Investigation On The Microstructure And Performance Of 800H Alloys With Nickel Cladding

Molten salt reactor(MSR)is the only fluid fueled reactor among the Generation IV reactors,which has incomparable advantages such as inherent safety,water-free cooling,small modular design and so on.To date,Hastelloy N alloy(UNS N10003)is the only successful structural alloy used in a MSR.Its codified maximum allowable temperature is 704 ~oC.However,for some applications like hydrogen production,in order to enhance the power generation efficiency,the operating temperature is expected to above 850 ~oC.Development of a new material is characterized by high cost,long cycle and big risk.On the other hand,surface modification of a mature alloy for high temperature application is easier to access.For a near-term application consideration,800H alloy stands out as an attractive candidate material due to its better readiness for nuclear service,which has the great advantages of being well-investigated for the gas-cooled nuclear reactor.It should be noted that only five alloys are codified into the ASME BPV Code subsection NH of Section III,among which 800H alloy is the only one allowed to be applied above 850 ~oC with a rich database.Unfortunately,in spite of the excellent mechanical performance at high temperature,plenty of studies have shown 800H alloy has a poor corrosion resistance to molten salts.800H alloy with a thin liner of nickel alloy is supposed to possess both the high-temperature mechanical performance of 800H alloy and the excellent corrosion resistance of nickel.Among common methods,the weld-overlay is a commonly used technique in the nuclear power industry and can be industrialized and automated.However,investigations on the Ni-cladded 800H alloy is limited and few investigations on the corrosion resistance,microstructure and mechanical properties of the Ni-cladding 800H alloy by weld-overlay.In this study,800H alloy was cladded with nickel by gas tungsten arc weld(GTAW)and the welding parameter has been optimized.The corrosion tests of 800H alloy with and without Ni-cladding,as well as GH3535 alloy(UNS N10003),were performed in an isothermal static immersion setup at 850 ~oC for 300 h.The corrosion resistance and corrosion mechanism of the cladding were investigated by inductively coupled plasma optical emission spectrometer(ICP-OES),X-ray diffraction(XRD),scanning electron microscopy(SEM),electron probe micro-analyzer(EPMA),transmission electron microscope(TEM)etc.Meanwhile,the microstructure and service properties including mechanical properties and long-term stabilities were also investigated by optical microscope(OM),micro-sclerometer and universal testing machine.The results of this study are as follows:(1)800H alloy can be cladded with nickel by GTAW and the optimum welding current is 200 A.(2)Ni-cladding by GTAW method can greatly improve the corrosion resistance of 800H alloy to FLiNa K salts at 850 ~oC.The uniform corrosion of cladding-top layer by molten FLiNaK salts was observed and its depth of Cr-depletion layer is almost equal to that of GH3535 alloy(about 31μm),which is only about 1/10 of the 800H alloy without cladding(about 344μm).(3)In the molten FLiNaK salts with impurities like Fe ions,the corrosion of the alloy was mainly controlled by the redox reaction of impurity metal ions with Fe and Cr in the alloy,which form“diffusion paths”that promote the diffusion of Cr outwardly to the alloy surface.To be conservative,the chemistry criterion for a good alloy or cladding layer with good compatibility to molten salts should be Fe+Cr≤11 wt.%.(4)The Cr element inΣ3 type grain boundaries(GBs)can be hardly depleted during the corrosion process,suggesting increasing the fraction of low-Σcoincidence site lattice boundaries(CSLBs)in an alloy can also improve its corrosion resistance to molten salts.(5)The microstructure of the Ni-cladding from the matrix-cladding interface to the outside consists of plane crystal,cellular crystal,dendritic crystal and fine equi-axed crystal.(6)800H alloy with Ni-cladding performs prefect ductility with no visible crack after the side bend test.The cladding and the matrix are well bonded,and the interfacial shear strength are 490MPa and 134MPa at room temperature(RT)and850 ~oC,respectively.The appearance of fracture at RT has large amount of dimple.The fracture modes are ductile at RT and cleavage fracture at 850 ~oC,respectively.(7)Hardness values and chemical compositions of the cladding sample are changed slightly after aging at 850 ~oC for 1000 h,indicating that 800H alloy with Ni-cladding has a great stability.