Hot Zone Design for Vacuum Furnaces
Hot Zone Design for Vacuum Furnaces

Insulation/Heat Shield

Molybdenum and graphite are also used for vacuum-furnace insulation. In an all-metal hot zone the heat-shield package is made up of multiple layers of sheet metal with spaces between each layer (Fig. 3). In a standard furnace with a maximum 1315°C (2400°F) operating temperature, the heat shielding often consists of two layers of molybdenum sheet backed by three layers of stainless steel sheet. For higher operating temperatures, the number of molybdenum layers is increased as well as the thickness of each layer. For very high operating temperatures over 1650°C (3000°F), tantalum sheet can be used in place of molybdenum. The insulating properties of the all-metal design come mostly from the gaps between the layers of sheet metal. These gaps prevent heat from being conducted outward from the hot zone. The reflectivity of the inner molybdenum sheet also helps to direct the radiant heat from the elements inward toward the load. All-metal hot zones tend to be preferred when high-vacuum or very clean processing environments are required (medical-component processing). Care must be taken in operating furnaces with all-metal hot zones because molybdenum embrittles due to recrystallization after a single exposure to temperatures above about 1150°C (2100°F). Embrittled heat shields can be relatively easily damaged if struck by fixtures or parts. Due to the high cost of molybdenum, all-metal hot zones also tend to be more expensive than some other choices.

Another popular heat-shield design incorporates graphite-based materials. A standard graphite insulation package normally consists of a minimum thickness of 1.5 inches of graphite-based material in the form of layers of blanket or board. An inner face of graphite foil or foil-bonded carbon composite is added to enhance reflectivity. Because the insulating material is itself porous and permeable, no spaces are required between the layers. These graphite-based materials have excellent insulating properties, are usually cheaper than metal heat shielding and are very easy to work with. Graphite blanket has some advantages over graphite board in terms of construction cost, durability and ease of maintenance.

New Hot-Zone Design

Through production experience, the manufacturer had determined that all-metal insulation does not stand up well in aggressive brazing environments. As condensed metallic constituents from the brazing filler metal build up, the metal insulation begins to curl and crack due to differential expansion between the insulation and the deposit from the filler metal. This eventually causes a deterioration of the hot zone’s insulating properties. Metal insulation is also particularly vulnerable to braze-alloy spillage. Molten braze alloy will often dissolve metal shielding, leaving holes through which heat can escape. The use of thicker sheet and special molybdenum alloys in the insulation will extend the life of an all-metal hot zone but also add considerable expense.

As a manufacturer and user of vacuum furnaces, VAC AERO needed a hot zone designed to withstand aggressive brazing applications and therefore decided to evaluate the performance of graphite-based insulation and heating elements. Tests confirmed that a properly maintained hot zone incorporating these materials can be used to successfully and consistently join difficult-to-braze materials such as nickel-based superalloys. Indeed, the results were better than anticipated. The hot-zone insulation consisted of three layers of graphite blanket with an inner face of foil-bonded carbon composite. While there is little doubt that graphite insulation is more likely to absorb moisture than metal insulation, regular burn-out cycles will rid the insulation of moisture and contaminants that are detrimental to the brazing process. The heating elements were lightweight, curved graphite. http://www.hscf-group.com/



graphite insulation
graphite insulation
graphite insulation