Study On CTOD Fracture Toughness Of X70M Pipeline Steel Weld Joint

Pipeline transportation is an important form to transport crude oil and gas with long distances.However,oil gas fields are generally located in remote areas where the climate is relatively harsh,requiring the pipeline with good service performance under low temperature and high pressure,especially for the favourable weldability.Compared with the base metal of the pipeline,the welded joint is the weakest portion in the pipeline,which are prone to cracks under stress,causing the safety accidents.So the adequate toughness of the welded joint is essencial.This dissertation adopt the welded joint of Baosteel's X70M grade pipeline steel as the test sample,performing CTOD testing to evaluate toughness with different welding methods.Researching into the joints of the CTOD value regularity by the microstructure and fracture morphology characteristics and choosing the best welding method.Based on the optimized welding method,the CTOD values of the welded joints with the different welding input and the tube thickness are analyzed.Finally,the distribution of CTOD values in three regions of the welded joints is studied by statistical analysis.The main results are as follows:The welding methods of pipeline steel include:RMD+SMAW?RMD+FCAW?STT+GMAW and GMAW+SAW.As the microstructure of the welded joints in the heat affected zone is relatively more complex,more attention is often needed.Through the microstructure and fracture morphology,the toughness of the heat affected zone is analyzed.it is revealed that,under the GMAW+SAW welding method,the average value of the toughness is the highest among the four welding methods.According to the uniform microstructure and the largest number of toughening structure,which is means that its toughness is the best one.The three zones of welded joints with different thickness and welding heat input are compared and analyzed by microstructures and fracture morphology.The conclusion are as followed:In a certain range,with increasing thickness and welding heat input,the grain grows coarse.But with the high content of AF and the"tempering"phenomenon of the thick joints,the M-A is resolving and the toughness is increasing.When the range of applicable welding heat input is 40-53kJ?cm^-1,the fracture toughness of the test sample with the thickness of24mm is the best.CTOD values of welded joints under GMAW+SAW method are processed in statistical methods.The corresponding density functions are established according to the various welding zones,which make the the safety assessment of welded joints much more scientific and systematic.