Study On Microstructure And Welding Technology Of X80 High Strength Steel

With the development of economy, the growing demand for oil and gas, speeds up the transportation requirements. It also stimulates the progress of the tendency that the pipeline construction of oil and gas to advanced steel, wide radius and long distance transportation. The second west-east natural gas transmission pipeline project has utilized a large amount of X80pipeline steel in China. Welding is regarded as a partially fast heating and cooling process. Because of the non-distribution heating in pipeline, the different properties of the welding joint are generated. Moreover, the differences will contribute to the possibility of the weakness of the whole project.Due to the high frequent accidents of oil leaking in pipelines, the welding production is paid more attention in order to avoid the occurrence of these accidents.This paper firstly utilizes the Gleeble-1500thermal simulation testing machine to simulate the microstructure of welding heat-affected coarse grain zone of X80steel under different cooling rate, and studies the microstructure characteristic, toughness and fractograph of these microstructure. Using the analysis results of thermal simulation experiment combined with welding metallurgy to design the welding technology of X80steel and then welding test. Investigating the micro structure characteristic and mechanical properties of welding joint, judge it whether meets the engineering requirement. The experiment conclusions are as follows:(1) The microstructure of heat-affected zone (CGHAZ) of X80steel is mainly composed of granular bainite and lath bainite. The amount of lath bainite decreases and granular bainite increases with the rising t8/5time, and the shape of lath bainite tends to be coarser and parallel. The M/A constituents in granular bainite gradually increase with the spacing decreasing and area increasing.(2) The impact toughness of CGHAZ firstly increases and then decreases with the increasing t8/5time. The granular bainite dispersed-distributs in large amount of lath bainite when t8/5time is7s, the dispersed granular bainte refines, the grain boundaries among the lath bainte and increase, and has refinment and strengthenness. The large amount of tiny dimples in the impact fractures and is typical ductile fracture mechanism.(3) Through impact experiment, the rigidity experiment and stretching experiment to test X80steel welding joint and microstructure analysis, the welding technology of100℃preheating and8.7-27.1kJ/cm heat input for can meet the requirements for X80steel of 《Mainline welding technical specification for second west-east natural gas transmission pipeline project》. Preheating100℃and heat input is at approximate13kJ/cm can provide relatively good toughness ability and combination property of HAZ. This is recommended in the pipelines installation process.It is hopeful for providing reference for welding technology design of X80high strength steel by the experimental research of this paper, improve the overall performance of X80pipeline and enhance the security of oil and gas pipelines, to ensure that the oil and gas transportation.