A Study On The Fracture Toughness And Testing Methods Of Pipeline Steel Weld Joints

Pipeline transportation,as one of the most important means of oil and gas transport,plays an important role in the modern energy and economic system.With the economic development and the increasing energy demand that follows,higher requirements have been put forward for the performance of pipeline steel.To ensure the fracture resistance performances in the pipeline design and integrity assessment process,it is necessary to accurately evaluate the fracture toughness of the pipeline steel.It is also theoretically and engineeringly significant to study on the more simple and effective pipeline steel fracture test methods.In the context of the Second West-East Gas Pipeline Project in China,this study uses the fracture mechanics method and microscopic analysis method to evaluate the fracture toughness of X70 and X80 installation weld joints.Miniature-specimen was used to obtain the precise strength distribution in the weld joints.The fracture properties are determined for various regions of the pipeline weld joints,and the influence factors of fracture toughness differences in different regions in weld joints are obtained,which provide important theoretical support for the process control of pipeline manufacturing installation welding process.At the same time,the fracture test methods of pipeline steel were analyzed and studied.The standard three-point bending fracture test(SENB)was performed using the unloading compliance(UC)method and normalization(NM)method and the application of two methods in different regions of welded joint are discussed.For the first time,the NM method was applied in the non-standard single edge-notched tension test(SENT),and a complete set of UC method and NM method parameters in SENT test were summarized and modified,and the application of the two methods at different depths was obtained.Through the study,it was found that grain size and M/A second phases grow occurs in the coarse grained heat affected zone and the weld metal on account of the welding heat input,which greatly impaired the fracture toughness.Due to the difference of the M/A second phase and inclusion content in different regions of weld joints,different fracture features are exhibited during the fracture process.Comparing the UC method and NM method in SENB tests,it is found larger difference emerges in the regions with ductile-brittle fracture features.Through the SENT tests using UC method and NM method under different crack depths,it is found that the UC method has a large error in crack length prediction under the shallow crack depths,and the NM method has better consistency and accuracy in both deep and shallow crack depths.