Fracture Toughness Of X80 Pipeline Steel Under Tensile Predeformation

Oil and gas energy play a significant role in daily life and industrial manufacture.They are important strategic materials of national economy,the comprehensive development of oil and gas industry are promoted with the growth and structural optimization of energy adjustment.The local demand of oil and gas is growing with the increasing volume of the world economy,science and technology development and progress which made wide range of oil and gas energy use since the 21 st century.According to the oil and gas transportation experience and actual situation at home and abroad,pipeline transportation has become the main oil and gas energy transportation mode in the world due to its economic,practical,safe and efficient advantages.Compared with other methods,pipeline transportation has the advantages of large throughput,low cost,high safety,convenience and high efficiency.At present,in the case of increasing distance of oil and gas transportation and the geological environment is becoming more and more complex and changeable,it is easy to cause safety accidents of pipeline steel in service.This is because the change of geological environment or the installation and construction will cause the deformation of pipeline steel and the change of its mechanical properties,which may lead to large cracking of pipe cracks of the large pipe crack and cause significant safety accident.In case of major safety accidents,it is necessary to ensure that the pipeline steel has high and stable fracture toughness.X80 pipeline steel is widely used in China's west-east gas transmission project,which involves a wide range,many lines and a long distance.Therefore,in the service process of X80 pipeline steel,the above phenomenon will also occur: local deformation occurs due to sudden change of internal pressure or external force on the pipeline.In order to study the influence of the deformation on the fracture toughness and crack growth mechanism of X80 pipeline steel,this paper tests the fracture toughness of X80 Pipeline Steel under four different tensile pre deformation conditions by tensile and single specimen unloading flexibility method.In this paper,when carrying out the fracture toughness test,the single sample flexibility method is adopted,which greatly improves the test accuracy and can reduce the test cost compared with the multi sample method.After the test,metallographic structure of the sample and the deformation area at the crack tip of SEB sample were observed and analyzed.The results show that the yield strength and tensile strength of the specimen increase and the yield strength ratio also increases with the increase of pre deformation while the fracture toughness values ?0.2 and J0.2 decrease gradually and the fracture toughness becomes worse.There are more secondary cracks and void defects in the deformation zone at the crack tip,and the delamination phenomenon appears.The specimen is more prone to brittle fracture.