Welding Technology Research For Girth Of Domestic X80 Grade Line Pipe

The development of domestic and foreign gas pipeline tend to pipe with high-strength grade and large diameter. Many projects will use line pipe steel of X80 grade, for example Second West-East Gas Pipeline being build, China-Burma Pipeline being planed and China-Russian Gas Pipeline. The field welding is a key factor in pipeline construction. Taking X80 grade pipe as the object of study, this dissertation researches the crack-resistance of X80 grade line pipe steel, executes the performance test and analysis of semi-automatic weld joint of X80 line pipe welded with self-shield flux cored wire. The research result provides comprehensive and systematic welding procedure schedule of semi-automatic welding technique using self-shield flux cored wire on X80 line pipe.Take in consideration that the influence of hydrogen content of filling material, pre-heating temperature, welding process and other various factors on root bead quality of girth seam of pipeline, this dissertation analyzes the crack-resistance of X80 line pipe according to actual construction condition of root weld in long-distance transportation pipeline by the slit type cracking test and implant cold crack test. The research result indicates the higher hydrogen content in cellulose electrode will gain lower crack-resistance, in case of insufficiency pre-heating temperature, the cold crack will occur. Therefore, this dissertation think for the first time that the cellulose electrode can not be permitted for welding root bead in welding construction of long-distance transportation pipeline with X80 grade line pipe. The low-hydrogen electrode and solid cored wire of low hydrogen content are of good crack-resistance. Due to the heat input of STT welding is low, the root face should be designed properly to avoid incomplete fusion on root toe.The performance test, research and analysis result of weld joint indicate, the structure of weld metal and HAZ is PF pro-eutectoid ferrite, acicular ferrite, granular bainite. The acicular ferrite interlaced in columnar zone and lattice will be found clearly in weld metal. Due to multi-pass and multi layer technique during welding, the structure on root bead is tiny, structure on cap bead is gross, and on filling bead, the tiny and gross grain will be interlaced. The strength of weld joint is matched to its of base metal. The hardness, toughness, CTOD and HIC resistance etc. can meet the safety requirement of X80 grade pipeline. Meanwhile, according to the technique requirement of X80 grade pipeline and the CTOD of weld joint, the acceptable maximum imperfect size of plane type is specified for corresponding welding construction and acceptance technical specification. This research result has been incorporated to relevant enterprise standard of Second West-East Gas Project.The research result indicates regarding this weld joint of X80 pipe, which uses welding process of low-hydrogen electrode for root bead and self-shield flux cored wire for filling & cap bead mentioned in this dissertation, each performance index can satisfy all requirements specified in relevant technical standard and design document. This welding procedure also can make sure well welding construction quality. This research achievement is being applied to the national key project"Second West-East Gas Pipeline"which is focused in the world.