Vacuum furnaces currently are widely used to process materials where conditions of low oxygen partial pressure and high temperature are required. In these furnaces, a vacuum pump system is used to remove the atmosphere (air) and to achieve oxygen partial pressures in the closed chamber in the range of 10-4 to 10-6 atmospheres. Metal oxides decompose under conditions of high temperature and low oxygen partial pressure to isolate oxygen, thus forming solid or liquid metal. Oxygen partial pressure is constant and fixed when metal and oxide exist together at a specific temperature, as shown in a diagram of standard free energy of metal oxide formation (Fig.1). Metal-oxide dissociation also occurs in an inert atmosphere having a very low oxygen partial pressure. Thus, commonly used refractory insulating materials, such as silica and alumina brick, also will dissociate when exposed to such an atmosphere. This precludes the use of refractory oxide materials as insulating materials in a furnace designed to achieve low oxygen partial pressures at high temperatures.
A high-temperature, continuous furnace system (called Oxynon(r)) developed by Kanto Yakin Kogyo Co. Ltd. (Hiratsuka, Japan) provides extremely low oxygen partial pressures without the need for vacuum pumps or a hydrogen atmosphere. The furnace is ideal for high-temperature processing of metals and ceramics requiring a neutral/ reducing atmosphere at temperatures to 2600C (4710F). When equipped with a conveyor belt, the furnace is an economical, efficient system for high-temperature processing of metals and ceramics.
The Oxynon furnace does not require refractory oxide insulation or firebrick, and the resulting furnace atmosphere has a very low oxygen partial pressure. The small amount of oxygen (if any) in the inert atmosphere readily reacts with the provided getter. Exclusion of free oxygen from the purging atmosphere ensures 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 chromium evaporate readily at high temperatures and are deposited in cooler regions of the furnace, often with undesirable results. However, the Oxynon furnace achieves a low oxygen partial pressure at atmospheric pressure, which avoids the problem of metal evaporation and allows processing metal alloys containing alloying elements such as zinc, chromium, silicon and manganese.
Furnace design features
The basic design of the furnace is a long tunnel-shaped chamber having an entrance and exit at opposite ends. Part or all of the furnace walls are made of a carbon insulating material to maintain a high temperature inside the furnace.
A carbon fiber-reinforced carbon, or carbon-carbon (C-C), composite conveyor belt (Fig. 2) used to carry the workload through the furnace can operate at ultrahigh temperatures to 2600C carrying a high load. By comparison, continuous furnaces using metal mesh belts are limited to operating temperatures lower than 1120C (2050F). Some refractory ceramic and metal belts can operate at furnace temperatures as high as 1600C (2910F), but they are expensive and fragile. Figure 3 shows the comparative strengths of some belt materials. A conventional pusher-tray furnace has certain disadvantages such as substantial heat loss due to the extra thermal mass of setter trays. The C-C conveyor belt, by comparison, has a low thermal mass and can support conventional work trays directly.
The high-purity inert furnace atmosphere (free of oxygen and water vapor) protects the carbon materials and maintains a thermochemical equilibrium with the work. In this atmosphere, oxides are easily dissociated, eliminating the need to use combustible gases. Although oxygen is removed from the inert atmosphere, a small amount of oxygen contained in the work could affect the atmosphere and react with the carbon material of the furnace wall to produce carbon monoxide. The amount of carbon material consumed by this reaction is small and does not affect the long-term stable operation of furnace.
The atmosphere prevents air intrusion from the ends of the furnace and exhausts vaporized contaminants to the outside. Optimally, it is best to use the smallest volume of gas practical to maintain a positive pressure inside the furnace, while uniformly flowing gas out the entrance and exit. This also exhausts vaporized contaminants generated by the work to the outside. Contaminated gas can affect operation of the furnace if the volume becomes too high to exhaust, a situation that might require presintering of the work.
Contaminants generally are created from additives required for powder molding. Light-metal oxides (from zinc, for example) should be particularly avoided because they deposit as solid oxides near the entrance of the furnace. If the temperature at that location is high enough, the oxide will dissociate and form liquid metallic zinc. The liquid metal is then transferred into the furnace where it evaporates into a gas at the higher temperature and returns to the entrance as a gas to be oxidized again. The process is repeated, eventually destroying the atmosphere and furnace structure.
