Study On Corrosion Fatigue Performance Of Riser Welded Joints And Its Optimization

Drilling riser is prone to corrosion fatigue under the synergistic effect of seawater electrolyte and alternating loads formed by sea current.At present,the studies on X80 steel base metal corrosion fatigue properties obtained through experiment method have been reported,but researches on corrosion fatigue of welded joints are relatively rare.This thesis focused on corrosion fatigue behavior by comparing X80 steel base metal and welded joints.Utilizing microstructure analysis and electrochemical tests,the loophole was pointed out.Meanwhile,the failure mechanism was discussed.The impact and mechanism of aluminum coating on the corrosion fatigue properties of X80 steel joints were analyzed by comparing corrosion fatigue properties between the coating specimen and uncoated ones.Corrosion fatigue test was operated by exploiting a reciprocating cantilever bending loading,and specimen's general morphology was obtained under the condition of equal strength beam theory.In order to prevent stress concentration,arc transition was used from tapered beam portion to the clamping part.Considering the effect of the arc radius size on stress distribution,transition region was calculated with the help of ANSYS software to ensure the welded joint a maximum stress value in equal-stress region.Corrosion fatigue fracture was located on the heat affected zone(HAZ),indicating that HAZ in welded joints functioned the weakest of Corrosion fatigue properties.The pitting of HAZ was more serious than the base metal and weld zone by morphology observation.The linear polarization and micro-electrochemical corrosion test results also showed that HAZ had the worst corrosion resistance,which means joint corrosion factor was of importance in corrosion fatigue.The pits were severe in HAZ by number and depth.Stress concentration caused in the pit bottle made cracks easier to inoculate,and thus formed sample edgetortuous and multi-planar morphology.Because HAZ had a greater crack resistance than the base material under the same circumstances,fatigue strip of welded joint specimen in stable expansion area was smaller than base metal.Aluminum coating could significantly improve the corrosion fatigue lifespan of welded joints specimen,but the fraction still started from heat affected zone.Corrosion process was effectively control as showed from the aluminum coating specimen,thus the crack inoculation time was effectively curbed.Corrosion fatigue failure in heat affected zone was mainly influenced by mechanical properties for the impact of the corrosion was significantly weakened by coating layer.Cathodic protection from aluminum coating produced side effects as hydrogen-induced embrittlement in plastic deformation zone and plastic zone,therefore secondary cracks in early stable expansion region of welded joints specimen coated by aluminum coating was wider and longer than uncoated ones.In latter stable expansion region and rapid expansion region,residual coating could has a beneficial effect for hydrogen embrittlement effects drastically reduced.