Study On Corrosion Fatigue Performance Of X65 Steel Considering The Influence Of Welding Residual Stress

It is inevitable to introduce welding residual stress in subsea oil and gas pipeline before and during pipeline laying.At the same time,as it is in the corrosive environment of seawater all the year round,the fatigue failure often occurs under the synergistic action of cyclic stress and corrosive environment,that is,corrosion fatigue.Based on X65 steel which is commonly used in oil and gas pipeline and considering the influence of welding residual stress,this paper studies the corrosion fatigue crack propagation performance of this steel,mainly from the following three aspects.In the first part,the process and distribution of welding residual stress are described by welding thermal simulation.The finite element analysis software ABAQUS was used to describe the welding process of steel plate by simulating the welding heat source based on the DFLUX subroutine of double ellipsoid body heat source and thermoelastoplastic method.Besides,the sensitivity analysis was carried out for the related parameters that affect the residual stress of welding,the results showed that the residual stress of steel plate is sensitive to welding speed and arc efficiency;it is not sensitive to radiation coefficient and ambient temperature.Finally,the residual stress distribution of CT specimen after welding was obtained by using the method of CT specimen cut after welding.The second part develops the corrosion fatigue test program considering the influence of welding residual stress.Based on the YYF-50 corrosion fatigue tester,the problem of data fluctuation in the corrosive solution environment was reduced by applying coated insulating glue,and the technical improvement work was completed.A steel plate welding scheme was prepared,standard compact CT samples were used and the test scheme was formulated according to the research objectives.In order to study the influence of residual stress of welding on the corrosion fatigue crack growth performance of the X65 steel,the specimens with gaps in the heat-affected zone of welding and the base material were selected as the research objects,and the tests were planned in the air and 3.5%Na Cl corrosive solution environment,respectively.In the third part,based on the experimental data processing results and fracture observation.The influence of residual stress on corrosion fatigue crack propagation was studied,what's more,the influence of corrosion solution and loading frequency on fatigue crack propagation of specimens with residual welding stress was studied.The seven-point incremental polynomial method was selected for data processing,the threshold of crack growth in the heat-affected zones and base metal of X65 under different environments was obtained by linear extrapolation method,and the differences of Paris constant C and mvalues with the environment and samples were analyzed.By comparing da/d N??K images,it is concluded that the compressive residual stress of welding will increases the stress intensity factor threshold?K_th,reduce the crack growth rate in the stable growth stage and improve the fracture toughness of X65 steel;What's more,the corrosive solution will increase?K_th,improve the stability of extension stage of the fatigue crack propagation rate and the fracture toughness of X65 steel.