| With the rapid development of China's national economy, the need for engergy sources, especially for petroleum and natural gas which are directly related to national security and the people's welfare has been greatly increased. Pipeline, due to its numerous advantages such as safe, efficient, engery-saving, non-stop and economic, has become the priority option to transport oil and gas. Pipeline transportation has been so rapidly developed worldwide that the total length of oil and gas pipelines in the world has reached (200-300)×104 km and increased annually by the rate of 2%-3% to date. Because China doesn't get a head start in pipeline construction, the total 6×104 km pipelines now in China apparently can't live up to the rapid development of national economy. In recent years, the world-famous national priority project -- West-to-East Gas Transportation Pipeline as well as the Second Line of West-East Gas Transportation Pipeline which is still under construction are the typical examples of China's strategy to develop the Great West, adjust national energy source structure, and to develop a low-carbon economy.In those pipeline projects, advanced high strength pipeline steel and automatic welding technologies have been adopted. The characteristics of current pipeline construction are long distance, high pressure, large contents, mostly in harsh natural environment like extremely cold and seismic areas. High strength and tenacity pipeline steel are widely used in pipeline construction under high-moisture weather conditions, deep water or under the sea. In combination with the urgent needs of X80 high strength pipeline steel pipeline construction in winter or cold areas, this paper focuses on the research on cryo welding technologies which has been of great theoretical and academic significance as well as great applied value in giant projects.X80 pipeline steel is the major material used in the research with proper supplement of X60, X65, X70. Comprehensive research have been done on the Charpy impact toughness under 0℃, -10℃, -20℃,-30℃,-40℃as well as the SEM (Scanning Electron Microscope) analysis of the fracture microstructure in order to sum up the pattern of toughness variation of those different pipeline materials in cold weather conditions. In addition, Y-groove cracking tests have been done on the four materials under 0℃, -10℃, -20℃respectively to sum up their cold crack sensitivity in cold weather conditions. The results prove that the crack resistance of the pipeline steel in cold weather conditions could be guaranteed with certain necessary measures. Therefore, the make-up of cryo welding procedure specifications (WPS) has got important experimental verification and foundation.Various welding experiments have been done on the girth weld of the four afore-mentioned pipeline steel under -20℃as well as mechanical property tests on the welded joints. The results show that the cryo welding joints of the four pipeline materials all meet the technical standards of Q/SY GJX 0110-2007.In accordance with the practical pipeline construction, PQR of X80 pipeline steel under -20℃, metallographic microstructure and SEM analysis as well as mechenical property tests of the welded joints have been finished. WPS of X80 pipeline steel under -20℃has also been worked out on the basis of the research which has been successfully applied on the pipeline construction in the cold areas of North China. |
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