By Dan Herring

Typical all-fiber-lined hot zone

This is the eighth in a series of articles in our Vacuum Heat-Treatment Series. This part talks about vacuum hot zones, their history, construction and maintenance. The type of hot zone construction is often important both to the material and to the process.

The first commercial vacuum furnace was sold to industry in 1929. In these early years vacuum furnaces were hot-wall retort designs; that is, alloy retorts placed inside atmosphere furnaces in which a vacuum was pulled on the retort interior. By the late 1950s, vacuum furnaces were gaining wider acceptance, particularly within the commercial heat-treatment industry. Larger sizes were especially in demand, prompting furnace manufacturers to consider alternative designs. The early 1960s saw the introduction of the first all-graphite-felt hot zone with graphite (cloth) heating elements. This was followed a few years later by a hot zone construction consisting of a molybdenum hot face backed with Kaowool insulation and graphite (tubular) heating elements. These early designs suffered from a combination of ills (leaky vacuum vessels, poor element life, workload contamination and contact carburization concerns) as the industry struggled to understand how vacuum furnaces needed to be built.

Construction Methods

Figure 1. Examples of hot zone designs [3]

In the mid 1960s the first all-metallic hot zone was introduced. The hot zone consisted of 2-3 molybdenum shields backed by 2-3 stainless steel shields and molybdenum heating elements. Other combinations soon followed. These designs contained no Kaowool or graphite, and by this time vacuum vessel technology had improved with excellent vacuum tightness and good leak rates. Although all-metal hot zones appealed to metallurgists concerned with such things as (gaseous and contact) carburization (from the carbon monoxide and carbon dioxide generated when air infiltrated into the hot zone) and alloy depletion in the parts being treated, it wasn’t long before the price and availability of molybdenum, (relatively) short life and power usage had customers looking for alternatives.

Several updated versions of the all-graphite hot zone were introduced in the l970s. One such design combined graphite-felt insulation with solid graphite elements. Another used a graphite foil hot face backed by graphite felt then Kaowool and had molybdenum elements in the form of 360º bands. These designs had the insulating characteristics of a graphite furnace without the power losses associated with an all-metallic design. By the early 1980s the Kaowool was replaced and graphite board insulation became popular.

Today, most vacuum furnaces contain graphite felt or graphite board hot zones with graphoil hot faces and solid graphite (bands or hexagonal-shaped) heating elements. All metallic heating chambers are also popular in certain applications as are all fiber-lined units.

Insulation Types

The most common insulation designs and materials (Fig. 2-5) can, in general, be classified as:

• All metallic (radiation shields or shield pack)
• Combination (inner metallic shield separated or backed by ceramic or graphite insulation)
• All graphite (board, fiber, carbon-carbon composite)
• All fiber

It is important that the hot zone support structure be designed to prevent distortion of the insulation, which would cause warpage, cracking or gaps through which radiant energy can leak. The structure must be simple and allow a fastening system that avoids undue conductive heat losses while holding the assembly rigid. Hot zone superstructures can be as simple as steel expanded metal mesh or as complex as stainless steel enclosures. The critical factor is to help ensure proper temperature uniformity in the workload area and minimize heat loss to the shell.

Fig. 2a. Typical all-graphite hot zone (graphite board/graphite felt/curved graphite elements)[4]

Another important factor in hot zone design is thermal expansion and contraction, especially important in today’s high-pressure gas quench designs. The expansion rates and temperatures must be taken into careful consideration in the design stage to allow for proper clearances around element supports, nozzles or restraint systems so that the insulation remains flat with minimal buckling or cracking.

Radiation shields can be manufactured from:

Most all-metallic designs consist of a combination of materials. For example, three molybdenum shields backed by two stainless steel shields would be typical for 2400°F (1150°C) operation. Radiation shields are made with (relatively) expensive materials and are labor-intensive to construct. When compared to purchase other types of insulation, their heat losses are high and become higher with loss of emissivity (reflectivity) due to the gradual oxidation and contamination of the shields. Properly designed, all-metallic hot zones have two distinct advantages: surface area is small (relative to fiber insulation) so absorbed and desorbed gases are reduced, facilitating pumpdown; and heat storage is low, promoting faster cooling.

