Inleakage during Metal Melting in a Vacuum Induction Furnace

M. I. SLOSMAN

(Presented by Professor, Doctor A. N. Dobrovidov)

One of the most important characteristics of a vacuum furnace is the value of the residual pressure, usually indicated in the furnace passport. However, there is another characteristic of a vacuum furnace, no less important, on which, in fact, the properties of the metal being melted depend: this is the amount of inleakage. The point is that a vacuum furnace practically can never be hermetic. In it, as in other vacuum systems, however carefully they are made, it is impossible to achieve the complete absence of inleakage. One can only ensure that the inleakage is minimal.

Since inleakage has a very substantial effect on the quality of the metal being melted, it is necessary to quantitatively evaluate the tightness of a vacuum furnace, i.e., to measure the rate of inleakage.

This paper gives a measurement of the rate of inleakage in the IV-52 vacuum induction furnace, installed in the Department of Physical Metallurgy for carrying out research work on the problem of vacuum steel.

The pressure in a vacuum furnace may increase due to two sources: a) due to the inleakage of gases from outside through various leaks; b) due to the release of gases from the metal and the refractory materials of the furnace. If the minimum pressure \((P_0)\) is reached in the furnace and the pump is disconnected from the furnace, then the possible variants of pressure change, depending on the above-mentioned sources of inleakage, may be represented in the following figure.

In the ideal case, when there is no inleakage from outside and no gas evolution in the furnace, the pressure should practically not change for a sufficiently long time (line 1). In the presence of gas evolution in the furnace and in the absence of leakage from outside, the pressure in the furnace changes along curve 2. \((P_{\mathrm{H}}\)—the pressure of saturated vapors.) In the presence of leakage from outside and in the absence of gas evolution in the furnace, the pressure changes along line 3. And, finally, in the presence of both inleakage from outside and gas evolution simultaneously (which is usually the case in practice), the pressure changes along curve 4. The tangent of the angle of inclination of the rectilinear section of this curve indicates the magnitude of inleakage from outside. The difference between the measured rate of inleakage and that calculated from the angle of inclination determines the amount of gas evolution.

Since the gases entering the furnace as a result of inleakage and the gases desorbed from the crucible and furnace walls may interact with the molten metal, it is necessary to take into account the effect of both of these sources.

Fig. 1.

The possibility of saturation of the metal with oxygen and nitrogen due to inleakage can be estimated quantitatively as follows. The operating conditions of the vacuum induction furnace under study are: 1) volume of the evacuated space—500 liters; 2) residual pressure in the furnace—\(5 \cdot 10^{-2}\) mm Hg; 3) melting time—1 hour; 4) melt weight—10 kg; 5) measured inleakage rate—500 ml/sec. Let us assume that all the oxygen and nitrogen from the gas phase passes into the metal. At an inleakage rate of 500 ml/sec, during a melting time of 1 hour, 1800 liters of air will leak in. In this quantity of air, at a pressure of \(5 \cdot 10^{-2}\) mm Hg, there are 0.0335 g of oxygen and 0.114 g of nitrogen, which amounts, for a 10 kg melt, to 0.004% oxygen and 0.012% nitrogen.

The value of the inleakage rate measured on a “cold” furnace is not definitive. When the furnace is operating, the crucible heats up and a rather intense evolution of gases begins due to their desorption. Measurement of the inleakage rate on the operating furnace showed that the main source of inleakage is gases and vapors adsorbed by refractory materials.

Measurement of the inleakage rate as a function of the temperature and condition of the furnace

From the table presented it is evident that, under melting conditions, with slight inleakage from outside, the inleakage due to desorption from the refractory lining significantly exceeds the inleakage due to leaks in the vacuum system. Operating experience with the indicated furnace showed that rela-

... sufficiently satisfactory degassing of titanium occurs only after a considerable number of vacuum melts.

When the furnace was pumped down over the course of three days, the inleakage changed as follows: if on the first day the inleakage in the cold furnace was within the range of 20–30 ml/sec, then when the furnace was pumped down on the following day (the furnace was not opened overnight), the inleakage was 10–15 ml/sec, and on the third day it decreased to 4 ml/sec after two hours of pumping. Since in practice it is not possible to pump down the furnace for several days before each melt, it may be considered that vacuum melting in induction furnaces with a refractory lining is melting in an atmosphere containing a certain amount of desorbed gases and vapors. This proposition is confirmed by a number of studies indicating substantial gas evolution from the refractory material [2].

Conclusions

1. The oxygen content in the metal depends on the rate of inleakage. Consequently, the amount of oxide inclusions, as well as the quality of the metal, also depends on the rate of inleakage.

2. The principal source of gases entering the metal during the period of its melting is the gases and vapors adsorbed by the refractory lining of the crucible.

Literature

1. B. I. Korolev. Fundamentals of Vacuum Technology. Gosenergoizdat, 1950.

2. V. E. Hopkins, C. H. Jenkins, H. E. Stone. J. Iron and Steel Inst. 168(4), pp. 377–383, 1951.