Study Of Stress Corrosion Cracking Susceptibility Of Nickel-based Welds In Safe-end Dissimilar Metal Weld In Pressurized Water Reactor

The safe-end dissimilar metal weld(DMW)is regarded as the weak part in the primary coolant system of pressurized water reactor(PWR),and the operation history indicates stress corrosion cracking(SCC)is one of main failure mechanisms.The assessment on SCC susceptibility of materials in DMW is of great importance for both the operation safety of current DMWs and the design and manufacture of future ones.The object of this study is the full-size mock-up of domestic third generation safe-end DMW,and the factor of material is highlightened among the three factors of SCC.The scanning electron microscopoy(SEM),transmission electron microscopy(TEM),focused ion beam(FIB),electron back-scattered diffraction(EBSD),nanoindentation,in-situ tensile,3D X-ray tomograpghy(3D-XRT),3D atomic probe(3DAP),transmission electron back-scattered diffraction(t-EBSD),slow strain rate tensile(SSRT),exposure test in simulated primary water,dynamic high temperature high pressure water recycling loop and in-situ rapid scratching electrode technology are applied to assess the SCC susceptibility of nickel-based welds of DMW in a multiply scale,including the ductility-dip cracking(DDC),welding inclusions and the repassivation behaviors.The microstructure,mechanical properties and corrosion behaviors in simulated primary water of welding defacts are systematically studied.The formation mechanisms of different welding defects and their potential influences on SCC susceptibility are also clarified.The feasibility of quick assessment on SCC susceptibility by repassivation parameter is explored based on the fundamental study on repassivation behaviors of nickel-based weld.The microstructure,mechanical properties and corrosion behaviors in simulated primary water of DDC are studied,and the results confirm that a DDC-concentrated zone(DCZ)in width of about 3 mm was observed adjacent to the 316L/52Mw fusion boundary(FB)in 52Mw.DDCs are random-shaped and disc-like cavities with corrugated structure of inner surface and are parallel in groups along straight high-angle boundaries of columnar grains,ranging from micrometers to millimeters in size.Large-size M23C6(M=Cr)carbides dominate on the grain boundaries rather than MC(M=Nb,Ti),and thus the bonding effect of carbides is absent for the straight grain boundaries.The yield strength,tensile strength and elongation to fracture of 52Mw-DCZ(400 MPa,450 MPa ahd 20%,respectively)are lower than those of 52Mw-MZ(460 MPa,550 MPa and 28%,respectively).The intrinsic high-restraint weld structure,the additional stress/strain caused by the thermal expansion difference between 316L and 52Mw as well as the detrimental carbide precipitation and the resulting grain boundary structure all add up to cause the occurrence of DCZ in the dissimilar metal weld.DCZ is the apparent degradation zone of mechanical properties,where the potential SCC risk is higher than other areas in nickel-based welds.The microstructure,mechanical properties and corrosion behaviors in simulated primary water of Cr inclusions are studied.The results indicate that the Cr inclusions originate from large-size undissolved Cr particles in 152 welding electrode flux,and they are only existed in inner 152 cladding.Cr inclusions are 50～150 μm in size in most cases,and there is a continuous transition zone of 2～5 μm in width between the Cr inclusion core and 152 cladding matrix,and the transition zone consists of Ni&Fe-enriched dendritic austenite and Cr23C6 and Cr matrix.The transition zone has the highest nanoindentation hardness(7.66 GPa),and the in-situ microscopic tensile tests confirm the highest cracking susceptibility.Considering the location and cracking susceptibility,Cr inclusions are of higher possibility for SCC initiation than cladding matrix during long-term operation.The microstructure,mechanical properties and corrosion behaviors in simulated primary water of ferroniobium topologically close-packed(TCP)phase welding inclusions are also studied.The ferroniobium TCP-phase inclusions originate from large-size undissolved ferroniobium supplement particles in 152 welding electrode flux.Two morphologies are observed for the inclusion,including with core and without core,and they are 100～200 μm in size.The inclusion with core has a homogeneous core of rhombohedral Fe7Nb6(μ phase),and a duplex transition zone consisting of columnar grain zone(mianly Fe7Nb6)and isometric grain zone(mainly Fe2Nb).The eutectic zone comprises hexagonal Fe2Nb(Laves phase)and austenite.The Fe2Nb is formed by eutectic metallurgical reaction between inclusion core and 152 cladding.The average hardness of inclusion core(17.89 GPa)is over 4.5 times as high as cladding matrix(3.91 GPa).The inclusion core is of higher cracking susceptibility than 152 cladding,and acts as the initiation of brittle cracks during in-situ tensile test.In simulated primary water,the general corrosion rate of Fe7Nb6 is about 7.3 times of eutectic austenite.The triple-layer oxide film of inclusion core consists of outer hexagonal polyhedral oxides,nano polycrystal porous oxides(mainly Nb2O5)and inner compact amorphous oxides(mainly NbO),and the phase of outer hexagonal polyhedral oxides is unknown,but its space group is deduced as P6/mmm by three-dimensional lattice symmetry.The ferroniobium TCP-phase inclusions are of higher SCC susceptibility than 152 cladding given the complex chemical compositions and crystal structures.The repassivation behaviors of 52M nickel-based alloy in simulated primary water are finally studied,and the results indicated its repassivation behaivors can be explained by place exchange model and high field ion conduction model.The preliminary stage of repassivation corresponds to the place exchange model,and the end stage meets the high field ion conduction model.The repassivation parameter cBV is positively correlated with SCC susceptibility,which can be refered to assess the SCC susceptibility for materials in a quick way.cBV increases along with the increase of polarization potential under different polarization potentials(OCP+200 mV～OCP+600 mV),and so does the SCC susceptibility.cBV has the maximum at 260 ℃ under test temperatures(200 ℃～300 ℃),where the SCC susceptibility reaches its climax.cBV increases along with the increase of dissolved hydrogen(DH)concentration under different DH concentrations(0 ppm～3.0 ppm),thus the SCC susceptibility is the highest under no hydrogen condition.