Study on Dissimilar Metal Welding Between SAF2205 and X65 Pipes

Abstract

Kela-2 Gas Field is the main supply for the West-East Gas Pipeline which runs across China with length of 4000 kilometers. Natural gas from Kela-2 contains CO2/H2S sour components and 10% condensation water, which makes the medium present very strong corrosivity. In avoidance of corrosion failure, DSS SAF2205 line pipes were introduced to transmit the gas from the Gas Field to gathering center and processing factory. The DSS pipeline is totally about 13km and it is the first time DSS pipes were used largely amount in oil and gas pipeline around the world. As it is known, DSS SAF2205 pipes and traditional linepipes X65 were welded together between the gathering lines and mainline. Properties of welding joints, especially corrosion resistance, are very important for the safety of the pipeline. In this paper, the dissimilar metal joints were welded by Metal Inertia Gas Welding (MIG) with ER2209 electrodes to study and optimize welding procedure. Optical microscope, scanning electronic microscope and energy dispersive spectrometer (EDS) were employed to observe microstructure and analysis alloy elements distribution in the jointing zone between the dissimilar steels. Along the dissimilar joint, the microstructure changes from duplex characteristic combination of Ferrite and Austenite to that with Ferrite and a few Pearlite or MA. Smooth and notch tensile test and low temperature impact tests were also used to investigate mechanical properties of the dissimilar joints. Corrosion resistance of dissimilar metal joints were tested and analyzed. Weld metal produced with ER2209 presents excellent resistance to general corrosion in 1M NaCl solution. It is found that the ER2209 electrode is more suitable for producing the dissimilar weld joint according to this job.