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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Conducting Temperature Uniformity Surveys per CQI-9

When discussing furnace compliance, we might ask ourselves the following question:

“If my thermocouples and control instruments are calibrated, and my SATs are performed successfully, does that mean the temperatures inside my furnaces are correct?”

Your immediate response might be “Yes”, however the correct answer is “Not necessarily”. A successful calibration and SAT only tells us that our furnace’s temperature system is accurate at one location: the tip of our sensor. Unfortunately, we don’t yet know the temperatures of any other locations inside our furnace.

This is why we must evaluate temperature uniformity, meaning the temperature variation of several locations within the furnace work zone (expressed in ±degrees) with respect to the temperature set point. To do this we must perform Temperature Uniformity Surveys (TUS)

A TUS is a test or series of tests where calibrated instrumentation and sensors measure temperature variations inside the furnace. By identifying where in the work zone these temperature variations exist, heat treaters can answer important questions such as:

  • Where are the cold and/or hot spots in my furnace?
  • Why are there cold and/or hot spots, and how do I fix them?

You may like the article: Pyrometry, the key for reliable heat treat in Automotive and Aerospace

Repeatability, repeatability, repeatability!!!!!!!

The TUS test helps ensure the entire work zone produces a repeatable metallurgical result. It also encourages proper maintenance of thermal processing equipment by identifying where non-homogeneous temperatures exist within your work zone. Above all else, performing TUSs tests reduces the likelihood of scrap and/or quality issues that could potentially lead to product RECALLS.

Since TUSs are a proactive measure (not reactive), the true costs related to performing TUSs can only be measured by NOT performing the TUSs and waiting for the bad news.  Unfortunately, far too many companies have “paid the price” for NOT performing TUSs. This decision has oftentimes resulted in recalls measured in the many millions of dollars!

In conclusion, if TUSs are not performed, the heat treater needs to quantify what potential liabilities might he/she inherit if metallurgical related recalls were to take place as a result of poor temperature uniformity.

 

Victor Zacarias

Heat treat management expertise: CQI-9, AMS2750

More Posts - Website