Research On Fracture And Fatigue Of X70Pipeline Steel Weld Joint

As an important way to transport oil and gas, construction of pipe workdepends on the research and development of pipeline steel. Because of itsgood mechanical properties, X70pipeline steel has been widely used in theoil and gas pipelines. There is a need of knowledge of pipeline residualstrength and life assessment to ensure the safe operation and life-saving ofthe pipeline, besides, manufacturing of high strength and toughness pipe,design of pipe work can’t be achieved without fracture, security andreliability theory. The particularity of the transported substance in pipelinemay lead to catastrophic accidents, so pipe work and pipeline steel has beenpaid great attention.Based on having learnt research status and mechanic test methods aboutpipeline steel and weld joint, making use of available research instruments,fracture toughness and low cycle fatigue testing are applied to both X70pipeline steel and its weld joint, to provide some experimental foundation forunderstanding the fracture and fatigue characteristics of materials in thesespecific location. Metallographic observation helps to analysis the affectionof organizations on fracture toughness properties.Considering the pipe shape and the actual direction of weld, two kindsof three-point bend specimens are designed. Crack tip opening displacementtesting technology is applied to test fracture toughness of metal in weld jointand different regions of the heat affected zone at-10℃. The results are asfollows, the weld metal has significantly stable crack extension region, goodtoughness with an average value of0.346mm, showing uniform mechanicalproperties; With the discrete test results proving obvious disparity in theorganization of heat affected zone, there is a need for further discussion of thedifference between the heat-affected fracture toughness of fine-grained andcoarse-grained region; the heat affected fine grain region has good toughnesswith an average CTOD value of0.346mm; heat affected coarse grain regionhas initial unstable crack propagation during the test, lead to far smaller fracture toughness than that of the weld metal and other heat affected zone;metallographic analysis show that form of organization and grain size haveaffection on the fracture toughness.Constant section specimen is designed to test low cycle fatigueproperties of X70steel and weld joint. According to the cyclic stress-straincurve and strain-life curve, it’s found that mechanical properties of weldjoint is weaker than that of pipeline steel’s.