Heat Affected Zone Softening Susceptibility Test

Abstract

Abstract There have been a number of unexpected girth weld failures in pipelines, both in-service and during pre-service hydrostatic testing. Investigation of these incidents indicated that the line pipe met industry standard requirements, such as API 5L and the welding procedures had been qualified to API 1104 and construction welding/inspection had been completed according to industry standards. The investigation of these failures indicated that they were not related to hydrogen cracking or misalignment, however, line pipe chemical composition and tensile properties were identified as having contributed to these failures. Low carbon and low carbon equivalent (CE) higher strength line pipe materials produced with thermo-mechanical controlled processing (TMCP) practice is believed to have contributed to the failures. Higher heat input welding in these lean chemistry steels can result in heat affected zone (HAZ) softening that produces a lower strength zone adjacent to the girth weld, making it susceptible to tensile failure from axial loading. In the presence of thermal or geohazard loading, these softened or lower strength HAZ’s become a concern where there has been pipe settlement or in strain-based assessment. To understand the factors that influence the susceptibility to HAZ softening and identify those materials that can be considered at risk, a material testing procedure has been developed. This new testing procedure is a weld bead-in-plate test that was designed to maximize the potential for HAZ softening in an effort to identify those line pipe materials that may be susceptible to HAZ softening in construction or during in-service welding. This paper presents the results of experimental trials comparing the susceptibility of leaner chemistry materials to HAZ softening. The testing procedure is described along with opportunities to enhance the procedure to develop a standardized test for HAZ softening assessment. This test has the potential to be used to identify materials at risk to HAZ softening and may also be used to qualify line pipe to demonstrate its resistance to HAZ softening.