Influence of martensite-austenite (MA) on impact toughness of X80 line pipe steels

Varying cooling rates are used to investigate the influence of martensite-austenite (MA) microconstituent on the mechanical properties of X80 linepipe steel after intercritical reheating. It is shown that air and water cooling forms MA, while furnace cooling does not. The increase in cooling rate decreases carbon diffusion to promote MA during air or water cooling. Faster cooling using water results in a change of MA morphology from slender to blocky. Both water and air cooled samples exhibit poor impact toughness in comparison to furnace cooling, with lower Charpy impact toughness occurring with a high percentage of blocky MA following water quenching. It is shown that MA deteriorates toughness by facilitating debonding, cracking and crack initiation during impact testing.