| Lead bismuth eutectic alloy(LBE)has the excellent properties of low melting point,strong heat carrying capacity and good neutron characteristics.It is the first candidate material for the spallation target and coolant of accelerator subcritical driving system(ADS).However,lead bismuth alloy can cause serious corrosion to the cladding material exposed to liquid metal.As in the cooling circuit in nuclear engineering,nuclear main pump impeller and the liquid metal in high speed relative motion to local area on the impeller pressure mutation zone,gas evolution after the formation of bubbles in the impeller surface,a large number of bubble collapse caused by cavitation corrosion on the metal surface,resulting in part to reduce the service life and maintenance costs increase.316 L steel has excellent mechanical properties and corrosion resistance.It is commonly used as the cladding material of ADS cooling loop and the main pump impeller material.In the nuclear engineering,many structures are assembled and manufactured by the method of welding,and the weld structure usually has some defects such as segregation and coarse structure.Therefore,the study of cavitation corrosion behavior of 316 L steel welded joint in liquid lead bismuth alloy can provide a theoretical basis for the application of ADS system and the future fusion reactor to be more economical and safe.In this paper,316 L stainless steel was used as base metal,316 L steel and Cobalt based alloy was used as filler metal for TIG welding and heat treatment was carried out after welding.The liquid metal ultrasonic cavitation equipment was used to conducted the cavitation erosion(CE)tests of 316 L base material,316 L weld metal before and after heat treatment and cobalt based alloy weld before and after heat treatment.SEM and AFM were used to characterize the surface morphology of the specimens in different time after CE,and the variation of the surface morphology with cavitation time was analyzed;The changes of surface roughness and the depth of corrosion pits of the samples were measured at different time after CE to reveal the differences of CE resistance and different influence of filling material and the heat treatment parameters on CE resistance in the liquid LBE.In addition,weld cross-section morphology of the oxide layer and the element changes after both static corrosion and CE 100 h were contrasted,and the combined action mechanism of dissolution and oxidation corrosion and CE on welded joints surface in liquid LBE was studied.The test results show that 316 L weld and base material surface roughness and the maximum depth of corrosion pits increased with prolonging test time of etching empty and,base material showed the CE resistance was much better than that of the weld,CE resistance was mainly correlated with both microstructure.The CE resistance of 316 L steel weld was improved aftersolid solution treatment,but was still not as good as the cavitation resistance of 316 L base metal.In addition,the changes of welding section elements compared to 316 L steel weld 100 h static corrosion and CE 100 h,dissolution and oxidation corrosion and cavitation erosion in liquid LBE will promote each other,and their combined effects cause more serious damage on the weld.Cobalt based alloy is a kind of metal with strong corrosion resistance,so the TIG butt welding of 316 L steel was carried out by using cobalt based alloy as filling material,the cobalt based alloy welds were treated with different temperature after welding,and then the cavitation erosion tests were carried out at different time.The experimental results showed that the CE resistance of cobalt based alloy welds was better than that of 316 L steel,and the CE resistance of cobalt based alloy welds were improved after heat treatment at different temperatures.After750℃heat treatment,the cobalt based alloy weld has the best corrosion resistance,followed by the heat treatment at 700 ℃,and the corrosion resistance of cobalt based alloy is the worst after heat treatment at 800℃.In the process of cavitation corrosion test,all the four samples of 316 L steel weld,non-heat treated cobalt base alloy welded joint,before and after solid solution treatment of 316 L had different degrees of work hardening in the process of cavitation erosion.The work hardening of 316 L base material was the highest,cobalt based alloy weld hardening degree was the minimum;cobalt based alloy weld of three groups after heat treatment were almost no work hardening,the hardening degree increased slightly in 20 h,andafter 20 h the surface hardness decreased slightly.Through comprehensive analysis,among all the materials studied in this paper,the cavitation erosion resistance of the base metal was the best,followed by the cobalt based alloy weld after 750℃ of heat treatment,and then the solid solution treated 316 L weld metal. |
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