Brazing Techniques: Pure Atmosphere Brazing

Brazing is the process of joining metal pieces by liquefying filler metal, distributing it between the pieces, and cooling the assembly until the metal solidifies. There are several techniques for brazing, with the technique of choice depending on (a) the type of metal that will be joined and (b) the size of the assembly. When distinguishing between brazing techniques, the first area of consideration is brazing atmosphere - litmbt salama erally, the atmosphere in which metal is brazed.

The Use of Atmospheres

Atmospheres can be separated into three categories: a vacuum atmosphere, which is devoid of any gases; pure atmospheres, which typically contain two elements or less; and impure atmospheres, which contain multiple elements. Because oxygen can cause metal to oxidize as it is brazed, metals that form a stable oxide layer in a low oxygen atmosphere require a vacuum atmosphere or a pure atmosphere that is carefully controlled. One such metal is stainless steel, which is prone to oxidization due to its high chromium content. When stainless steel is brazed in an impure atmosphere, two problems can result:

A layer of oxidization between the base metals can weaken the jointThembt ema surface of the finished assembly can appear drossy

Depending on the finished assembly's application, one problem may be more worrisome than the other. But buyers of stainless steel expect both problems to be avoided.

Examples of Atmospheres

Pure atmospheres prevent metal from oxidizing as it is brazed. Today, the commonly used pure atmospheres for brazing stainless steel are: a nitrogen atmosphere, a hydrogen atmosphere, and a nitrogen/hydrogen atmosphere mix.

Regardless of which atmosphere is used, maintaining its purity depends on using the right furnace, one that prevents oxygen and other gases from entering its heating chamber. In most cases, a continuous furnace (i.e. belt furnace) that features the following design elements is used:

Curtain boxes that deter atmosphere lossDoor design that deters atmosphere lossDirectional flow control that prevents air from entering the furnaceStacking design that separates stacks from the furnace hood

A furnace that features these elements facilitates "controlled atmosphere" brazing, with the control resulting from the design elements listed above. A non-oxygenated atmosphere can also be achieved by using a vacuum furnace, which applies a vacuum to remove all gases from the heating chamber(s). Vacuum furnaces are often used when heat treatment techniques (e.g. age-hardening) must be performed during brazing, the assemblies are not large, and the brazed joints must be kept especially clean such as joints used in aerospace equipment.

Vacuum furnaces are more expensive than most types of continuous furnaces - a trait that, combined with their batch style of operation, designates them as specialty furnaces.

Conclusion

Different metals require different brazing techniques. For stainless steel, controlled atmosphere brazing and vacuum brazing are the techniques of choice. Due to the expertise and expensive equipment that brazing requires, most companies that require brazed assemblies choose to outsource brazing. For more information on brazing stainless steel and other metals, contact a metal treatment service.