The restraint intensity （RI） of the pipeline girth welding joint was investigated using finite element method and experimental method to predict the cold cracking susceptibility of pipeline steel. The distribution of RI along the girth weld was investigated to study the influence of welding position on the RL Subsequently, the effects of outer diameter （OD） and wall thickness of pipeline on the RI were studied with ABAQUS software. The results show that the RI is almost independent of welding position. The RI increased with the increasing wall thickness but decreased with the increasing OD. A prediction model of Rl was developed based on the effects of the OD and the wall thickness. It has been found that the predicted RIs were in good agreement with the experimental values. The maximum fractional error between the predicted RI and the experimental values was just about 10%. h was indicated that the errors were mainly caused by the heterogeneous of the weld bead.
The intercritical heat-affected zone（ICHAZ） of X80 pipeline steel was simulated by using the Gleeble-3500thermal/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 cooling 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.
