Effect Of Heat Treatment On The Structure And Tribological Properties Of WC-12Co Reinforced Ni-based Coatings On Inconel 718 Alloy

Steam safety valves play a vital role in the design and operation of nuclear power plants.Applying protective coatings is essential to ensure the long and stable operation of steam safety valves.In this study,WC-12Co reinforced nickel-based coatings were first fabricated on Inconel 718 alloy using High-Velocity Air Fuel（HVAF）for the service conditions of high-temperature moving components such as steam safety valves.Meanwhile,the as-fabricated coatings were heat-treated at different temperatures.Subsequently,the microstructure and mechanical properties of the as-fabricated coatings and the coatings heat-treated under different temperatures were investigated in detail.Finally,the effects of service temperature,friction load,and rotational speed on the tribological properties of Inconel 718 alloy,as-fabricated,and heat-treated coatings were investigated systematically.The microstructure and mechanical properties of the as-fabricated WC-12Co reinforced nickel-based coatings,and the enhancement of tribological properties of Inconel 718 alloy in a wide temperature range（25°C,180°C,380°C,580°C,and780°C）were investigated,systematically.The results showed that the mechanical bonding between the as-fabricated coating and Inconel 718 substrate was dominant,and the internal porosity of the coating was about 1.30%.The HVAF technology enhanced the hardness of the Inconel 718 substrate from 247.7 HV to 410.7 HV and enabled the hardness of as-fabricated coating to reach 996.9 HV.The tribological properties of the as-fabricated coatings were stable in the temperature range of 25°C to 780°C.Meanwhile,the wear rates were significantly lower than those of Inconel718 alloy at the corresponding temperature conditions.It indicated that the as-fabricated WC-12Co reinforced nickel-based coating provided good protection against Inconel 718 alloy.The microstructure,mechanical properties,and tribological properties at room temperature and 580°C of the coatings heat-treated under 970°C,1000°C,1030°C,and 1070°C were investigated systematically.The results showed that the heat treatment facilitated the formation of the metallurgical bond between the coating and Inconel 718 substrate and reduced the porosity of the coating.The coatings heat-treated at 1000°C had the lowest average porosity of 0.47%.The coatings heat-treated at 1000°C exhibited the best comprehensive mechanical properties and had the lowest wear rates at both room temperature and 580°C,1.04×10^-6mm³N^-1m^-1at room temperature and 3.87×10^-5mm³N^-1m^-1at 580°C.The protective effects of the as-fabricated coating and the coating heat-treated at1000°C on the Inconel 718 substrate under different loads and rotational speeds were investigated,respectively.The results showed that the average coefficient of friction of the as-fabricated coating under different loads was not very different.Meanwhile,the wear rate at the corresponding load and rotational speed was reduced by one order of magnitude compared to that of Inconel 718 alloy,which indicated that the as-fabricated coating provided good protection for Inconel 718 alloy.Compared with the as-fabricated coating,the average friction coefficient of the coating heat-treated at1000°C was slightly reduced under the corresponding load and rotational speed.Meanwhile,it showed better load carrying capacity at both high loads and low rotational speeds.