Study On Optimization Of Heat Treatment Process And Thermal Ageing Behavior Of M5Zirconium Alloy

M5zirconium alloys belong to Zr-Nb alloy series and are mainly used for fuel cladding tubes with the design burn up from55to60Gwd/tu in nuclear reactor. Fuel cladding tubes take service in the harsh environment of high temperature, high pressure, neutron irradiation, boron ionic aqueous corrosion and the inflation, corrosion and brittle fracture of fuel cladding tubes will occur after long term service which directly threaten to the safety of nuclear power plant. Moreover, with tendency of the nuclear reactors to high fuel consumption and long nuclear fuel cycle, there are higher demands on fuel cladding tube materials in mechanical properties and corrosion resistance. At present, there are few researches on Zr-Nb alloys series in our country and the researches have not been reported on the structures and mechanical properties of M5zirconium alloys quenched in β phase zone.The optimization of the obvious orthogonal experiment results have been conducted by the single factor test and the effects of the different quenching and tempering parameters and the thermal ageing treatment at400℃for600h,1000h and1700h on the microstructure and mechanical properties of martensitic M5zirconium alloys have been investigated in order to provide a theoretical reference data for the safety performance and the localization of this zirconium alloys.The results of the single factor test show that the optimum process is quenching at920℃and then tempering at610℃for8h. The grain size of M5zirconium alloy after annealing is very tiny with equiaxed shape and the grain boundary is straight. There are high dense dislocation and a lot of uneven second phase particles in the grain. The microstructure of M5zirconium alloy after quenching and tempering is mainly the rod twinned martensitic structure. The grain is coarse and nonuniform with local serrated grain boundaries and there are a few dislocations and no secondary phase particles in the grain. The fragmentation of twinned martensitic and the density of the dislocation increase with the prolonging of the thermal ageing time.The axis and hoop tensile strengths and the of plasticities M5zirconium alloys after quenching and tempering are obviously higher and lower than those of M5zirconium alloys after annealing, respectively. The hoop tensile strengths and the hoop tensile plasticities individually decrease and increase with the thermal ageing time and the thermal ageing time has almost no effect on the axis tensile properties. The microhardness of M5alloy after quenching and tempering which is227HV is higher than that of M5alloy after annealing which is197HV and it remained fundamentally unchanged before and after thermal ageing treatment. SEM observation indicates that the tensile macrofractures of M5zirconium alloy involved are gray fibrous fractures and the fracture mechanisms are microvoid-aggregation ductile fracture. Due to its better strength and toughness, M5zirconium alloys after quenching and tempering can replace annealing M5zirconium alloy and can be applied in nuclear reactor.The white and compact oxide films are formed in the inner and outer walls of thermal aged M5zirconium alloy cladding tubes after quenching and tempering. The film thickness gradually increases and the color of oxide film turns from white to ivory with the increase of thermal ageing time. When the oxide film reaches a certain thickness, it will happen to crack.