Choosing the Right Furnace, part 1: Heating Method

On a practical basis, in heat treatment operations where a large variety of products is being processed, it is rarely possible to select a furnace that is ideal for each specific product. Selection is often narrowed to that equipment which offers the best performance for high-production.

However, in this entry we would like to discuss the fundamentals of heat transfer to help customers select the furnace which is the most economical, but also practical for a given job. Any heat treat furnace must be built strong enough to 1) support the load of the work, 2) have sufficient heating capacity to produce the desired weight per unit time, and 3) at the desired temperature, produce compliant parts.

Classification of Furnaces

  • Temperature range is a logical means of classifi­cation in many shops because a furnace designed for operating over a temperature range from slightly above room to about 1100°F (595°C) is different from equipment designed for operation in the higher temperature ranges, the reason being that materials of construction are selected for their suitability to a given temperature range.
  • Method of operation also is a common means of classifying heat treating furnaces. There are two general groups, batch furnaces and continuous furnaces.
  • Heating Medium. One common, although very broad, means of classification is the heating medium used. This may be a gaseous medium (which may include vacuum), or a liquid bath such as molten metal or salt. Source of energy, whether gas or electricity, is another means of classifying furnaces.

The following infographic shows the different methods of heat transfer in heat treatment operations and provides a quick guide on the sources of energy available

Victor Zacarias

Heat treat management expertise: CQI-9, AMS2750

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Furnace Brazing: 3 important factors to have in mind

BRAZING comprises a group of joining processes in which coalescence is produced by heating to a suitable temperature above 450 °C (840 °F) and below the solidus temperature of the base metal. The most common heating methods available for brazing can be summarized in the following table:

Furnace brazing is by far the most popular method due to the comparatively low equipment cost, furnace adaptability, and minimal required jigging. It is a low-cost process relative to other processes specially when a high-volume production output is the primary factor.

It is a common process used both in automotive and aerospace industries but highly misunderstood so, we would like to share 3 important factors in this entry to grasp better this procedure

Furnace design and uniformity

The brazing temperature, which is significantly higher than those used in heat treatment of steel, imposes special considerations on furnace design, including the degree of  temperature uniformity that can be maintained, the time required to heat the workpieces to the brazing temperature, and the weight of the load that can be supported at 1100 °C (2000 °F) without sagging of furnace fixtures.

Atmospheres

The atmospheres used in furnace brazing serve essentially to protect the steel assemblies from oxidation or scaling and to assist the flow of filler metal by promoting wetting of steel surfaces. Both functions require a gas atmosphere that is reducing. When required, the atmosphere may also serve to maintain the carbon content of the steel by preventing carburization or decarburization at elevated temperatures.

High-temperature brazing using a BCu or  high-temperature copper alloy filler metals can be conducted in hydrogen or dissociated ammonia atmospheres. Low-temperature brazing using BAg series filler metals also is possible when hydrogen or dissociated ammonia atmospheres re used. Endothermically generated atmospheres containing 14-16% hydrogen can also be used for properly cleaned carbon steels.

Filler Metals

Selection of the proper filler metal depends on strength and temperature requirements. The AWS BAg series of filler metals generally has good long-term strength at temperature up to 200 °C (400 °F). Some of the higher temperature filler metals, such as BAg-13 and BAg-13a, can be used for joints that require high strength up to 425 °C (800 °F). The BCu filler metals, which are the most commonly employed, are generally used for service up to 480 °C (900 °F).

 

At Global Thermal Solutions, we can help you to conduct temperature uniformity surveys and temperature profiles to achieve consistent results in brazing processes. To learn more, suscribe to our blog or follow us in LinkedIn

Victor Zacarias

Heat treat management expertise: CQI-9, AMS2750

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