Research On Mechanism And Influence Factors Of Carbon Migration For Pearlitic Steel And Austenitic Steel Dissimilar Joint

With widely applying of dissimilar material welding for pearlite and austenitic steel inproject engineer, more and more cases indicate that the main reason cause weld seam failure iscarbon migration on fusion line between pearlite steel and austenitic steel. This paper tookpower plant boiler system as background,weld classic joint by simulating actual workingstatus. Carbon migration was tested and recorded by using scanning electron microscopy(SEM). After analyzed the carbon distribution around fusion line, diffusion was from pearlitesteel to austenitic steel. Chemical potential gradient was calculated according to the measuredcomposition of carbon and other alloy elements. Carbon migration formula for pearlite andaustenitic dissimilar joint was derived by using Fick's law and calculated chemical potentialgradients.Four commonly used welding consumables,A102,A402,R307,Ni317, were selected tocreate an effective welding consumable that could restrain carbon migration. Test specimenswere welded for all the above welding consumable in the stage of welding/agingtreatment/creep. Microstructure and carbon migration for different specimens were compared.The main effect factors were discovered for carbon migration,a new ultra-low carbon weldingconsumable was created base on the discovery. The same test specimens were welded andmachined. Comparison for specimens' microstructure/facture/carbon migration between newultra-low carbon and four common welding consumables are performed.Examination results showed that carbon migration was main factor lead to creep rupture.Rupture location for A102/A402/R307test specimens occurred around fusion line of pearliteand austenitic. Obvious carbon migration was founded for above test specimens after analyzecreep rupture fracture and observe microstructure and carbon migration on rupture position.Brittle rupture features were discovered on facture. Harmful hard and brittlemicrostructure,compose of cementite and carbon chromium compounds,were founded incarbon migration layer.Carbon migration was almost not occurred for test specimens ofultra-low carbon and Ni317. Plastic rupture features were founded on facture. Only a smallamount of carbon diffusion occurred around fusion line because of difference carbonconcentration gradient on both sides. Creep rupture time for ultra-low carbon electrode was longer than A102/A402/R307,only slightly shorter than Ni317by comparing the five testspecimens.It could be concluded that the new type ultra-low carbon electrode wouldeffectively restrain carbon migration. High temperature mechanical properties of ultra-lowcarbon electrode were better than the commonly used austenitic and pearlite weldingconsumables. Although slightly worse than the nickel-based materials, but could meet most ofthe majority engineering request. This new type welding consumable had a great priceadvantage than the austenitic material,not to mention nickel-based material. So this kind ofwelding consumable wasa very practical,with a high cost-effective.