Research On Fatigue Property Of Pipe Body And Weld Joint Of Three ERW Pipe

ERW(electrical resistance welding)pipes are widely used in oil and gas transportion,oil well casings and structural pipes due to their high production efficiency,excellent weld microstructure,high dimensional accuracy and low cost.In the long-distance transportation process of oil and gas,fatigue fracture is one of the common failure modes.Studying the fatigue performance of ERW welded pipe has great significance for improving the service life of pipeline steel.In this paper,tensile testing machine,micro hardness tester and metallographic microscope were used to test the mechanical properties and metallographic structure of the tubes and welds of L360,L415 and X70 ERW welded pipes,and use MTS-Landmark 370.The servo-hydraulic fatigue testing machine tested the fatigue performance of three kinds of welded pipe surface treated and untreated welded pipe and base metal.The results show that the mechanical properties,fatigue properties and microstructure of X70 are the best,followed by L415;the overall mechanical properties of the weld and heat affected zone are lower than that of the base metal;the metallographic structure is ferrite + pearlite,and the X70 contains a small amount.In acicular ferrite,the grain size gradually increases from the base metal to the weld,and the band-like structure appears in the heat-affected zone due to element segregation;under the same stress conditions,the fatigue life of the base metal is 0.5-2 times higher than that of the weld.The surface treated base metal and weld bead are 1-3 times higher than untreated;improving the microstructure and mechanical properties of the welded joint is necessary to increase the fatigue performance of the welded pipe.The fatigue cracks and fracture morphology of the three welded pipes were analyzed by X-ray diffraction(XRD),scanning electron microscopy(SEM)and electron backscatter diffraction(EBSD).The results show that the surface cracks and fatigue cracks of the untreated base metal and weld are all caused by surface defects;the surface cracking and expansion of the surface treated and untreated weld specimens occur in the weld and heat with weak microstructure and properties.Affected area;the fatigue cracks of the base metal,surface treated base metal,weld and surface treated weld of the three welded pipes are similar in the fracture area of the stable expansion zone and the unstable expansion zone,so the fatigue life of the ERW welded pipe mainly depends on the fatigue The crack initiation life and the improvement of the surface quality of the welded pipe can effectively improve the fatigue life.