Microstructure and Mechanical Properties of Intercritical Heat-affected Zone of X80 Pipeline Steel in Simulated In-Service Welding

The intercritical heat-affected zone (ICHAZ) of X80 pipeline steel was simulated by using the Gleeble-3500 thermal/mechanical simulator according to the thermal cycle of in-service welding. The microstructures of ICHAZ with different cooling rates were examined, and the hardness, the toughness and corresponding fractography were investigated. Results show that untransformed bainite and ferrite as well as retransformed fine bainite and martensite-austenite (M-A) constituents constitute the microstructure of ICHAZ. The two different morphologies of M-A constituents are stringer and block. Second phase particles which mainly composed of Ti, Nb, C, Fe and Cu coarsened in ICHAZ. Compared with normal welding condition, the toughness of ICHAZ is poor when the cobling time is short under in-service welding condition because of the large area fraction and size of M-A constituents that connect into chains and distribute at the grain boundaries. The Vickers hardness of ICHAZ that decreases with the increase in the cooling time is independent with the area fraction of M-A constituents.