| In order to realize the full localization of nuclear zirconium,the existing production of Zr-4 ingots on ?220??440 mm must be expanded to ?720 mm to achieve the internationally advanced melting level of nuclear zirconium alloys.In the development of ingots on ?720 mm,the Fe element exceeded the standard,and the ingot was added and removed according to the requirements of ASTM B495.The yield was less than 85% at the last.If implementing national standard GB/T8767-2010,the ingot will be directly scrapped.Therefore,the precise control of Fe elements has become a bottleneck in the domestic production of large Zr-4 ingots.The precise control of Fe elements is a great problem for the manufacturers of nuclear zirconium materials all over the world.And macro segregation is one of the main factors that lead to the low yield of zirconium alloy ingots.On the contrary,as the size of the ingot increasing,the control of the Fe element is more difficult.For example,the maximum difference between the Fe element at the head of the ingot and the bottom of the ingot can reach500 ppm,while the content of the Fe element in the Zr-4 alloy is 1800 ? 2400 ppm.The control range is very narrow,Ingots are often removed by the machine because of uneven composition.In this paper,the preparation of Zr-Fe master alloy,the treatment of raw material distribution and VAR melting are used to focus on the key technologies affecting the homogeneity of Fe in Zr-4 ingot: the addition of Fe element,the treatment of raw material homogeneity and the vacuum melting technology of large-size Zr-4ingot.The main conclusions are as follows.(1)Using vacuum argon arc welding,the mixture of sponge zirconium and Fe particles is fully melted,and three kinds of mixtures of Zr6 Fe,Zr9Fe and Zr12 Fe are obtained,which can produce master alloys with excellent alloying effect.The experimental ingot(?160 mm)of Zr-4 alloy was produced by using a master alloy with a ratio of less than 9% of Fe element,and the results of chemical composition analysis of Fe elements on the side wall and the bottom of the ingot showed that the extremely poor content of Fe in different parts was only 0.01 %,and the overall distribution of Fe elements was uniform.(2)Through field experiments,the effects of motor frequency of distributor and silo excitation force on uniformity of distribution system are compared.The recommended optimum process conditions are that motor frequency must be greater than 30 Hz and silo excitation force should be strictly controlled between 50% and 60%.The difference in the weight of the sponge zirconium partition between the 12 connecting barrels is the smallest,and less than 1% of the total material weight.The content of Fe elements in different parts of the pure zirconium experimental ingot produced by the material after the fabric is extremely poor at 0.0074 %,which is far lower than the pure zirconium ingots of the same specification produced without the fabric.(3)The effects of melting current,stirring coil and cooling intensity on the uniformity of chemical composition of Zr-4 alloy ingot were studied.L9(3)4 orthogonal test method was used to study the influence of different factors and the same factor on the uniformity of Fe elements.Nine experimental schemes were selected for trial production of Zr-4 and the content of Fe elements in different locations of ingots was analyzed.It was found that with the increase of melting current and cooling intensity,the uniformity of Fe element(extremely poor distribution)both show a pattern of first lowering and then rising,and with the increase of stirring coil,the distribution of Fe element was more uniform.The orthogonal test method was used to analyze the results of the research.It was found that among the three influencing factors of melting current,stirring coil and cooling intensity,the effect of stirring coil on the homogeneity of Fe element in Zr-4 ingot was the greatest,followed by melting current and cooling water intensity.After repeated production verification,the optimized melting parameters are melting current: 22 KA,magnetic stirring current: 6 A,cooling intensity: 80 m3/h.(4)The Zr-Fe master alloy preparation and the reverse segregation of the master alloy,the homogenization of raw materials by using the distribution system and the optimization of recommended melting parameters were adopted.The industrialized Zr-4ingot with a large size of 720 mm was successfully produced,and the yield is 92%.The internal structure of the ingot was equiaxed grains,which facilitated the subsequent processing.Although there is a certain positive segregation trend,the overall distribution is uniform;the original 720 mm ingot is with requirement for one-time analysis of Fe,resulting in a low yield of products has been improved,achieving the720 mm one-time sampling qualified,without material removal phenomenon caused by the unqualified element of Fe,greatly promoting the technical level of producing Zr-4alloy ingot.And more,a large number of thick columnar crystals existing in the typical non-equilibrium sequential solidification structure,and the large size Zr-4 ingot developed is mostly isoaxial crystals that are conducive to subsequent processing.The above research represents that the melting technology of Zr-4 alloy ingots in China has reached the international advanced level.At the same time,through this study,the first internal Fe element distribution model and internal organization model of Zr-4ingots in China have been established.It reveals the distribution of elements inside the molten pool and the internal microstructure of the molten pool,which promotes the combination of theory and practice,and provides technical support for the improvement of zirconium alloy smelting technology and the research and development of new zirconium alloys. |
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