A Study On The Fracture Toughness And Testing Methods Of X80 Pipeline Steel

With the rapid development of economy,pipeline transport has gradually become the main method of energy transport.From economic considerations,pipeline showing the characteristics of large diameter,high pressure and large flow rate.Based on this background,due to the excellent strength and toughness,pipeline transport got rapid development.The X80 pipeline steel of the Second West-East Gas Pipeline Project found many cracking phenomenon near the welded joints.So the crack resistance of pipeline steels have a great effect on pipeline safety operation.Taking X80 pipeline steel of the Second West-East Gas Pipeline Project Xinjiang section as the object of study,this dissertation researches microstructure observation,the hardness distribution test and the uniaxial tensile test at room temperature.We also adopt standard three point bending and non-standard single edge notch tension specimens to test resistance curve of different regions in two X80 pipeline welded joints,and through micro fracture analysis to explain the fracture toughness result.From the research we found that X80 is characterized with acicular ferrite organization,which will lead to high strength and toughness.The outer welding after first internal welding lead to the higher toughness in the outside of welding,which will lead to a lower toughness.Due to the effect of welding heat input,the toughness of weld metal and heat affected zone is lower than that of base metal.In heat affected zone,the toughness of fusion zone is lowest because of the uneven characteristics,while the fine grained heat affected zone has the best toughness because of the smaller and more uniform grain size than other regions.Due to three point bending specimen are closer to plan strain state,so the resistance curve obtained from it below that obtained from single edge notch tension specimen.We obtained the strength of X80 pipeline different regions through small specimen,which can't achieved through standard specimen,means a lot to actual engineering evaluation.We also compared the results obtained through small specimen with that obtained through standard specimen.