| Nickel-based heat-resistant C700R-1 alloy has good tensile strength,yield strength,plasticity,corrosion resistance,hot workability and weldability,and is considered to have over 100 MPa of 105 h creep rupture strength at the serving temperature(700℃)and pressure(35 MPa).This superalloy has been selected as an alternative material for advanced ultra-supercritical steam turbine rotors.However,solidification segregation,including microsegregation and macrosegregation,generally exists in the solidification process of nickel-based alloys.The former will lead to uneven distribution of elements in the dendrite,which increases the difficulty of subsequent homogenization and heat treatment;The latter will lead to a wide range of uneven composition in the alloy(freckle is common in nickel-base alloys).The freckle can usually reach the millimeter level,which will seriously damage the alloy performance and cannot be eliminated through heat treatment and other processes.The material can only be scrapped.This study was financially supported by national project "research and development of key heat-resistant materials and components for supercritical steam turbines,No.2021 YFB3704102".This subject mainly aimed to solve the problem of internal solidification segregation of large-sized ingots.First,the thermodynamic transformation of C700R-1 alloy under equilibrium and non-equilibrium solidification conditions was Calculated through Thermo-Calc software,and the segregation behavior and solidification characteristics of C700R-1 alloy under different temperatures and cooling rates were analyzed.Thermo-Calc software was used to adjust the alloy composition,and comprehensive research was carried out from the temperature range of the solid-liquid,the segregation of elements in the liquid phase,the relative Rayleigh number and the precipitation rule of the second phase,to obtain the best C700R-1 alloy composition;Meltflow software was used to fit and verify the smelting process of 660 mm ingot.The key links of 810 mm ingot smelting molding under different cooling conditions and smelting rates were simulated.The restrictive links in 920 mm ingot smelting process were explored.The homogenization treatment system and high temperature element diffusion mechanism of C700R-1 alloy at different cooling rates were studied.All work will be targeted to improve the triple process,homogenization treatment and other key smelting and processing processes,so as to provide a reliable guarantee for manufacturing large-sized turbine rotor ingots without macrosegregation.Firstly,the thermodynamic results of the solidification phase transition of C700R-1 alloy were Calculated according to Thermo-Calc software.The solidification behavior of C700R-1 alloy at slow cooling rate(5℃/min)and different cooling rates was studied by using high-temperature digital microscope,differential scanning calorimeter.The results showed that the dendrites nucleated and grew rapidly at the beginning of solidification,while the residual liquid between dendrites solidified very slowly.The main phase transformation of solidification was as follows:(1)L→γ(1370℃);(2)L→γ+MC(1298℃),where M represents Ti,Nb and Mo;(3)L→γ+μ(1217℃).With the start of the solidification process,Al,Cr,Ni and W segregated in the solid phase,while Ti,Nb,Mo,C,Zr and B accumulated in the residual liquid phase,resulting in the formation of MC carbides and μ phase.In the final solidification stage,the segregation of Mo and Cr lead to the formation of multiple precipitates(μ,M6C,MC and M3B2)and very low solidus.The solidification process of the alloy at different cooling rates shows that with the increase of the cooling rate,the dendrites are gradually refined,and the dendrite morphology becomes irregular.The prediction formula of the secondary dendrite spacing and slow cooling rate solidification can be expressed as:λ=192 X(GR)-0.41.With the increase of cooling rate,the size of precipitates decreased significantly.The degree of segregation of W,Ti,Nb and Mo gradually increased,and then decreased.The segregation degree of Al,Ni and Co was small,and Cr was basically non segregated.The effective partition coefficients of Ti,Nb and Mo decreased linearly with the increase of cooling rate,and then increased again.The minimum value appeared at the cooling rate of 6.8℃/min.During solidification,the concentration of Ti,Nb and Mo in the dendrite nucleus increased with the increase of solidification time,which is due to the existence of solid phase back diffusion.The Equilibrium module in Thermo-Calc software was used to Calculate the influence of alloy composition on the solidification solid and liquid zone,the degree of element segregation in the liquid phase and relative Rayleigh number.The Scheil-Gulliver module was used to Calculate the influence of Ti,Nb and W on the precipitation type and precipitation order of the second phase of C700R-1 alloy during solidification.The order of the ability to narrow solid and liquid region was:Nb>Cr>Al>C>Ti>B;The order of capacity of expanding solid and liquid region was Zr>Co>W>Mo;C had a significant effect on positive segregation elements.Increasing the content of C could significantly reduce the segregation of Ti,Nb,Mo and Zr in the liquid phase.Reducing the content of Ti,Nb and Mo could also increase the segregation degree of positive segregation elements;Increasing the content of Al and Nb could reduce the probability of occurrence of freckle in the solidification stage of the alloy,and increasing the content of Cr and Ti could not only reduce the probability of occurrence of freckle in the solidification stage of the alloy,but also delay the temperature of occurrence of freckle;The addition of Ti and Nb made the tendency of MC carbide precipitation gradually increased during solidification,while the tendency of M6C and M23C6 formation gradually decreased.In order to reduce the content of MC as much as possible,the content of Nb should be less than 0.3%.The addition of W has no effect on the type and order of precipitates.Subsequently,based on the previous experimental results and the feedback data from the factory,the solidification of 660 mm,810 mm and 920 mm ingots was simulated by Meltflow software.When the melting rate of 660 mm ingot was less than 5 kg/min,the internal freckle could be effectively avoided;When the melting rate was 5.5～5.9 kg/min,there was no macrosegregation in the 810 mm ingot,which was the best process;Although different melting rate were adopted,there were still serious freckle in 920 mm ingot;Introducing helium gas into the smelting process could effectively improve the heat dissipation of ingots and reduce the volume of freckle.The increase of heat transfer coefficient was limited with the increasing helium pressure.Finally,the high-temperature diffusion mechanism of solute elements in C700R-1 alloy system was studied based on the remelting experiment designed with a cooling rate of 0.5～10℃/min.The best temperature of homogenization was 1190℃.After holding at 1190℃ for 80 hours,the dendrites were completely eliminated,and the large precipitates had been completely dissolved,but there was no effect on TiN.The actual diffusion coefficients of Ti,Nb,Mo and W in C700R1 alloy at 1190℃ Calculated by experiments were D[Ti]=2.352×10-12·m2·s-1,D[Nb]=6.291×10-13·m2·s-1,D[Mo]=6.070×10-13·m2·s-1,D[W]=1.587×10-14·m2·s-1.The actual diffusion coefficients of Ti,Nb,Mo and W in C700R-1 alloy at 1170℃Calculated by experiments were D[Ti]=1.647×10-12·m2·s-1,D[Nb]=5.415×10-13·m2·s1,D[Mo]=4.353×10-13·m2·s-1,D[W]=1.066×10-14·m2·s-1. |
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