Role of Microstructure and Heat Input on Crack Nucleation and Propagation in Submerged Arc Welding of API X70 Pipeline Steel

Abstract

Weld toe crack failure in submerged arc welding of API X70 PSL2 pipeline steels represents an important defect; the underlying mechanisms, especially the role of heat input under real manufacturing conditions, are not well understood. Here we study the process conditions that minimize this phenomenon while maintaining compliance with PSL2 specifications and at the same time perform analysis aiming to advance the understanding of the mechanisms involved. Two API X70 PSL2 steel plates 15.6 (0.614 in) and 18.7 mm (0.736 in) in thickness were welded using SAW with nine different HI values (192-287 kJ/in). Characterization included chemical and microstructural analysis, Vickers hardness testing, Charpy V-notch impact tests at 0 ℃, guided bend tests, and scanning electron microscopy fractography. Optimal HI of 219 kJ/in produced microstructures with a mixture of allotriomorphic ferrite (AlF), Widmanstätten ferrite (WF), and acicular ferrite (AF), achieving toughness up to 451.5 N-m (333 lb-ft), no cracks in guided bend tests and featured a ductile fracture surface. A critical AlF grain size threshold of 10 μm was identified: grains greater than 10 μm promote brittle propagation. Oxygen content variations of 25-32 ppm significantly altered AF grain size (13-14 ASTM grain size numbers). Paradoxically, the presence of WF increased toughness despite serving as crack nucleation site. Cracks nucleate at WF tips due to geometric stress concentration, then propagate either in ductile manner through AF or brittlely through AlF. Weld toe crack failure originates from elastic incompatibility between weld deposit and heat-affected zone (HAZ).