Microstructure and Mechanical Properties of Girth Weld HAZ in X80 Line Pipe With High Deformability for Strain Based Design Applications

Abstract

X80 line pipe with high longitudinal deformability (X80HD) has been developed and applied in the Strain Based Design (SBD) of pipelines in harsh environment such as seismic areas, permafrost areas, fault zones, etc. For SBD pipelines it is critical that the pipeline girth welds overmatch the tensile properties of the pipe material to avoid local strain accumulation in the girth weld during a strain event. Also, it is important that pipeline girth welds that may experience high strains in operation have sufficient toughness to ensure adequate resistance to failure by fracture. The objective of this research was to gain a better understanding of the influence of chemical composition and essential welding variables on microstructure and properties of the HAZ regions formed in X80HD pipeline girth welds. In this study, by using the weld thermal simulation approach, the peak temperatures (Tp, representative of the distance to the fusion boundary) and the cooling times, particularly between 800 °C and 500 °C (t8/5, representative of the weld heat input), identical to those occurring in the girth weld HAZ of three different X80HD pipe steels, were artificially reproduced. It should be noted that t8/5 is influenced by both heat input and preheat temperature. The weld peak temperatures, Tp, from 500 °C to 1300 °C, in 100 °C increment, whereas the cooling times t8/5 from 5 to 30 seconds were in 5, 15, and 30 seconds, associated with the heat input range of self-shielded flux cored arc welding (FCAW-S). The thermal simulation specimens on tensile properties, Charpy impact toughness, and Vickers hardness were tested and analyzed. Microstructures of these simulated HAZ were characterized by optical microscopy (OM) and scanning electron microscopy (SEM). Finally, the actual FCAW-S girth welding experiments were carried out. These girth welds were subjected to different testing for evaluation of microstructure and mechanical properties of X80HD girth welded joints. These included transverse weld tensile testing, microhardness map of the weld joint, Charpy V-notch impact testing of weld metal and HAZ, and microstructure analysis. The results demonstrated that softening occurs in the fine grained HAZ (FGHAZ) and the inter-critical HAZ (ICHAZ) of X80HD line pipe girth welds. The severity of HAZ softening depends on the steel chemistry and the heat input applied during girth welding. The metallurgical design of the X80HD pipeline steel and the optimization of the girth welding procedures were proposed.