It is possible for the atmosphere brazing process to be carried out in three various environments, including open air, controlled atmosphere of inert gas and high vacuum or high pressure environment. Each of these environments have its own benefits and drawbacks.
Frequently with induction brazing, an open air oxygen atmosphere is used. It has the benefit of simplicity and cost efficiency. However, when metals are heated to high temperatures in normal atmospheres, the result is a chemical change like oxidation, carbon buildup or scaling.
The application of flux can be used to improve the flow of braze material and reduce oxidation. On the other hand, it can also weaken the joint. The parts may need to be cleaned in an acid bath, which can be both time consuming and costly. However, in some cases, open oxygen atmosphere may be the best choice.
When joint quality is a priority and cycle times must be kept short, controlled atmosphere brazing is the best option. In this highly controlled environment, carbon buildup, scaling and oxidation are no longer an issue.
The controlled atmosphere can be achieved with an atmospheric bell jar, sealed glove box or vacuum furnace. When the bell jar system is used, the parts are put into place and then the bell jar is lowered to create the controlled atmosphere. The glove box is the best process when hands-on heating is required.
Inert gases, such as argon, dissociated ammonia, hydrogen and nitrogen are popular options for controlled atmosphere brazing. Argon provides the highest level of control and is more inert than nitrogen, but it is usually more expensive.
The process temperature can have an effect on the performance of the gas that is use. While nitrogen is typically the most economical choice, it can react with some steels when it is heated above certain temperatures. Hydrogen is a strong deoxidizer that has high thermal conductivity, so it is used often for annealing and brazing copper. Dissociated ammonia is relatively inexpensive and can be used for several different annealing and brazing jobs.
Atmosphere brazing in a high vacuum environment offers the greatest process control. In addition, it also produces clean products that are free of scaling or oxidation. The is the ideal brazing environment for hardening medical devices, brazing aerospace and all other applications where the highest quality of parts is required.