Metallurgical Behavior Simulation And Analysis For In-service Welding On High Pressure Natural Gas Pipeline In City

With the rapid development of the economy, the demand for natural gas is becoming higher and higher in China town and the total length of the pipeline increases year by year. The technology of in-service welding is indispensable for repairing the damaged pipeline and modification of gas pipeline. It is very dangerous to welding on the pipeline transporting flammable hazardous materials, such as natural gas and oil. Therefore it is very important to research the in-service welding on damaged operating pipeline to provide theoretical basis and guidance for future work.In this paper, self-designed in-service welding experimental device was utilized, during which the natural gas was substituted by flowing water as the cooling medium to conduct the in-service multi-pass welding of a sleeve fillet weld. In accordance with the practical welding process, the parameters of double ellipsoid heat source for each bead were determined by a fitting method for heat source, and the molten pool morphology simulated was coincided well with that of welded fillet bead. Using the heat source adjusted above, the processes of in-service welding were simulated by finite element method (FEM), and the temperature fields of multi-pass fillet welds were obtained. The Leblond phase transition model was used to predict the microstructure composition and the maximum hardness of the heat affect zone analyze the effect of heat input on the phase transition of HAZ during cooling process.The results show that the heat transfer condition has a significant effect on thermal cycle characteristic at the inner wall of pipelines, but a less effect on that at the outer surface of pipelines. For the different pass sequence, it is found that a faster cooling rate will appear if a fillet weld is as the first pass, compared with the situation when a surfacing bead on the pipeline is as the first one. For in-service welding of sleeve-repair, it should be concerned that the effect of pass sequence on thermal cycling characteristics. Heat affect zone is composed of a large number of bainite and small amount of martensite. With the increase of the heat input, the amount of bainite generated is becoming larger and larger, on the contrary, the amount of martensite is reduced, the hardness of the heat affected zone decreases accordingly. It is an effective method to prevent the hydrogen induced cracking of heat affect zone to control the heat input and reduce the amount of martensite generated during cooling process.