The effect of high Nb linepipe steel chemistry on girth weld heat-affected zone (HAZ) softening and toughness

In this his study, the effect of high niobium alloying (Nb > 0.07 wt.%) on heat-affected zone (HAZ) softening and toughness of girth welds in linepipe steel were investigated. Laboratory heats were cast with varying concentrations of C and Nb and then thermo-mechanically controlled processing (TMCP) rolled to the desired thickness. Physical simulation of both unaltered and reheated HAZ microstructures was performed for different cooling rates and the HAZ softening was compared between the different chemistries. Further experiments involving girth welding simulation using multi-pass gas metal arc welding (GMAW) were performed on flat plates with selected – low C, high Nb – composition. Through macro hardness mapping of weld cross-sections, as well as Charpy impact V-notch and Crack Tip Opening Displacement (CTOD) testing, it was observed that the HAZ exhibited a limited level of softening and maintained excellent toughness.