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All metals want to react with oxygen as the metals are heated. The higher the temperature, the greater the thermodynamic driving force to have those oxides form. This is true for all metals, even though the oxides of some metals are not as stable as the oxides of other metals.  Gold and nickel are examples of metals whose oxides are not stable at any temperature we would encounter in our daily activities, and thus, do not concern us at all. Copper oxides and iron oxides are examples of metals whose oxides are not stable at higher temperatures, in that those oxides are easily and quickly dissociated at elevated temperatures. Chromium-oxide, however, is an example of a fairly stable oxide (up to about 1850F/1000C before dissociating in a typical brazing atmosphere furnace), whereas aluminum-oxide will be extremely stable in a brazing furnace, and is beyond the capability of any standard brazing atmosphere to reduce that oxide. Titanium-oxides behave in a very similar fashion to aluminum oxides in typical brazing furnace atmospheres. By Dan Kay

In June's article, we'll look at how creating "partial-pressures" in vacuum furnaces by back-filling vacuum furnaces with an inert atmosphere is sometimes necessary to achieve successful brazements!

As mentioned in previous articles, more and more brazing shops are using vacuum furnaces.  These furnaces are quite complex, offering more options for heating, cooling, partial pressure, or multi-bar pressure (pressure capabilities above one atmosphere) for high-speed cooling.

The overall effectiveness of the equipment, however, still lies with the people who program and run the furnaces. Figure 1 (below) shows a series of vapor pressure curves for a number of common metallic elements.  Each curve shows the melting point of the element (indicated by a small circle along the curve) and, where the curve crosses the dotted line across the top of the chart, its approximate boiling point (where it wants to turn to a gas) at one standard atmosphere. By Dan Kay

Over the years I’ve helped many brazing shops resolve common brazing problems (such as leakers, non-wetting surfaces, etc.). In evaluating these situations, it is not uncommon to discover that sub-components (such as brackets, or fittings, etc.) from outside suppliers can actually be the trouble-makers!. By Dan Kay