Failure Analysis Of Stainless Steel Bellows Expansion Joint And Stress Corrosion Research Of Stainless Steel

Bellows expansion joint,as a kind of important displacement compensation structure in the modern thermodynamic pipeline,is the key component of thermal compensation in In addition to the displacement compensation,it’s role have both the function of the vibration,noise reduction and sealing.In recent years,it has been widely used in petroleum,chemical industry,metallurgy,electric power,cement,construction,Urban district heat supplying and other industries.However,in practical application,expansion joint applying environment is complex.The case that bellows expansion joint fracture happened frequently.Aiming at this situation,this article taked the failure bellows expansion joint provided by HuXian thermal power plant as the research object,using scanning electron microscopy(SEM),X-ray diffraction(XRD)and metallographic analyzed its failure reason.Then on this basis,studied the stress corrosion behavior of different welding forms of austenitic stainless steel 304,316 and ferritic stainless steel 404 in neutral etchant solution.Through the macroscopic corrosion test and electrochemical methods researched the corrosion of three kinds of stainless steel,and compared corrosion condition with natural condition of Q235 carbon steel(no welding and no stress).rovided the basis for the new expansion joint material selection and design.The results of the study show that:(1)The corrosion resistance of 304 stainless steel under the condition without stress and welding is far greater than carbon steel in corrosion solution.However,The fracture of bellows expansion joint belongs to the stress corrosion cracking.The twinning in stainless steel,residual stress,corrosive chlorine ions in the medium,and coupling effects of these factors caused the expansion joint to stress corrosion cracks,the fracture mechanism conformed to the slip dissolution mechanism.(2)Under the condition of stress,in the neutral corrosion liquid of pH = 6,304 stainless steel corrosion rate was greater than 316,404 stainless steel corrosion rate was the least,The naturally Q235 carbon steel corrosion rate was much greater than the three kinds stainless steel;Under the condition of stress and transverse welding(the stress direction was vertical with welding line),404 ferrite stainless steel corrosion rate was 50%of the austenitic stainless steel 304,64%of the 316 stainless steel,naturally Q235 carbon steel corrosion rate falled in between 304 and 316 stainless steel;Under the Condition of stress and longitudinal welding(the stress direction was parallell with welding line),the corrosion rate of three kinds stainless steel were less than the transverse welding specimens,but greater than no welding specimens.404 stainless steel showed the better corrosion resistance.Corrosion rate of naturally Q235 carbon steel was consistent with 304 stainless steel,but less than 316 stainless steel.(3)In the process of stress corrosion,The position where three kinds of stainless steel touched the carbon steel clamp appeared serious galvanic corrosion.Stainless steel 304 was the most serious,404 was relatively lighter.The corrosion pits in longitudinal welding specimens were significantly more than the transverse welding specimens.(4)Under the Condition of welding,the corrosion rate of three kinds stainless steel specimens with longitudinal welding were consistent with the transverse welding specimens.The corrosion resistance of 404 stainless steel was better than 304 and 316 stainless steel,naturally Q235 carbon steel corrosion was the worst.Compared with the existence of stress,the corrosion rate of three kinds stainless steel were all reduced obviously.(5)The carbon steel main pipe in heat pipe network is relatively stress free compared with expansion joint.The corrosion rate of existing 304 stainless steel expansion joint welding line is consistent with carbon steel main pipe,but the expansion joint exists the galvanic corrosion,machining stress etc,which makes the expansion joint rupture preceded the main pipe.The corrosion rate of 404 stainless steel welding line is less than the carbon steel main pipe,suggest improvement expansion joint using 404 stainless steel materials.