Effect Of Ultrasonic Impact Treatment On Localized Corrosion Behavior Of Stainless Steel Welded Joint Used For Nuclear Power Plants

The stainless steel used in nuclear power plant spent fuel pool has attracted increasing attention on localized corrosion.In particular,the stainless steel welding parts have poor corrosion resistance due to the high heat input and rapid cooling during welding,and a certain installation stress during the engineering installation.Ultrasonic impact treatment(UIT)can produce strong plastic deformation on the surface of welded joint,refine the crystal grains,release welding residual stress,bring beneficial compressive stress and improve component surface integrity,mechanical properties and corrosion resistance.This paper used metallographic method,hardness method,residual stress measurement method,immersion method and electrochemical method to study and compare the stainless steel cladding welded joints about microstructure,roughness,residual stress,hardness and the changes of pitting corrosion,crevice corrosion and stress corrosion cracking(SCC)under related water chemical environment.The test materials were 304L/ER316L austenitic stainless steel and S32101/ER2209 duplex stainless steel UIT front and rear welded joints and pure base materials.Both types of stainless steel are used for the inner wall of the spent fuel pool of nuclear power plants.The related solutions for the corrosion test part were:a)The stress corrosion cracking test part includes:42%boiling Mg Cl₂solution;a simulated solution that simulated the service environment of the stainless steel-clad concrete side of the spent fuel pool,saturated boric acid+0.5 mol/L H⁺(hydrochloric acid)+1 mol/L Cl^-(Na Cl)solution;high temperature and high pressure water environment solution 2 mg/L Li⁺+1200 mg/L B³⁺+100 mg/L Cl^-solution;b)pitting and crevice corrosion test part included:6%Fe Cl₃solution,saturated boric acid solution,saturated boric acid+200 mg/L Cl^-solution;test temperature included 250,143,60,40 and 35?.The main contents and conclusions were as follows:1.Characterisation on the changes in microstructure,roughness,hardness,and residual stress of the two types of materials after UIT.After UIT,the microstructure of the upper surface of the two types of stainless steel occurred significant deformation marks and the grains refined.The deformation layer of about 900?m was observed at the cross-sectional position.The roughness significantly reduced,and the base metal area and weld area of the welded joint decreased by about40%.The Vickers hardness value was significantly increased,the hardness value gradually decrease as the distance from the UIT surface layer on the cross section was farther,and the value gradually stabilized at about 1 mm;the residual stress transformed from tensile stress to compressive stress,and as the depth increaseing,the residual compressive stress gradually became smaller,and the depth of influence was about 1mm.2.Researched with U-bend specimens and double beam bend specimens at high and low stress levels which immersed in boiling Mg Cl₂solution to compare the stress corrosion resistance of the two stainless steel welded joints before and after UIT.(1)Tests with U-bend specimens showed that UIT did not significantly reduce the SCC sensitivity of the two welded joints.The SCC sensitive areas of 304L and S32101welded joints were the base metal area+heat-affected zone and the fusion line respectively.The crack types were intergranular+transgranular mixed and transgranular cracking.S32101-BM had the best stress corrosion resistance.(2)Tests with double beam bend specimens showed that UIT significantly reduced the SCC susceptibility of the two welded joints.The SCC sensitive areas and crack types of the two welded joints were the same as the U-bend specimens,and the SCC sensitivity of the 304L specimens increased with the increase of stress.3.Researched with U-bend specimens and the double beam bend specimens under high stress(?500MPa)level which were immersed in a simulated solution to compare the stress corrosion resistance of the two materials before and after UIT.(1)Tests with U-bend specimens showed that UIT did not improve the stress corrosion resistance of the two types of welded joints significantly.The SCC sensitivity ranking of the six state specimens was:304L-UIT=304L-AW=S32101-BM<304L-BM<S32101-UIT=S32101-AW.(2)Tests with double beam bend specimens showed that under high stress(?500MPa),UIT significantly improved the stress corrosion resistance of 304L welded joints,while no stress corrosion cracks were observed after S32101 welded joints immersed for 80 days.The effect of UIT was unknown.The SCC sensitivity ranking was:S32101-BM=S32101-AW=S32101-UIT<304L-UIT<304L-AW=304L-BM.4.Researched static load tensile SCC in high temperature and hpressure water environment with tensile specimens at 250?,2 mg/L Li⁺+1200 mg/L B³⁺+100 mg/L Cl^-solution.The welded joints before and after UIT only showed uniform corrosion after being immersed for 14 days,and the surface of the welded joints after UIT had fewer bright corrosion products.It can be preliminarily believed that after UIT,it seemed that the corrosion resistance of the material in high temperature water had been improved.5.Researched with ferric chloride immersion and electrochemical method to study pitting and crevice corrosion behavior.(1)Tests with ferric chloride immersion showed that the pitting corrosion resistance of the two stainless steel welded joints decreased after UIT.The pitting corrosion mainly occurred in the base metal area and the heat-affected zone,and the weld metal had the best pitting resistance.(2)Tests with polarization curve showed that in saturated boric acid+200 mg/L Cl^-solution at 40?,UIT significantly improved the pitting resistance of the base metal area of the 304L and S32101 welded joints;In saturated boric acid solution at 60?,all regions showed excellent pitting corrosion performance.As the Cl^-concentration and temperature increased,the pitting corrosion performance of the welded joints decreased.The pitting sensitivity of the two types of stainless steel welded joints was ranked as follows:weld zone>base metal zone>Mixed zone.(3)Tests with crevice corrosion potential method showed that in 40?saturated boric acid+200 mg/L Cl^-solution,UIT increased the crevice corrosion potential of the base metal area of the 304L welded joint and reduced the crevice corrosion potential of the base metal area of the S32101 welded joint.