Characterization of Crystallographic Texture of ERW Welded and Heat-Treated API X70 Pipeline Steel

The impact toughness of high frequency electric resistance welded (ERW) line pipe depends on the steel chemical composition, welding procedure, and post-welding heat treatment. Among several microstructural factors that may influence the impact toughness of high frequency electric resistance welded bond line, the crystallographic texture factor is often assumed, but not sufficiently studied. The evolution of texture during high frequency electric resistance welding and simulated post-welding heat treatments (PWHT) of API X70 pipeline steel was characterized using X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). Results of a texture factor calculated using the θ/2θ XRD scans indicated that following the post-weld normalizing treatment, the texture factor of {100} planes for the weld bond line appeared to have increased with the increase in post-weld normalizing temperature. The texture factor results obtained from θ/2θ XRD scans were further verified by using the EBSD on the CVN tested samples in the regions near the fracture path. The XRD texture factor was also used to correlate the evolution of crystallographic texture and Charpy toughness for the PWHT samples. Based on the observations from both XRD and EBSD, the {100} crystallographic orientation of planes in the notched direction correlates well with the cleavage fracture planes of the Charpy impact tested samples. Therefore, the post-weld heat treatment should be designed to minimize any preferred crystallographic texture with {100} planes in the bond line, to promote ductile fracture and improved bond line impact toughness at low temperatures.