CVN and CTOD Toughness of Girth Weld HAZ for X80 Pipes Subjected to Gleeble Simulation

Abstract The aim of this study was to determine the major factors controlling the heat affected zone (HAZ) performance of three X80 pipe steels by means of physical simulation techniques, determination of Charpy-V-notch (CVN) impact transition behaviour and crack-tip opening displacement (CTOD) fracture toughness, in conjunction with advanced characterization of both pipe steels and simulated grain-coarsened (GC)HAZs. The first X80 pipe was JCO double-submerged welded while the other two pipes were manufactured using a spiral/helical forming mill and contained lower and higher carbon contents than the JCO pipe steel. Continuous cooling transformation (CCT) behaviour of the single cycled GCHAZs produced with a peak temperature (Tp) = 1350°C and cooling times, Δt800-500 = ∼ 1 to 100 s was evaluated. The simulated GCHAZs, consisted of a relatively coarse prior austenite grains that transformed to different fractions of mainly low carbon lath martensite/lower bainite or fine lath bainite as well as mixtures of upper bainite and/or granular bainite as a function of increasing cooling time. A consistent trend of decreasing microhardness with increasing cooling time was observed for all three pipe steels. Advanced characterizations by means of scanning electron microscopy with electron backscattered diffraction and transmission electron microscopy was used to analyze features of the transformation products, fraction of high angle boundaries (HAGBs) and the nature of microconstituents, including carbonitride precipitates and inclusions. Gleeble® Simulated GCHAZs with cooling times of t8-5 = 5 and 15 s were assessed by means of CVN impact energy transition curves, which revealed shifts towards higher temperatures relative to the pipe base metals. From the energy transition temperatures (ETT) it was evident that JCO X80 pipe steel exhibited highest impact energies and the lowest ETT values followed by the high carbon X80 and low carbon X80 spiral/helical pipes. It is noteworthy that for the high carbon X80 pipe, despite having a richer chemistry and lower base metal CVN impact toughness properties, the CVN toughness of the GCHAZ was dramatically improved relative to the low carbon X80 spiral/helical pipe. Interestingly, the CTOD fracture toughness results for the tested pipe steels were more consistent with fully ductile behaviours observed. The GCHAZ CTOD fracture toughness results revealed a similar trend as the GCHAZ CVN results, with lower fracture toughness observed for the low carbon X80 spiral/ helical pipe. Decreases in transformation temperature promoted formation of lath aggregates with different crystallographic variants in GCHAZ microstructure that contained a high fraction of high angle grain boundaries and exhibited improved toughness performance. Other factors contributing to the variations in impact and CTOD fracture toughness of the respective GCHAZs were the differences in the amount of high carbon microphases, hardness, and the presence of Ti-N precipitates found in cleavage fracture initiation sites for the low carbon X80 pipe steel.