Someone recently suggested to their aluminum-brazing client that they should conduct a high-temperature burnout cycle in their vacuum furnace by “heating the furnace to 1600°F to ensure that all the oxides are removed.” Unfortunately, this statement is not correct since 1600°F (870°C) is not high enough to remove (dissociate) oxides of metals such as aluminum, magnesium and/or titanium from any vacuum-furnace walls or hot zone. Additionally, many vacuum furnaces used for aluminum brazing are simply not designed to be able to reach temperatures as high as 1600°F.

When using furnaces designed for brazing aluminum, any so-called “burnout” cycles (clean-up cycles) are not actually designed to remove oxides. Instead, they are supposed to remove any surface contaminants (oils, lubricants, fingerprints, etc.) that may have volatilized during a brazing cycle and then condensed onto the water-cooled furnace walls during normal brazing operations. To remove these kinds of surface contaminants, burnout cycles using very high temperatures are not required. However, implementing clean-up cycles using temperatures of only 1300-1400°F (700-760°C) should be more than adequate.


Fig. 1. Metal/metal-oxide curves for metals (published in the AWS Brazing Handbook, Fifth Edition, 2007, pp. 120) // Credit: Dan Kay

Removing Oxides
Figure 1 shows the well-known metal/metal-oxide (M/MO) chart. Each of the curves shown on the M/MO chart represents an oxide of a particular metal. Although the chart was originally created to be used for hydrogen atmosphere furnaces, the curves have been shown to be relatively accurate for a variety of atmospheres (argon, nitrogen and helium) and for vacuum.

To remove (dissociate) any particular oxide, the furnace conditions must be such as to allow the furnace to operate well to the right of that particular oxide curve by at least the diagonal of one of the little boxes that makes up the graphical section of the chart. Note that each of the oxide lines represents the peak of a bell curve, so to speak, and this “peak” can shift slightly for different atmospheres. Therefore, to take into account all the previously mentioned atmospheres, a furnace should be operated well to the right of any particular metal-oxide curve.

For example, when brazing 304L stainless steel in a hydrogen atmosphere furnace, a major concern would be the formation of chromium-oxide when heating because chromium reacts quickly with oxygen when heated to form stable chromium oxides, which gives 304 stainless its corrosion resistance when used in a variety of end-use service conditions.