Titanium transfer from molten steel to the lining in a vacuum induction oven

Abstract

The lattice in an oxide is disrupted by ions and electrons diffusing from a hot zone to a cold one, which favors metal-component mass transport in linings, ioe., from the contact layer into the lining~ The density of the thermal diffusion current i is [1] i~_vj.n2 ( e$ dT dq~ ee dT d~ T dx +q-'d~x )+v~n~ (--~.-+e--~'-x),in which v i and v e are the ion and electron mobilities, with n i and n, the concentrations of the ionic and electronic defects, while ~i and se are the corresponding activation energies, with q and e the charges on the positive ions and electrons and dT/dx and d~o/dx are the temperature gradient and the potential gradient. Refractory oxides are considered as virtually stoichiometric compounds, but this is so only for moderate temperatures; they become nonstoichiometric at high temperatures~ For example, zirconium dioxide at 1200~ has a component ratio corresponding to ZrO1.86, not ZrO 2 [2]. Therefore, one can estimate the currents flowing between phases in the metal-lining system by using both parts of (1), which characterize the contributions from the ionic and electron defects to the thermal-diffusion current density.The ionic-defect and free-electron concentrations are represented by the same type of equation [1]:3.If the lining or stack in an oven is not in contact with the liquid metal but is subject to thermal stress, the thermal diffusion current is gradually attenuated because of the concentration-dependent diffusion flux. When the lining contacts the liquid metal, such balance is not attained because ions from readily ionized atoms continuously enter the contact layer.(1) indirectly implies that the transfer rate under otherwise equal conditions is governed by the oxide disorder, i.e., the defect formation energies, so we consider the kinetic relationships for the metal-lining transfer from the viewpoint of disordering, which can be estimated from the resistivity.The experiments were performed with steel containing 2% titanium at 1600-18500C in a vacuum induction furnace having a capacity of 1 kg and crucibles made from fine-grain powder (fraction around 100 #m) of aluminum, beryllium, zirconium, and magnesitim oxides. The final sintering for the crucibles was for 2 h at 1700~ in an oven with graphite heaters. The crucible wall thickness was about 10 mm (top) or 15 mm (bottom)~ The initial porosity was determined from the theoretical density together with the mass and volume, and the values were 38, 39, 34, and 36% correspondingly for the aluminum, beryllium, zirconium, and magnesium oxide crucibles. These high porosities occur because the refractory materials for making the crucibles were powders of about the same fraction size. The firing temperature and time were insufficient for the crucibles to sinter further. The impregnated-zone thickness was dependent on the material and on the temperature and time in the experiments; it was 2-10 mm. We found that the impregnation and blackening occurred throughout the wall and bottom of the crucible at 17500C and above.The liquids were kept...