High-purity nitrogen atmosphere is used in most cases, but argon may be substituted when the material might react to form a nitride. For example, chromium in stainless steel reacts with the nitrogen atmosphere to form chromium-nitride precipitates, which affect both the corrosion resistance and magnetic characteristics of the metal. This is avoided by using argon instead of nitrogen.
The melting reaction that can occur between the C-C composite belt and iron-base parts at temperatures above 1150C (2100F) 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 ensure that they are free of impurities, which could damage the furnace or change the process chemistry. Some pretreatment might be necessary.
In the case of a stationary or batch type furnace, the workload entering the furnace at room temperature usually is heated up using a program matched to the heat source, or the work is loaded into a preheated furnace and kept there for a predetermined period of time. By comparison, three or more independent temperature control zones are provided in the Oxynon furnace according to production requirements. The temperature of the control zones can easily be set according to the material, weight, configuration and production volume. For instance, various temperature zones are programmed according to the belt speed and processing time through the furnace to maintain a continuous flow of work. Because this heating method closely controls the heat load, the continuous furnace is ideally suited to produce a high volume of work having very consistent quality.
Lubricant and binder removal
The removal (burn-off) of hydrocarbon based lubricants and binders from the green powder compacts is carried out in a presintering chamber with the off-gases either exhausted or decomposed. Inert gas is used both as an atmosphere and to purge the furnace. A C-C composite belt is used at furnace operating temperatures higher than 1120C (2050F), while a metallic belt is used at lower temperatures. Inert gas must be used in both the presinter and high temperature furnaces when using a C-C composite belt, while air can be used in the presintering chamber when using a metallic belt.
If the presintering zone is located close to the sintering zone, off-gases generated in the presintering chamber can readily flow into the sintering chamber. However, the gases are returned to the presintering chamber and exhausted outside by feeding pure gas into the sintering chamber to raise the furnace pressure. The off-gases contain a significant amount of polymerized oily mist, which can cause unsafe furnace operation if chamber and exhaust-duct components become filled with the high-viscosity material.
Carbonization of the organic binder in the green molded powder parts, which needs to be decomposed and exhausted, is minimized by evaporation at comparatively low temperature. Thus, the furnace as a whole must be kept at an adequate temperature and filled with appropriate atmosphere to prevent the volatile material from reaching the sintering zone. The composition of the gas exhausted from the presintering process, and the correlation between exhausted volume and temperature significantly affect the furnace structure and atmosphere.
Burn-off also can be accomplished in a presintering and effluent gas-decomposition chamber. It is desirable to decompose the polymerized oily material generated by the presinter operation into carbon monoxide and water and exhaust the decomposition products. This is accomplished using an oxidizing and combustion zone (controlled to contain 5% free oxygen) in the bottom part of presintering chamber. The polymerized material is exhausted after being held for several seconds at a temperature of 850C (1560F) or higher. The circuit connecting the presintering zone to the gas-decomposition zone is kept at a negative pressure to prevent the gas from escaping. The heat source used to aid the combustion can be either fuel gas or electric.
Typical applications for the furnace include sintering metals and ceramics and oxide dissociation of powders. Metals sintered include alloy steels, chromium steels, high-alloy and stainless steels, copper alloys and others. Other applications include debinding and reaction sintering of oxide, nitride, carbide, boride and other ceramics; brazing of carbon and alloy steels, copper-alloyed steel, stainless steel and aluminum alloys; diffusion bonding of metals to ceramics; and firing and sintering of inorganic fibers. Representative microstructures of a sintered stainless steel and titanium alloy are shown in Fig. 4.
The first Oxynon furnace delivered to the U.S. is being installed at the Pennsylvania State University Center of Innovative Sintered Products, or CISP (tel: 814-865-2121; Internet: www.cisp.psu.edu) in University Park, Pa. and is expected to be operational by mid-summer 2002. The four-zone furnace (Fig. 5) is capable of continuous operation at temperatures to 2000C (3630F).