Brazing Atmosphere

The choice of brazing atmosphere is an important criteria for creating efficient bonds. For different brazing processes, you can use three different types of environments. Each one of them have their own advantages and disadvantages. They are:

1) Open Air
An oxygen atmosphere is the most suitable and frequently used atmosphere for induction brazing. Its main advantage is its economy and simplicity. But heating metals to high temperatures in open environments make them prone to certain chemical changes like scaling and carbon build up on the parts and most importantly their oxidation. However oxidation can be minimized by the application of different types of fluxes. They facilitate the flow of filler materials but they weaken the strength of joints. The post brazing cleaning can be done with the help of acid baths but these steps can prove time consuming and expensive as well. Apart from all the obstacles, the open air atmosphere is the most suitable choice for some brazing processes where oxidation, scaling, and carbon build up don't affect the joining process. In some cases they even prove beneficial.

2) Controlled Atmosphere Brazing
Those brazing processes, which emphasize on the brazed joint quality and minimization of number of cycles require controlled atmosphere under normal or close-to-normal atmospheric pressure. This type of environment provides high degree of control over the entire process and it completely eliminates the open air problems, such as oxidation, scaling, and carbon build up.

The most common choices for controlled atmosphere brazing are the inert atmosphere created by:

Nitrogen
Nitrogen is the most economical choice. But above certain temperatures it is known to react with some steels.

Argon
Argon provides more control than nitrogen because it is more inert. But it is usually more expensive.

Hydrogen
Hydrogen is a strong de-oxidizer having high thermal conductivity. It is often used for copper brazing and annealing steel.

Dissociated Ammonia
Dissociated ammonia is “75% hydrogen + 25% mononuclear nitrogen”. It provides relatively inexpensive atmosphere and can be successfully used for several types of brazing and annealing processes.

A controlled atmosphere can be produced in various environments like:

A vacuum furnace
It is mainly used for vacuum brazing processes.

A sealed glove box
Glove box systems are best suited for processes which require hands-on heating control.

An atmospheric bell jar
In the bell jar system, the parts are positioned prior to the bell jar is lowered into place and the controlled atmosphere is created.

 

Selection of Gas
The selection of a gas for atmospheric brazing depends on different process requirements like:

Purity
The atmospheric gases are usually available in different purity grades. The lower grade gases contain small amounts of water vapor or oxygen mixed with the pure gas. The high grade gases are more expensive but they are a safe bet because even a small amount of impurity may be just enough to contaminate a tightly-controlled process. The gases are found in dewars bottles, cylinders or in liquid bulk.

Heating Temperature
The heating temperature of the process can affect the performance of the gas chosen.

Cost Considerations
The cost of gases is an important point to consider while selecting the gas.

3) Vacuum Brazing
Brazing in a high vacuum environment creates the more efficient and stronger joints than produced in open air or controlled air atmosphere. Vacuum brazing provides the most process control and produces the cleanest parts, which are free from any scaling or oxidation. In vacuum systems, the joining parts are heated in a fully enclosed, stainless steel chamber. The pressure inside the chamber can be reduced to 10-6 Torr. For automatic loading and unloading, special fixturing can be designed. This type of system is generally used for brazing steel or nickel alloys with steel.

The brazing environment is preferred for those joining processes that require the absolute highest part quality, such as: