Furnace brazing
by Sue Dunkerton
The process
Brazing is the process of joining metallic or ceramic materials using a molten filler metal, drawn into the joint by capillary action or preplaced with preform. Usually, filler metal becomes molten well below the melting point of the materials being
joined, although in some cases, the melting points are close but again the filler is the lower melting point material.
Various methods are used to braze components, and this summary provides background on the use of furnace brazing.
Furnace types
The popularity of furnace brazing stems from the clean atmosphere used, which mostly eliminates use of fluxes and also eliminates post-braze cleaning. Various types of furnace are used for brazing, mostly employing either a gaseous atmosphere or
vacuum. The overall furnace construction is based on either batch type or continuous operation.
Batch operation includes retort type furnaces used for hydrogen brazing, and vacuum chambers for vacuum brazing.
In a retort furnace, an inner heat resistant alloy container retains and protects the enclosed hydrogen atmosphere. Hydrogen provides the active ingredient to clean the braze components, and eliminates the need to use flux. To speed up brazing
cycles, these furnaces would typically use a nitrogen purge at the end of the cycle to accelerate cooling rates. Although care needs to be taken with potentially explosive mixtures of hydrogen and air, equipment is designed and safeguarded to
prevent this. Alternatively, lower percentages of hydrogen are used by using mixed gases based on hydrogen with nitrogen, argon or helium.
Vacuum furnaces are widely used, and often companies will utilise them for heat treatment as well as brazing, dependent on capacity. Furnaces today are based on a cold wall construction, with internal heating elements, usually carbon or
molybdenum. The cold wall refers to the water cooled doubled skin construction, to keep external temperatures down to room temperature (or less). Either horizontal furnaces with side loading are used or vertical furnaces loaded from the top or
bottom. Furnace temperatures up to 1650°C are readily available.
Vacuum furnaces are high capital investments, but they are versatile, safe and produce a high quality product.
Materials
Most materials can be furnace brazed, although high vapour pressure elements should be avoided in vacuum brazing (zinc, cadmium, lead, etc). All materials need to be cleaned prior to insertion in the furnace to remove surface scale, grease and other
contaminants (high quality in, high quality out). The most widely used fillers for furnace brazing are based on silver, copper, nickel and gold, the latter two being most applicable to stainless steels, and heat and corrosion resistant alloys.
Applications
Furnace brazing is more applicable to value added components because of the high capital outlay and less productive duty cycles compared with other 'lower cost' brazing alternatives. However, applications still include high volume components
for the electrical and electronic sectors. Other large markets include aeroengine components, power generation components (nuclear and gas turbine) and various marine and other engineering applications. The last decade has seen increasing vacuum
brazing of aluminium heat exchangers because of the clean processing possible, and the advent of large brazing furnaces (greater than 2x1m).
Benefits
Furnace brazing has the key advantages of a controlled heating cycle, and that clean parts may be processed with no need for post-braze cleaning.
Precautions
There are no significant risks associated with vacuum brazing other than the normal for such equipment (electrical, thermal). If volatile materials should be placed in the furnace, extraction should be examined to ensure venting into a safe area. In
furnace brazing, there may be specific precautions to take where a hydrogen atmosphere is used.
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