Characterization of mechanical properties, fatigue-crack propagation, and residual stresses in a microalloyed pipeline-steel friction-stir weld

Highlights: • Structure–property relationships of friction-stir welded pipelines are investigated. • Microstructures in the friction-stir welds vary significantly from base metal. • Residual stresses are quite low compared to the base metal yield strength. • Fatigue crack propagation is improved in weld material due to residual stresses. • A simple engineering model of friction-stir weld strain hardening is provided. - Abstract: The influence of the friction-stir welding process on microstructure and mechanical properties of API 5L X80 skelp was investigated. Friction-stir welds were produced using welding parameters optimized to promote weld toughness. The solid-state welding process produced microstructures that significantly varied from those observed in the base metal, namely the redistribution and resizing of Martensite–Austenite constituent in the heat-affected zone and stir zone regions of the welds. Mechanical properties of the welds and base metal were evaluated with uniaxial tension testing and microhardness testing revealing overmatching welds and a hard zone within the weld stir zone. Residual stresses were determined in several directions with respect to the joint revealing that stress in the longitudinal direction is highest, yet well below material yield strength. Fatigue-crack propagation behavior was characterized in the different weld regions and base metal by testing with the compact tension specimen configuration showing that welds have impeded fatigue-crack growth compared to the base metal mostly due to welding-induced residual stress fields interacting with the crack.