Vacuum Induction Melting of Superalloys
Vacuum induction melting is the first step in the melting of all superalloys (including deformation, casting and powder superalloys). Its technological advantage is that the oxidizable strengthening elements Al and Ti can be added to the alloys to improve the properties of the alloys on the premise of avoiding metal oxidation. Because of the continuous chemical reaction between the superalloy melt and the refractory crucible wall in vacuum induction melting at about 1550 C, ceramics inclusions such as Al2O3, ZrO 2, CrO 3 and rare earth oxides are formed. At the same time, inorganic inclusions in the refractory runner system are also introduced into the process of casting the electrodes. A lot of research work has been done on improving the purity of high-temperature alloys in vacuum induction melting process at home and abroad. The process measures including foam ceramic filtration, calcium oxide (CaO) crucible refining, electromagnetic stirring, rotary ingot casting, composite molten salt purification, electromagnetic soft contact molding and purification have been proposed. Among them, ceramic foam filtration is a commonly used and economical method at home and abroad. The principle of removing ceramic inclusions in alloy melts includes: larger inclusions filtered from alloy melt and blocked to filter screen; smaller size inclusions are adsorbed onto the inner surface of ceramic filter screen or the inclusions captured by filter net. At present, the ceramic filter materials used for smelting superalloys at home and abroad include Al2O3, ZrO 2, mullite (3Al2O3.2SiO 2), cordierite (2MgO.2Al2O3.5SiO 2), etc.
Accurate control of composition of Superalloy by vacuum induction melting process has been widely accepted at home and abroad. The main and trace elements of Wrought Superalloys can be strictly controlled within the required standard range by induction melting under vacuum (including refining under argon protection, electromagnetic stirring, etc.). Even harmful trace elements (Pb, Sn, Bi, etc.) and gas elements can be reduced to several ppm levels (i.e. 10-6), through ceramic filtration, adding additives (C, CaO-based slag, etc.) and The oxidation inclusions and O, N, H in the alloy can be significantly reduced by optimizing the process.
Reducing the content of harmful gases (H, O, N) in vacuum induction melting is an important process. The low solubility of hydrogen (H) in the melt of superalloy under vacuum and the low affinity between H and alloy elements result in the rapid diffusion of hydrogen to the gas-liquid interface during the melting process, which is also the reason why the content of H in the deformed alloy by vacuum induction melting is very low and can meet the requirement; O element exists in the form of solid solution or oxide impurities in the superalloy, and oxide inclusions decrease the combination. The fatigue and creep properties of gold include carbon (C) deoxidation and oxide deoxidation. The former is the main method. C has a strong ability to deoxidize in vacuum. Based on the principle of reaction kinetics, [C]+[O]= CO (g), because of the continuous evaporation of CO gas and the shift of reaction balance to the right under vacuum, the O content in the alloy decreases continuously. In addition, the content of O in the alloy can also be reduced by adding Al and Ca metals. Reasonable melting process parameters combined with C, Al, Ca and other elements can remove most of O from alloy melt and reach < 10ppm. N in superalloy exists in two forms: solid solution and nitride (TiN, Ti (CN), AlN, etc.). To reduce the content of N in alloy, on the one hand, low N raw materials, especially metal Cr, are needed, on the other hand, refining process needs to be optimized. The results show that the higher the refining temperature, the longer the refining time, the better the effect of N removal. In addition, electromagnetic stirring of alloy melt can also promote the reaction of N removal.
Another important function of vacuum induction melting process is the pure recovery and reuse of superalloy returns. Car scraps account for a large part of the superalloy returns, so it is significant to study the pure melting of car scraps. Y2O3 crucible was used in vacuum induction melting abroad. The content of O and N in GH4169 scrap alloy after remelting was studied experimentally. It was found that with the increase of vacuum, the content of N and O in the alloy decreased (from 72 ppm to 7 ppm; the content of O decreased from 28 ppm to 8 ppm); with the increase of melting temperature, the content of N in the alloy melt did not change significantly, but the content of O and the amount of deoxidizer C did not change significantly. There is a certain relationship.
Although the vacuum induction melting process has prominent advantages in precise control of Superalloy composition and recovery of returned materials, there are segregation of internal shrinkage and porosities, ceramic inclusions and so on in the prepared ingot. The master alloy or electrode rod prepared by vacuum induction melting needs remelting in order to further improve the metallurgical quality and mechanical properties of the material.