Experimental Study On The Reactivity Of Low Fluoride Slag System And Nickel-based Superalloy

As a typical of high titanium low aluminum nickel-based superalloy,Incoloy825alloy is widely used in aerospace and energy fields.The content of Al and Ti in the alloy directly affects the microstructure,mechanical properties and corrosion resistance of the alloy.Electroslag remelting is the main refining method for Incoloy825 alloy.Due to the redox reaction between slag and metal in the process of refining,Al and Ti in the alloy are oxidized by TiO₂ and Al₂O₃ in the slag.Loss of Ti to increase Al or loss of Ai to increase Ti.Therefore,it is necessary to design an appropriate slag system to control the content of Al and Ti elements in the alloy.At present,most of the slag systems used in electroslag remelting are high-fluorine slag.The volatile fluoride gas generated during the smelting process not only pollutes the environment,but also fluctuates the content of components in the slag,causing secondary oxidation of alloying elements and worsening the electricity Slag ingot quality.The low-fluoride slag system has a low CaF₂ content and a small amount of fluoride volatilization,which is conducive to the stable slag-gold reaction.Therefore,it is of great significance to develop a low-fluorine slag system suitable for smelting Incoloy825 alloy.The phase diagram of the quaternary slag system CaF₂-CaO-Al₂O₃-TiO₂ was drawn with the help of FactSage thermodynamic software,and the low temperature range of the low-fluoride slag system was obtained.The order of influence of the group members on the viscosity of the slag system is:CaF₂>CaO/Al₂O₃>TiO₂>MgO.The order of influence of each component on the conductivity of the slag system is:CaF₂>CaO/Al₂O₃>MgO>TiO₂.The designed low-fluoride slag ratio is 23%Ca F₂,CaO/Al₂O₃=0.9,2%MgO,4%Li₂O,and 0?12%TiO₂.In order to clarify the thermodynamic mechanism of Al and Ti oxidation in Incoloy825 alloy,a thermodynamic model of slag-metal reaction was proposed.The model results show that the nature of the oxidation of Al and Ti in the alloy is the effect of the components in the slag on the activity and activity ratio of Al₂O₃ and TiO₂.With the increase of Ca O and Al₂O₃ content,the equilibrium Ti content decreases.As the TiO₂content increases,the equilibrium Ti content increases.The influence of components in the slag on the equilibrium Al content is opposite to that of the equilibrium Ti.The content of CaF₂ and MgO has little effect on the equilibrium Al and Ti content in the alloy.When the initial Al and Ti contents in the alloy differ greatly,the Ti element is susceptible to oxidation during the reaction.When the phase difference is small,the Al element is susceptible to oxidation during the reaction.The kinetics model of low-fluoride slag system and Incoloy825 alloy was proposed based on the two film theory.The calculated values of Al,Ti and Si elements in the alloy and TiO₂ in the slag are in good agreement with the measured values.The dissolved oxygen in the alloy reacts with the elements Al,Ti,and Si in the alloy to reduce the content.The measured values of Al₂O₃ and SiO₂ in the slag are lower than the calculated values due to the reaction with CaF₂.The controlling step of reaction4[Al]+3?TiO₂?=3[Ti]+2?Al₂O₃?was the mass transfer of Al and Ti in the liquild alloy.The controlling step of reactions[Si]+?TiO₂?=[Ti]+?SiO₂?and[Al]+?SiO₂?=[Si]+?Al₂O₃?was the mass transfer of SiO₂ in the molten slag.