Microstructure Evolution in the HAZ of Girth Welds in Linepipe Steels for the Arctic

A critical aspect of building pipelines to transport natural gas will be development of suitable high strength steels and new economic welding procedures, e.g. dual torch welding, without compromising the pipeline’s structural integrity during its in-service performance. The objective of this project is to predict the microstructure and mechanical properties of the weld heat affected zone (HAZ) of an X80 linepipe steel as a function of its temperature-time history. The approach taken involves a combination of experimental techniques and advanced modelling approaches. On the experimental side, dual-torch weld trials for assessment of spatial and temporal variations of temperature in the HAZ were conducted. To simulate and investigate the microstructure evolutions in the HAZ, i.e. precipitate dissolution, austenite formation, grain growth and decompositions, Gleeble thermo-mechanical simulations were performed. These simulations include rapid heating and cooling tests at rates of up to 1000 °C/s. Notably, real-time monitoring of austenite grain growth was possible by using a novel laser ultrasonic technique. Further, bulk samples were produced using the Gleeble adopting the experimentally determined temperature time history. These bulk specimens were subsequently subjected to tensile and fracture resistance tests. A concise overview of these novel experimental activities, highlighting new insights, is presented and challenges associated with the measurements are discussed.