Advances in Furnace Design
Continuous High Temperature Materials Processing
at Low Oxygen Partial Pressures
The vacuum furnace is currently the most widely used equipment for processing materials, which require a low oxygen partial pressure at high temperature. In the closed chamber a vacuum pump system is used to remove the atmosphere (air) and to achieve oxygen partial pressures in the range of 10-4 to 10-6 atmospheres.
In a high temperature chamber, where the oxygen partial pressure is low, solid or liquid metal is formed as metallic oxides decompose to isolate oxygen. The diagram of standard free energy of metal oxide formation (Fig.1) shows that the oxygen partial pressure is constant and fixed when metal and oxide exist together at a specific temperature. It is known that vacuum is not necessary to achieve a low oxygen partial pressure and that there are other methods to eliminate oxygen. If an inert atmosphere with a low oxygen partial pressure is used with a furnace, metal oxide dissociation proceeds according to the equilibrium partial pressure curve, (Fig. 2). It should also be noted that commonly used refractory insulating materials such as silica and alumina brick will dissociate when exposed to such an atmosphere. Thus, in the case of a furnace designed to achieve low oxygen partial pressures at high temperatures, refractory oxide materials are not suitable as insulating materials.
A high temperature, continuous furnace system developed by Kanto Yakin Kogyo Co. Ltd. which provides extremely low oxygen partial pressures without the need for vacuum pumps or hydrogen atmospheres. It is ideal for high temperature processing of metals and ceramics requiring a neutral / reducing atmosphere at temperatures up to 2600C.
The OXYNONÒ furnace requires no oxide insulation or firebrick, and the resulting furnace atmosphere has a very low oxygen partial pressure. The small amount of oxygen in the inert atmosphere, if any, reacts readily with the provided getter. The equilibrium partial pressure curve shows how a slight amount of resulting carbon monoxide will work on metal oxides. Careful furnace design and exclusion of free oxygen from the purging atmosphere insure long furnace and belt life.
A significant concern in vacuum furnace operation is the possibility of material evaporation at low partial pressures. Metals such as zinc and chrome evaporate readily at high temperatures and are deposited in cooler regions of the furnace, often with undesirable results. The OXYNONÒ furnace, however, achieves a low oxygen partial pressure at atmospheric pressure, avoiding the problem of metal evaporation and allowing the employment of metals such as zinc, chrome, silicon, and manganese as alloy constituents of the materials to be processed.
The basic design of the OXYNONÒ furnace is a long tunnel charged with inert gas. When equipped with a conveyor belt the furnace becomes a very economical and efficient system for high temperature processing of metals and ceramics. Continuous furnaces using metal mesh belts are limited to operating temperatures of less than 1120OC. Although some recently marketed furnaces use refractory ceramic or metal belts to temperatures as high as 1600OC, they are expensive and fragile.
An important characteristic of the OXYNONÒ furnace is the use of a carbon fiber reinforced belt for operation up to 2600OC. Until just recently this kind of belt technology did not even exist. The atmosphere prevents air intrusion from the ends of the furnace and exhausts vaporized contaminants to the outside. In most cases high purity nitrogen is used. Argon may be substituted when the material might react to form a nitride. For example, if oxidation resistant stainless steel is produced by a solid solution of chrome in iron in a nitrogen atmosphere, the chrome will nitride and precipitate. The resulting nitride affects both the corrosion resistance and magnetic characteristics of the metal. This can be avoided by using argon as the furnace atmosphere.
Melting reaction inherently produced between the carbon belt and the iron work above 1152OC can be avoided by placing the work on ceramic trays or otherwise separating the work from the belt. Care must be taken in the selection of the ceramic to insure that they are free of impurities, which could damage the furnace or change the process chemistry. Some pretreatment may be necessary.
Some uses of the OXYNONÒ Furnace
(1) Diagram of standard free energy of metallic oxide formation
(2) Diagram of equilibrium oxygen partial pressure of metallic oxide