Effect Of Water Jet Cavitation Peening On Stress Corrosion Cracking Of Nuclear Austenitic Alloys In High Temperature Water

Corrosion and stress corrosion cracking(SCC)are the main failure modes of nuclear austenitic alloys due to long-term service in high temperature water.Reducing the corrosion and SCC of nuclear austenitic alloys can not only improve the economic benefits of nuclear power plants,but also be of great significance to the safety of people’s livelihood.To effectively alleviate the corrosion and SCC of nuclear austenitic alloys in high temperature water,this paper intends to obtain by water jet cavitation peening(WJP).Compared with other surface stress improvement technologies,WJP is a green and environment-friendly technology,which is suitable for application in the nuclear power industry.Nevertheless,the effects of WJP on corrosion and SCC are not well understood.As such,in this paper,WJP was used to surface strengthening of the three nuclear austenitic alloys.The effects of WJP on the microstructure,stress state,corrosion,and SCC behavior of the three austenitic alloys were systematically studied.WJP was applied to surface strengthening of cold-worked 316 L stainless steel,which was widely used as the main pipeline material in nuclear power plants.X-ray micro-stress meter,Vickers hardness meter,laser scanning confocal microscope,scanning electron microscope(SEM),and transmission electron microscope(TEM)were used to investigate the WJP induced change in stress state and microstructure.The effects of WJP on corrosion and SCC were performed using exposure experiments and interrupted slow strain rate tensile(SSRT)tests in high temperature water.The results showed that the surface grain refinement and a high level of residual compressive stress were introduced after the WJP treatment.The maximum residual compressive stress was about-500.63 MPa,the depth of the strengthening layer was about 100μm,and the thickness of the surface lath-like ultrafine grain layer was about 200 nm.The exposure experiments suggested that the inner oxide film on the surface of 316 L stainless steel was changed from porous to dense,and the corrosion resistance of 316 L stainless steel was improved.In addition,the crack initiation strain increased from 2.5% to 5.0%,and the crack number,crack length,and crack depth decreased,indicating that the SCC susceptibility was decreased after the WJP treatment.Then,the effect of WJP on the stress state,microstructure,and SCC susceptibility of nickel based alloy 182 was studied.The results confirmed that residual compressive stress was introduced after the WJP treatment.The maximum residual compressive stress was about-300 MPa,and the depth of the strengthening layer was about 100 μm.In addition,the SSRT tests results showed that the crack initiation strain was not changed,the maximum crack length decreased from about 585 μm to about 338 μm,and the maximum crack depth decreased from about 35.2 μm to about 17.5 μm.The improved SCC resistance of alloy 182 was less than that of 316 L stainless steel.Finally,the influence mechanism of WJP on corrosion,intergranular oxidation,and SCC was revealed by stress relief annealing(SRA)treatment and surface polishing treatment combined with room temperature electrochemical tests,and exposure experiments and interrupted SSRT tests in high temperature water.The results showed that a high level of residual compressive stress can be introduced and an ultrafine grain layer can be formed in nickel based alloy 600 after the WJP treatment.The maximum residual compressive stress was about-520 MPa,and the depth of the strengthening layer was about 600 μm.The results of room temperature electrochemical tests showed that the corrosion potential shifted positively,and the corrosion current density decreased,suggesting that the corrosion resistance of alloy600 was increased.The results of exposure experiments showed that the formation of a complete oxide film on the surface can be promoted after 50 hours of exposure.After exposure of 500 hours,the thickness of the surface oxide film decreased from about 125 nm to about 35 nm,and the corrosion resistance was improved.The SSRT tests results showed that the crack initiation strain was increased from 2% to 12%,the surface crack density and crack length decreased,and the SCC susceptibility of alloy 600 was decreased.In addition,the results of SRA treatment and surface polishing treatment showed that the surface ultrafine grain structure was the dominant factor for alleviating the short-period corrosion,the high level of residual compressive stress was the key factor for mitigating the SCC,and the high level of residual compressive stress and the surface ultrafine grain structure were both helpful to inhibit the intergranular oxidation of the alloy.