Stress Corrosion Behavior And Damage Fracture Of Pipeline Steel

For long distance transportation,pipelines inevitably pass through geographically complex areas,so the dual-phase large deformation pipeline steel has been developed,which has good deformation ability on the basis of meeting the strength and toughness indexes of traditional pipeline steel.The research on stress corrosion failure and damage fracture of pipeline is directly related to the safe service and fracture control of pipeline steel.In this paper,the stress corrosion behaviors of granular bainite and dual-phase B+F X80 pipeline steel in simulated near-neutral soil solution were compared by slow strain rate test(SSRT)at different strain rates and polarization test at fast and slow scanning rate;the corrosion behaviors and SCC mechanism of B+F dual-phase pipeline steel in acid(p H=4.5),near-neutral(p H=7.8)and alkaline(p H=9.5)simulated soil solution were studied by polarization test,immersion test and SSRT;The initiation and development of voids in B+F X80 pipeline steel were studied by means of statistical analysis of the thickness profiles of necking zone or fracture of the specimen after the specimens were stretched to different strain points(14%?20%?fracture)in the strain range from the maximum force close to the engineering stress-strain curve to fracture.The conclusions are as follows:(1)In simulated near-neutral soil solution,the SCC sensitivity between granular bainite and B+F large deformation X80 pipeline steel was evaluated by using critical strain rate.The critical strain rate of B+F pipeline steel is slightly smaller than GB pipeline steel,so the SCC sensitivity of B+F pipeline steel in NS4 solution is slightly lower than GB pipeline steel.The crack propagation direction of the necking zone surface of granular bainite and B+F X80 pipeline steel both in NS4 solution is about 45° with the tensile axis direction.The cracks of B+F pipeline steel propagate in the transgranular manner.(2)In acid,near-neutral and alkaline simulated soil solution,the corrosion rate of B+F X80 pipeline steel in polarization test is consistent with that in immersion test.The corrosion rate in near-neutral and alkaline simulated soil solution is similar,but slightly higher in alkaline solution.The corrosion rate in acidic simulated soil solution is significantly higher than that in near-neutral and alkaline solution.After immersion test,the surface of the specimen in acidic solution has a layer of corrosion products which is firmly bound to the substrate but not dense.Corrosion pits on the surface of specimens in near-neutral and alkaline simulated soil solution are mainly found in bainite area and inclusion area.The SCC sensitivity of B+F pipeline steel in acid,near-neutral and alkaline simulated soil solution is as follows: in alkaline simulated soil solution > in acid simulated soil solution > in near-neutral simulated soil solution.In near-neutral and alkaline solution,the corrosion pits will be the initiation sites of stress corrosion cracks on the surface of the necking zone.The cracks on the surface of the necking zone of specimen in acid,nearneutral and alkaline simulated soil solution propagate in two stages,and are divided into two types: turning and no turning.In the first stage of crack propagation,the joint mechanism of micro-dissolution and micro-shear is found in acid,near-neutral and alkaline simulated soil solution.In the second stage of crack propagation,the joint mechanism of micro-dissolution and micro-shear is found in near-neutral and alkaline solution,while the hydrogen induced cracking under shear stress is found in acid solution.(3)The voids on the thickness profile of necking zone or fracture of B+F X80 pipeline steel specimen can be divided into three types according to the size: small void < 1?m,medium void 1?5?m,large void > 5?m.Medium and large voids are caused by inclusions.The occurrence locations of inclusions are: F-F grain boundary,B-F phase boundary,F grain intersection angle,in F and B.The inclusions can be divided into two categories: spherical or quasi-spherical inclusions include Al-Mg-Ca-O inclusions and Mn-Ca-S inclusions;triangular or quadrilateral plate inclusions are Fe-Si-C-O inclusions.On the thickness profile of the necking zone of the fracture specimen,it is generally indicated that the closer to the necking center,the more the number of small voids is.The porosity in the necking center(namely the fracture)of the specimen is about 0.038%?0.517%.Small voids appear in the following locations: in B,in F,F-F grain boundary,B-F phase boundary,interface between M-A island and matrix.Small voids are probably formed mainly by the separation of small spherular precipitates from the matrix(surrounding phase).