Figure 2b. Typical all-graphite hot zone (graphite insulation/ graphite foil hot face/curved graphite elements)[4]

Combination (or so-called sandwich insulation pack) designs are composed of one or more radiation shields typically with ceramic wool insulation between or behind them. Combinations of graphite fiber sheets and ceramic insulation wool are also used. These versions are cheaper to buy and maintain but adsorb higher levels of water vapor and gases (due to the very large surface area of the insulation wool). Their heat losses are considerably lower than those of radiation shields. Advantages of this style include low cost, good maintainability and good insulation value. Disadvantages include: a tendency for the blanket to shrink, leaving voids, which allows heat loss; dusting of the material, particularly after devitrification; and a strong tendency toward adsorption of water vapor. The systems must be supported by hangers, which project through the insulation adding to potential heat loss problems.

Graphite fiber insulation, especially in the form of fiberboard, has very low adsorption rates, ensuring fast pump-down speeds and reduced outgassing compared to ceramic fiber. The speed at which graphite-lined hot zones reach their ultimate vacuum and life depends strongly on the purity of the graphite. Designs typically cost more than combination insulation. Advantages include lower heating costs, ease of installation and extended life. The maximum operating temperature is around 3630°F (2000°C). In some applications, such as brazing, a sacrificial layer is used to protect the insulation beneath.

Fig. 3. Typical all-metal hot zone (three molybdenum shields / two stainless steel shields/ molybdenum heating elements/ceramic nozzles)[4]

It is also important to recognize, especially when doing a reline, that not all graphite-based material is the same. For example, carbon felt, which is fired at elevated temperature to produce graphite felt, has porous cells. The higher the porosity, the greater the fiber surface area. Thus, more air and moisture retention when chambers containing low-purity material are exposed to air. This means more outgassing and longer vacuum pump-down and degassing on heating in vacuum. The higher the firing temperatures, the better the purity and, in general, the more expensive the material.

All fiber-lined batch and multi-chamber designs are also used in many low- and high-temperature applications. In these designs, heat loss is extremely low and the furnaces can often be opened at temperature without damaging the insulation. For equal material thickness, kaowool is a better insulator than graphite by perhaps as much as 20%. Vacuum units with fiber insulation can be supplied with electric elements or be gas-fired.

Process Assurance

Figure 4a. Typical combination metal and graphite hot zone (molybdenum hot face/ceramic insulation /molybdenum heating elements /molybdenum gas nozzles)[4]

The process demands on vacuum furnaces are numerous – from hardening to brazing to sintering with a plethora of other thermal processes, from ultrahigh temperatures to temperatures barely above ambient, from ultrahigh vacuum to atmospheric pressures just below atmospheric. Irrespective of how the hot zone is insulated, the goal is to conserve the loss of thermal energy to the walls of the vessel and to protect parts being processed.

Hot Zone Maintenance

Figure 4b. Typical combination metal and graphite hot zone (graphite board/graphite felt/curved molybdenum heating elements)[4]

As with any piece of equipment, proper maintenance at regular intervals is essential for long service life and trouble-free operation. Maintenance considerations begin with good operating (process) practices. Start by eliminating all sources of part discoloration due to air (oxygen), dirt (debris, oil, cleaning compounds) and water vapor. Simple maintenance methods yield surprisingly good results. Cleaning dirty door seals, inspecting then cleaning/regreasing or replacing worn or cracked “O” rings, avoiding broken thermocouples, and making sure fittings properly seat and seal are just a few of the steps that can be taken to minimize air infiltration and lengthen the life of hot zone components.

