Mechanism of solidification cracking during welding of high strength steels for subsea linepipe

Weld solidification cracking is an important issue in fusion welding. If undetected, the cracking defects can act as stress concentration sites which lead to premature failure via fatigue, as well as offer favourable sites for hydrogen assisted cracking and stress corrosion cracking. For welded steel products such as deep sea oil and gas transportation pipes, such defects heighten the risk of catastrophic in-service failures. Such failures can lead to devastating environmental, economic, and social damage. In this thesis, a comprehensive review of literature associated with steel linepipe and solidification cracking defects is first presented. Fluid flow prior to solidification is then observed and quantified in situ using a novel synchrotron X-ray radiography approach. The flow is dynamic at velocities up to 0.52 m/s and primarily driven via Marangoni flow. The relationship between the microstructure and mechanical properties of the welded linepipe are extensively characterised, with a new equation derived to assess fracture toughness based on the size and distribution of carbonitride precipitates. Weld residual stresses are measured both before and after linepipe expansion in the U-forming, O-forming and expansion process for the first time using a neutron diffraction technique. To further understand the fundamental mechanisms of solidification cracking during welding of high strength steels for subsea linepipe, a novel small-scale Varestraint test rig was developed for use in synchrotron X-ray imaging experiments and a Transvarestriant test rig utilised for industrial scale weldability tests. Solidification cracking during the welding of steel is observed in situ for the first time using a micro-radiography approach and the 3D crack network is rebuilt using a micro-tomography technique. It is proposed that solidification cracks nucleate from sub-surface cavities associated with: i) residual liquid high in solute and impurity concentration (hot cracks), ii) Ti (C,N) precipitated during solidification (that induce ductile microvoids). Solidification cracks then propagate via inter-dendritic hot tearing.