Interaction of zirconia refractories with molybdenum ingots in induction furnaces

Abstract

Ingots of high-melting metals (Mo, W9 Nb, Ta, etc.) are treated at 2000-2500~ in a protective atmosphere or in vacuo in induction furnaces.An investigation of the interaction of the refractory lining of these furnaces with the metal being heated is relevant from the standpoint of the influence of the lining on the quality of the metal and that of the metal on the durability of the refractory, and will make it possible to select the best material for the construction of the lining.A thermodynamic analysis showed that the relative inertness of zirconia to high-melting metals enables this material to be used in high-temperature furnaces.Other workers [i] have used a ZrO2-based lining in a furnace for heating molybdenum ingots to 2000-2100~ and found the thermal strength of the refractory to be satisfactory.Published information deals with the contact interaction of Zr02 with molybdenum, and the temperature at which the interaction becomes significant is stated to be in the range 2000-2500~ [2, pp. 250-255;3-5].The statement that the interaction sets in at 1800~[6] was made, in the present authors' opinion, without taking the purity of the material into account.Presumably the interaction proceeds through the gaseous phase since direct contact exists only with the refractory support.The dissemination of vaporization products in a gaseous medium proceeds by virtue of convection and diffusion so that it is significantly slower than in vacuo.It is assumed [3] that the interaction develops in two stages, viz., the dissociative vaporization of the oxide which gives molecular oxygen, and the oxidation of the high-melting metal by this oxygen followed by the dissociation of the remanent oxygen into atoms:ZrO=(~ol)'~ZrO (gas)-~-l/2 02, Mo (sol)+~/20~=MoO (gas), Mo (sol)+O~----MoO~ (gas), Mo+~/~ O~=Mo03 (gas), O~=20.A mass-spectrometric analysis [7] of chemically pure Zr02 has shown that at a temperature above 2000~ the vapor over the material consists mainly of ZrO ++ ions and a considerably smaller proportion of ZrO2 molecules.A calculation from the equations given by Shchukarev et al. [7] showed that at 2000~ the vapor pressure does not exceed 5-lO-Tmm Hg, i.e., that zirconia can be used at high temperatures in vacuo.The present authors used Knudsen's equation to calculate that at 2000~ the vaporization rate of ZrO2 is about 6.10 -9 g/cm2.sec.For a similar ceramic stabilized with CaO the vaporization rate determined from the weight loss in vacuo [8] was found to be greater, viz., 5"10-7-9-10 -7 g/cm=-sec, evidently owing to the rapid elimination of the calcium oxide at that temperature.According to Espe [5], the vaporization rate of molybdenum at 2000~ is 5.3"10 -9 g/cm2.sec and the vapor pressure 4.10-7 mm Hg so that these indices for molybdenum agree approximately with those for zirconia.The presence of oxygen in the furnace results in the formation of molybdenum oxides on the ingot surface which begin to be noticeably sublimated at 540~ At a temperature above 770~ the rate of their formation is approximately equal to...