| The inner walls of the low-alloy steel(LAS)reactor pressure vessel(RPV)cylinder in pressurized water reactor(PWR)nuclear power plants are cladded with stainless steels,where in general the low-alloy steel(base metal)is at first cladded with a 309L layer and then a subsequent 308L layer.The stainless steel(SS)cladding layer is in contact with high-temperature water in the primary circuit,therefore,its service performances such as stress corrosion cracking(SCC)is crucial to the structural integrity of the pressure vessel.In this thesis,the microstructure of the low alloy steel A508Ⅲ-309L/308L SS cladding layers was characterized by microhardness tester,optical microscope,scanning electron microscope,electron backscattering diffraction,electron probe microanalysis,focused ion beam,and transmission electron microscope.The oxide films and SCC behaviors of 309L and 308L SS cladding layers in simulated PWR primary water environments were investigated by means of exposure tests,slow strain rate tests(SSRTs)and SCC growth rate tests with compact tension specimens.The effects of surface finishing conditions and PWR primary water temperature on the oxide film formed on 308L SS cladding were studied.The effects of electrochemically pre-charged hydrogen on the local oxidation and SCC susceptibilities of 308L and 316L SSs were studied.The main conclusions are as follows:1.The cladding layer was composed of the fine-grains area near the fusion boundary(FB)and the columnar grains.The grains in 309L were smaller than that in 308L,and the grain average misorientation of 309L was higher than that of 308L.There were lath martensite layers,type-I and type-II grain boundaries on the 309L side near the FB.The micro-hardness was higher,and the number and size of inclusions were larger than in 309L than in 308L.Welding dilution was found in 309L cladding,and lessδ-ferrite content was observed in 309L than in308L.The oxidation and SCC resistances were higher for 308L than for 309L SS in PWR primary water with dissolved hydrogen,which was attributed to the effects of networkedδ-ferrites with a high Cr content in 308L.Meanwhile,the presence of type-I and type-II grain boundaries,composition dilution and lessδ-ferrite content in 309L decreased the SCC resistance.2.After the exposure in PWR primary water,the inner oxide film formed on 309L surface was relatively thicker and more porous than on 308L surface,which resulted in the lower oxidation resistance for 308L than for 309L.Cr dilution and lowδ-ferrite content in 309L decreased its oxidation resistance,resulting in the local oxidation penetration below the oxide/matrix(O/M)interface.The SCC growth rates of the 309L cladding layer were higher than that of the 308L cladding layer in PWR primary water at 325℃and 350℃.The low SCC growth rate of 308L was mainly due to the networkedδ-ferrites that could mitigate the initial oxidation and hindered the crack advance by branching the primary crack.Local oxidation penetration and a more porous inner oxide layer on 309L would enhance its SCC growth.3.Compared to the mechanically polished surface,the ground 308L surface showed a higher oxidation resistance in PWR primary water.Ground treatment generated a fine-grained layer with a high density of dislocations and subgrain boundaries,which could increase the Cr outward diffusion flux to form an initial dense and protective oxide film.δ-ferrites with the special lattice structure and higher Cr-content exhibited a higher oxidation resistance than theγ-austenite in PWR primary water at 310 ~oC and 325 ~oC,while the difference in the oxidation resistance between austenite and ferrite near the PB at 350 ~oC was not significant,resulting from the thermally activation enhanced self-diffusion of Cr inδ-ferrite.The effect ofδ-ferrite on inhibiting the oxidation of adjacent austenite grain was more significant with the increase in temperature,which was mainly shown by the wider ferrite-affected oxidation zone at a higher test temperature.4.The SCC performances of 309L and 308L in deaerated and oxygenated high-temperature water at 325℃were evaluated.The results from SSRTs in deaerated water showed that there was no obvious indication of SCC on the fracture surface of 308L,while local SCC characteristics appeared on the fracture surface of 309L.After SSRTs in water containing 0.2ppm(wt.)dissolved oxygen,there was no apparent SCC indication on the 308L fracture surface,while the 309L fracture surface was featured by transgranular+intergranular cracking.After SSRTs in water containing 8 ppm(wt.)dissolved oxygen,the fracture surface of 308L mainly exhibited ductile cracking characteristics with dimple-like cracks and local cleavage-like transgranular cracks.The whole fracture surface of 309L showed obvious interdendritic cracking characteristics.The SCC susceptibilities of 308L and 309L increased with increasing dissolved oxygen concentration in high-temperature water.The 308L compact tension specimens were used for SCC tests under constant load loading,and the crack growth rate test results showed higher SCC susceptibility than SSRT results.The loading mode had a significant influence on the cracking mode of 308L in oxygenated water but did not have significant effect on the cracking mode of 309L.5.Pre-charged hydrogen increased the oxidation rate of 308L cladding in deaerated high-temperature water,as confirmed by the formation of thicker oxide film on the hydrogen-charged specimen.Hydrogen accelerated the outward diffusion of iron,thereby formatting the large iron-rich oxide particles.Hydrogen enhanced theδ-ferrite oxidation,grain boundary preferential oxidation and phase boundary penetration oxidation.The SSRT results show that pre-charged hydrogen in 308L increased the yield strength,reduced the tensile strength and elongation to fracture,and resulted in significantly increased the SCC susceptibility,as evidenced by the obvious intergranular SCC features on the edge of the fracture surface.The charged hydrogen in 308L cladding promotedδ-ferrite local denting oxidation in deaerated high-temperature water and embrittlement of hydrogen on grain boundaries would increase the susceptibility of SCC.6.The effects of pre-charged hydrogen on the local oxidation behavior and SCC susceptibilities of solution-annealed(SA)state and cold-worked(CW)state 316L SS were studied.Hydrogen increased the outward diffusion rate of iron,leading to the large iron-bearing oxide particles forming on the outer layer and causing local oxidation penetration beneath the oxide/metal interface.Cold-work by pre-deformation increased the oxidation rate of 316L SS in high-temperature water.The synergistic effect of hydrogen and cold-work significantly accelerated the oxidation of 316L SS.Hydrogen increased the SCC susceptibilities of SA316L and CW316L samples,which was consistent with the observed phenomenon that hydrogen increased the SCC susceptibility of 308L.The pre-charged hydrogen specimens showed high SCC susceptibility at a lower strain rate. |
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