Research On Co-free Ni-based Alloy Made By Laser Cladding Applied In Nuclear Valve Sealing Surface Hardening

Nuclear valve is an important nuclear equipment. Especially, the use performanceof its sealing surfaces directly affects the lifetime of nuclear valve. Co-based alloypowder has always been the optimal welding material used for sealing surfaces ofnuclear valve. But the use of Co-based alloy has two top issues: one is the element Co isone kind of rare, limited and expensive metal. Another is the element Co can be easilyactivated to be isotope Co60which can cause nuclear pollution, increase handling costand potential risk for its long radioactive half-life in nuclear radiation environment. Sodesigning and developing a new kind of welding material as sealing surfaces of nuclearvalve to replace of Co-based alloy is very emergent.In this paper a new kind of Co-free Ni-based alloy powder is designed. In this alloyformula element Ni is selected as the main composition, and Cr₃C₂, Cr, Mo, Si, B andY₂O₃and etc. are added as strengthening elements by solid solution strengthening,dispersion strengthening and grain boundary strengthening. Meanwhile, the harmfulphases are controlled by phase calculation principle in the alloy design phase. Theaverage electronic density scope of the new alloy powder formula can guide to adjustthe content of each element. Then the new-designed alloy powder is successfullydeposited on the stainless steel substrate by the hollow laser technique. The best alloypowder formula is confirmed through the crystal structure and phase compositions, andmechanical performance analysis of its laser cladding layer, and finally named Ni-4.The SEM microstructure analysis reveals that the bottom section of Ni-4lasercladding layer is constituted by cell crystals and dendrite structure which has obviousorientation; the middle section contains two typical microstructure, one is thin stripdendrites presented like sunflowers and another is relatively thick clubbed crystalsdistributed like chrysanthemums; the top section is mainly composed by the fineneedle-like dendrites. EDS and XRD test indicates that the Ni-4coating is made up by basic phase of γ Ni-based solid solution, carboborides like M23（C、 B）6, siliconcompounds like Ni₁₇Si₃and intermetallic compounds such as FeNi₃, Ni_2.9Cr_0.7Fe0.36etc.and as well as some hard phases like Ni3B, CrB, Cr7C3and so on.The microhardness test reveals that the average traverse microhardness of the Ni-4coating is695.3HV0.3while the average longitudinal microhardness698.6HV0.3inroom temperature, which is about200HV0.3higher than that of matrix alloy Ni45andalloy Stellite6. And there is no local hardness mutation in the Ni-4coating. Meanwhile,the traverse and longitudinal microhardness change of the alloy Ni-4shows regionalfeatures, which accords with its microstructure feature and distribution. The hightemperature microhardness tests at300℃,600℃and900℃reveal that themicrohardness of Ni-4are all higher than that of Ni45and Stellite6, and themicrohardness range of Ni-4changed by temperature is much smaller than them. Thetest also demonstrates that alloy Ni-4has its usability at the temperature of900℃.The results of friction and wear experiments at normal temperature,360℃and600℃show that the average friction coefficients of Ni-4are0.3740,0.2884and0.4445accordingly. What’s more, the friction coefficient of Ni-4at360℃is smallest andstable which has best wear-resisting property in that temperature. Compared with alloyNi45and Stellite6, at normal temperature,360℃and600℃, the friction coefficients ofNi-4are smaller than that of Ni45; at normal temperature and600℃, the frictioncoefficients of Ni-4are a little higher than that of alloy Stellite06. SEM shows that thegrinding crack of Ni-4is fine and smooth which belongs to abrasive wear. The abrasionloss test reveals that the volume loss of Ni-4is smallest than that of alloy Ni45andStellite6at the three kinds of temperature.The corrosion test indicates that in the acid corrosive the decay resistance of Ni-4isbetter than that of alloy Stellite6and more stable than that of Ni45at both normal andhigh temperature. While in the basic solution alloy Ni-4, Stellite6and Ni45havesimilar decay resistance, and finally reach small and steady corrosion rate at bothnormal and high temperature.