Effect of Base Metal Microstructure on Softening Behavior of the Heat-Affected Zone of X80 GMAW Girth Weld

Softening behavior in the heat-affected zone (HAZ) of two X80 pipeline girth welds with different base metal microstructure, i.e. acicular ferrite (AF) dominated (X80-AF) and granular bainite (GB) dominated (X80-GB), were investigated in the present study. Hardness tests, transmission electron microscope (TEM), and electron backscattered diffraction (EBSD) were employed to analyze the softening behavior and corresponding microstructural evolution in the HAZ. The results indicated that softening in the HAZ of two girth welds primarily occurred in the fine-grained (FG) HAZ, while hardening was found in the coarse-grained (CG) HAZ. Due to its high dislocation density and refined interlocking structure, AF could effectively inhibit phase transformation and grain growth during reheating which resulted in smaller grains and lower softening rate in the FGHAZ. In contrast, coarse GB in the base metal was more prone to grain coarsening and hence engendered more pronounced softening. Therefore, for the microstructural design of high strength pipeline steels, increasing the proportion of refined AF is beneficial to the softening resistance and thereby elevates the service safety of pipelines.