Study On The Smelting And Homogenizing Technology Of High Quality N08028 Alloy

Because of the high content of Cr?Mo and Cu in Fe-Ni based corrosion resistant alloys,the alloys have good resistance to pitting corrosion,intercrystalline corrosion and stress corrosion in both oxidizing and reducing media.At the same time,due to higher cost performance than Ni-based alloys,it is widely used in petroleum,chemical,natural gas and nuclear power industries,and it has good market prospect.In this paper,through the optimization of smelting process and the study of high temperature diffusion homogenization process,the manufacturing process of high quality N08028 Fe-Ni base corrosion resistant alloy is developed.The main research results are:(1)Through the study of VOD refining process,the content of C element in oxygen blowing stage is less than 0.01%.The effects of initial molten steel temperature,degassing stage reductant dosage and reduction time on the recovery of Cr and gas content are studied.The average recovery of Cr can reach 98.5%and the limit oxygen content can reach below 10 ppm.In order to improve the recovery of Cr and reduce the O content to the maximum extent during VOD oxygen decarbonization,the optimum initial temperature of molten steel is 16600?-1670?,the addition of Al particles is 200-260kg per furnace during the degassing process,and the reduction time under the limit vacuum should be longer than 10 minutes.(2)In order to further improve the purity of the alloy,three kinds of raw material treatment processes are compared,and it is concluded that the purity of the returned material treated by non-vacuum induction furnace+LF+VOD process is the best.Comparing three slag systems for electroslag remelting,it is determined that the purity of ESR ingot smelted by CaF:Al2O3:CaO:MgO=60%:20%:10%:10%quaternary slag system is the best.(3)There is a brittle and harmful ? phase enriched with Cr and Mo in the as-cast microstructure of N08028 alloy.The existence of this precipitate phase seriously affects the corrosion resistance of the alloy.After holding the same time at different temperatures for the forged samples,the ? phase precipitation peak is 900??1000?,and the complete solution temperature is 1200?.(4)The experimental results show that the segregation of Mo element is much larger than the Cr,but the Cr content(about 27%)is more than Mo element(about 3%),so the segregation degree of two elements is not significant.For the homogenization of Cr,the effect of increasing temperature and prolonging time on reducing segregation is also effective in the range of 1100??1200?;For the homogenization of Mo,prolonging the time is more effective than increasing the temperature to reduce the segregation.(5)Calculated the relationship between diffusion coefficient D and temperature T of Cr and Mo elements in N08028 alloy are:(6)By calculating the residual segregation index of Cr and Mo in N08028,it is concluded that the high temperature diffusion process is suitable for industrial production,i.e.the ? phase in the alloy can be eliminated when the diffusion time is 18-20 hours at 1200?.By means of high temperature diffusion process in pre-deformation and forging,the ? phase in N08028 alloy can be eliminated after only 10 hours at 1200?.