Contamination

Fig. 6. Debris buildup in a furnace hot zone (photo courtesy of VAC AERO International)

In addition to these problems, vaporization of volatile elements when heated under vacuum will eventually contaminate the furnace internals with undesirable residues. The reduction of this condition may be accomplished by deteriorating ultimate vacuum levels, introducing inert gas and operating in partial-pressure ranges. Dirt or debris in the form of metallic residues can cause electrical short circuits in hot zone heating elements. Over time, a considerable amount of contaminants can build up, often with no discernable change in operating pressures until disaster strikes (Fig. 6). Regular bake-out cycles must be performed to burn off contaminants before they begin affecting load quality or create electrical problems. Depending on the cleanliness of the work processed previously, a bake-out cycle may also be required immediately prior to processing a critical workload or materials that are particularly prone to contamination.

For example, partial-pressure systems should be set up in such a way as to avoid vaporization of elements like chromium. Equally important is to limit the heating rate, typically no greater than 25°F/minute (13.8°C/minute). Otherwise, dramatically shortened hot zone life results. Another good practice is to be aware of low melting point eutectics that can damage the interior. For example, molybdenum has a melting point of 4730°F (2610°C), but in contact with nickel results in a nickel-molybdenum eutectic at about 2310°F (1265°C). Air leaks are a major reason for the destruction of any hot zones and loss of efficiency in metallic-lined units. Brazing is another process that should be carefully considered. Having a sacrificial layer has often avoided costly replacement. Oil and even cleaning chemicals can outgas and create problems.

Safety

There are a number of safety issues that must be considered when maintaining vacuum furnaces. Standard safety practices must be adhered to in order to avoid injury, burns and electric shock (c.f. NFPA 86). In addition to these, there are several special considerations specific to vacuum furnace equipment. Maintenance of furnace chamber internals should only be conducted using approved confined space entry and electrical lockout procedures. Lockout procedures to prevent furnace operation must be in place before entering any furnace chamber.

Outgassing

Whenever the door to a vacuum furnace chamber is open, humidity from the air (even in the dry desert) will enter the chamber and water vapor will condense on the chamber walls and be absorbed into the hot zone materials. When the chamber is subsequently evacuated (before heating) and the furnace internals are exposed to this lower-pressure vacuum, “outgassing” of the entrapped moisture will occur. If sufficient moisture has been entrapped (such as in very humid environments), the outgassing effect will slow the pump-down process and may even give the appearance of a malfunction in the pumping system. Eventually, the outgassed moisture will be pulled out of the chamber by the pumping system and evacuation rates will improve. This same effect will be apparent when oily or contaminated workloads are placed in the furnace. It may be more pronounced in furnaces with ceramic felt or carbon-based hot zone insulation materials. To minimize the outgassing effect, it is important to keep the chamber door closed whenever possible. Ideally, the chamber should also be kept at least partially evacuated whenever the furnace is not in use. Maintaining the recommended temperature of the coolant entering the chamber cooling jacket is also important. Condensation of moisture is pronounced on cooler surfaces.

Part nine of this series discusses the types of heating systems available for vacuum furnaces and explains how temperature uniformity is achieved in various styles of furnaces.

Daniel H. Herring / Tel: (630) 834-3017) /E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance. He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.

References

1. William. R. Jones, CEO, Solar Atmospheres (editorial review)
2. Pritchard, Jeff, "Hot-Zone Design for Vacuum Furnaces," Industrial Heating, 2007
3. Heat Treatment of Steel, 2nd Edition, Chapter 7, "Vacuum Heat Processing," Totten, George E. [Ed.], 2006
4. Vacuum Furnace Systems (VFS), product literature
5. Metalsky, William J., "Hot Zone Considerations for Vacuum Furnace Processing," Industrial Heating, 1993
6. The Nature of Vacuum, SECO/WARWICK Corporation
7. Maintenance Procedures for Vacuum Furnaces, VAC AERO International, Inc.
8. Moyer, Michael, Keeping it Bright, ASM International Vacuum Maintenance Seminar, Anaheim, CA, 2009.