Microstructural analysis of fracture in heat-affected zone of two ×70 pipeline steel weldments

In the previous study, different crack propagation behaviours (ductile fracture and brittle cleavage fracture) were observed in two ×70 pipeline steel weldments (13.4 and 17.8-mm-thick) during single-edge notched bend testing. To further understand these two fracture behaviours, detailed microstructures of the base metal (BM), fine-grained heat-affected zone (FGHAZ), and coarse-grained heat-affected zone (CGHAZ) of these two ×70 pipeline steel weldments have been analysed. The results show that the initial structure of the two pipe BMs and different welding cooling rates owing to different thicknesses contributed to structural variations of the correlated sub-regions of the HAZ. For both weldments, the FGHAZ close to the BM has the highest fraction of the high-angle grain boundaries, the finest grain size, the lowest local strain levels, and the highest fraction of recrystallised ferrite grains. The CGHAZ of the 17.8-mm-thick pipe welds exhibits the lowest toughness with the highest hardness, a high frequency of deformed grains, the highest local strain level, and the highest density of preferred {100} cleavage planes than the other sub-regions in the HAZ. The high density of the {100}<011> texture components in the HAZ may cause the cleavage micro-cracks to propagate toward the BM at an approximate 45° angle to the original crack plane during bending tests.