Effect of Austenite Microstructure and Composition on Transformation Behaviour in The Heat Affected Zone of Pipeline Steel

Pipelines represent a challenge in oil and gas distribution in remote places with extreme weather. It is expected than in the coming years, the energy consumption will progressively increase to 28% by 2040. In this trend, the natural gas will increase 1.4% every year. Due to that scenario, pipelines are crucial to achieve the production improving mechanical properties in the steel without diminishing the toughness. The aim of this research is to analyse the effect of precipitates of niobium carbonitrides on the microstructure, in addition to the tensile strength, elongation and toughness in X80 steel. This research utilised five different austenitising temperatures between 900°C to 1300°C, together with three different cooling rates of 0.5°C, 5°C and 50°C following each of the austenitising treatments. The evolution of microstructure was analysed using optical microscopy, SEM and EBSD. The precipitation was evaluated using carbon extraction replicas in the TEM.