed in a research project supported by the Pipeline Research Council International (PRCI) is intended as a line pipe material susceptibility test rather than a weld procedure acceptance test which could consider the impact of the heating cycles of multi-pass weld procedures. The test development included the optimization of the welding procedure, sample configuration and support to produce repeatable results, as well as the definition of a testing procedure susceptibility metric considering local maximum and average through thickness softening. The performance of the testing procedure was demonstrated by considering 20 steels, ranging from X52-X70, steel chemical compositions and 1963–2020 manufacturing dates, designed to represent a range of HAZ softening susceptibilities. The development work demonstrated that the potential for HAZ softening cannot be exclusively related to steel chemistry and that the magnitude of softening is correlated to the size (e.g., width) of the HAZ. The single pass weld test developed and described in this work successfully demonstrated the relative susceptibility of line pipe steels. In addition, Cross weld tensile testing employing digital image correlation was used to illustrate the process of strain accumulation and localization promoted by HAZ softening and is presented in this paper to describe the failure process of affected materials and estimate their residual strength. The single pass weld test results presented may be used in a material acceptance test or identify materials that require attention in to weld application or alternate welding procedures. These results provide opportunities to learn and reduce the risks associated with HAZ softening and this developing sound weldments that can better resist axial load.