Research On CTOD Fracture Toughness Of X70 Pipeline Steel Welded Joints

With good economical effects and the high safety factor,pipeline transportation which is one of the best ways for transporting oil and gas,has become an essential part of the development of industry in our country.As a significant step of the pipeline project,welding has got decisive influence on not only the work efficiency of the pipeline construction but also the reliability of the pipeline industry.However,as an impact of welding thermal cycles,it is easy to find cracks on the welded joints of pipeline steel,where fractures tends to happen.Common techniques in pipe-making and field work,such as longitudinal submerged arc welding(LSAW),shielded metal arc welding(SMAW)and gas metal arc welding(GMAW),are applied in this paper.Fracture toughness of weld metal(WM),fusion line(FL)and FL+2 in X70 pipeline steel welded joints are investigated by three-point crack tip opening displacement(CTOD)test at-10℃.The experimental results show that the CTOD fracture toughness of longitudinal submerged arc welded joints is pretty good;and there may be pop-in effect at the shielded metal arc welded joints;besides,the CTOD eigenvalue of weld seam in the gas metal arc welded joints is not so large.According to the CTOD results for welded joints produced by different welding techniques,the relationship between microstructural changes that affected by welding thermal cycles and CTOD fracture toughness in X70 pipeline steel welded joints,are investigated by hardness distribution test,metallographic observation,fracture morphology observation and EBSD technology.The research revealed the main factors which influence the CTOD fracture toughness of X70 pipeline steel welded joints.Moreover,detailed explanations of the reason why the CTOD eigenvalue of fusion line of X70 pipeline steel welded joints turns to be unstable are given.