Weldline Charpy toughness improvements on High Frequency Induction (HFI) welded pipes

Typical oil and gas company pipeline specifications for HFI pipes transporting hydrocarbons require minimum weldline Charpy impact values at -20oC and the trend is towards lower specified test temperatures. Whilst such requirements can be largely satisfied by Tata Steel's 20" HFI Pipe Mill at Hartlepool, the occurrences of sporadic low values have made it difficult to guarantee these properties with a sufficiently high level of confidence and this is potentially losing business for Tata. Welding trials at the Hartlepool 20" Pipe Mill have shown that pipes with nominally similar weldline microstructures (as evaluated by optical metallographic techniques) can have very different levels of toughness, as measured by the Charpy V-notch impact test. The propose work would characterise the weldline microstructure to a greater extent taking into account such microstructural components as texture, (sub) grain size, dislocation density, grain boundary carbide size and distribution and inclusion content using a variety of investigative techniques as required, to achieve a reliable correlation with weldline Charpy toughness. Following on from this, the relationships between welding parameters during the production of HFI pipe and the resulting bondline microstructures require definition in order to manufacture pipe with consistently good weldline Charpy toughness. It is envisaged that Phase 1 of this work will be carried out in the laboratory to establish, develop and explore the fundamental relationships between welding parameters, weld microstructure, weldline inclusion levels and bondline toughness followed by Phase 2, a verification stage of works trials on a commercial mill.