Furnace brazing is a semi-automated process that is widely used in industrial brazing operations due to its ability to accommodate large-scale production and the use of unskilled labor. Furnace brazing has many advantages over other heating methods, making it ideal for large-scale production. One major advantage is that it can easily produce a large number of small parts that are easy to jigging or self-positioning. The process also has the advantage of controlled thermal cycling (allowing the use of components that may deform under localized heating) without the need for post brazing cleaning. Common atmospheres used include: an inert atmosphere, a reducing atmosphere or a vacuum atmosphere, all of which protect the components from oxidation. Other advantages include low unit cost for mass production, tight temperature control, and the ability to braze multiple joints simultaneously. Furnaces are usually heated using electricity, gas or oil, depending on the type and application of the furnace. However, some of the shortcomings of this approach include high capital equipment costs, more difficult design considerations, and high power consumption.
There are four main types of furnaces used in brazing operations: batch type; continuous; retort with controlled atmosphere; and vacuum.
Batch furnaces have relatively low initial equipment costs and can heat the load of each component separately. It can be turned on and off at will, which reduces operating expenses when not in use. These furnaces are suitable for medium to high volume production and offer a great degree of flexibility in the types of parts that can be brazed. A controlled atmosphere or flux can be used to control the oxidation and cleanliness of the part.
Continuous type furnaces are best suited to a steady flow of similar-sized parts through the furnace. These furnaces are often conveyor fed, moving parts through the hot zone at a controlled speed. It is common to use either controlled atmosphere or pre-applied flux in continuous furnaces. In particular, these furnaces offer the benefit of very low manual labor requirements and so are best suited to large scale production operations.
Retort-type furnaces differ from other batch-type furnaces in that they make use of a sealed lining called a "retort". The retort is generally sealed with either a gasket or is welded shut and filled completely with the desired atmosphere and then heated externally by conventional heating elements. Due to the high temperatures involved, the retort is usually made of heat resistant alloys that resist oxidation. Retort furnaces are often either used in a batch or semi-continuous versions.
Vacuum furnaces are a relatively economical method of preventing oxides and are most commonly used to braze materials with very stable oxides (aluminum, titanium and zirconium) which cannot be brazed in an atmosphere furnace. Vacuum brazing also uses a large number of refractory materials and other special alloy combinations that are not suitable for atmospheric furnaces. Because there is no flux or reducing atmosphere, component cleanliness is critical when brazing in vacuum. The three main types of vacuum furnaces are: single wall hot distillers, double wall hot distillers and cold wall distillers. Typical vacuums for brazing range from 1.3 to 0.13 Pascals (10-2 to 10-3 Torr) to 0.00013 Pascals (10-6 Torr) or less. The most common types of vacuum furnaces are batch types, which are suitable for medium and high throughput.