Studies On Heat Treatment And Welding Properties Of F91 Material For Main Steam Isolation Valve

The main steam isolation valve is the critical equipment of secondary circuit of high temperature gas cooled reactor, the quality of sealing pair is a very important to the nuclear power station. F91 heat resistant steel is used to sealing pair of the main steam isolation valve,because it has lower linear expansion coefficient and high temperature stability. Cobalt base alloy, which is with high wear resistance and high temperature resistant corrosion, is considered to be a very optimized surface strengthening alloy for sealing pair.Plasma surfacing technology has been widely used on the welding reinforcement of wear resistant alloy, because it has parent metal lower dilution rate, good weld-seam shaping.The heat treatment is used to improve the performance of F91 heat resistant steel.And F91 heat resistant steel is deposited with Co-based alloy hardfacing coating by plasma surfacing welding technology to improve its surface properties. Influence of different heat treatment on properties of F91 heat resistant steel and influence of different welding current on the microstructure and mechanical performance are measured by scanning electron microscope, X-ray diffraction(XRD), Tensile testing machine, impact testing machine,ear testing machine and hardness test,etc. The experimental results are as follows:(1) When the tempering temperature is 760℃, F91 heat resistant steel gets best mechanical performance.(2) Coatings of Co101 F and Co112 F consist of dendritic structure and eutectic structure with petal shape and cylindricity shape by scanning electron microscope. Analysis presents the existence of a Co-rich γ phase solid solution,carbides Cr7C3 and Cr23C6 in the coatings.(3) Coating of Co101 F is thin and bad weld-seam shaping,when welding current is 140 A.Dilution rate of coating is too large, so it affects the performance of coating, when welding current is 220 A.Performance of coating is optimal, when welding current 180 A.(4) Coating of Co112 F gets best performance,when welding current is 180 A.Weld heat input is too less,so the coating has porosities and non-fusion, when welding current is 140 A.Dilution rate of coating is too large, so it affects the performance of the surfacing, when welding current is 220 A.