Types of Brazing Atmospheres
The induction brazing process can be carried out in three different environments, each with its own advantages and disadvantages: open air; a controlled atmosphere of argon or other inert gas; or a high vacuum/high pressure environment.
An open air, oxygen atmosphere is frequently used for induction brazing. Open air has the advantages simplicity and economy. However, heating metals to high temperatures in a normal atmosphere causes chemical changes such as oxidation, scaling and carbon buildup on the parts. Applications of various types of flux are often used to reduce oxidation and improve the flow of the braze material, but at the same time they can reduce the strength of the joint. Acid cleaning baths are used to clean the parts as needed, but the extra cleaning step can be both expensive and time-consuming.
But for some brazing processes, an open oxygen atmosphere may be the best choice; the oxidation, scaling and carbon buildup may not affect the part’s performance or in some cases may even be beneficial. Or further machining and cleaning at a later stage of the manufacturing process make the whole issue irrelevant.
Controlled Atmosphere Brazing
For lean manufacturing environments in which more control over joint quality is required and cycle times must be minimized, the next step beyond an open-air environment is to use a controlled atmosphere under normal or close-to-normal atmospheric pressure. In this type of environment, a high degree of control over the overall process can be achieved and open-air issues of oxidation, scaling and carbon buildup can be virtually eliminated.
A controlled atmosphere can be produced in a vacuum furnace, a sealed “glove box” or with an atmospheric bell jar. With a bell jar system, the parts are positioned before the bell jar is lowered into place and the controlled atmosphere is created. The glove box system is ideal for processes which require hands-on heating control. Learn more about brazing equipment.
Inert atmospheres of nitrogen, argon, hydrogen and dissociated ammonia are common choices for controlled atmosphere brazing. Argon is more inert than nitrogen and therefore provides more control, but it is generally more expensive. The process temperature can also affects the performance of the gas chosen; nitrogen is often an economical choice but it is known to react with some steels above certain temperatures. Hydrogen - a strong deoxidizer with high thermal conductivity - is often used for copper brazing and annealing steel. Dissociated ammonia (75% hydrogen + 25% mononuclear nitrogen) is a relatively inexpensive atmosphere which can be successfully used for many types of brazing and annealing processes.
The selection of a gas for atmospheric brazing depends on a variety of process requirements including purity and heating temperature as well as cost considerations. The engineers at GH Induction Atmospheres have extensive experience in selecting the best atmosphere for a specific combination of material being brazed and filler metal, and have prepared a helpful Atmosphere Comparison Matrix. An additional Filler Metals Comparison Chart provides information about various filler metal characteristics.
Each atmospheric gas is generally available in different purity grades; the lower grades retain small amounts of water vapor or oxygen remain mixed with the pure gas. Using the highest grade is more expensive but a small amount of impurity may be just enough to contaminate a tightly-controlled process. The gases are available in cylinders, dewars bottles or in liquid bulk.
Brazing in a high vacuum environment provides the most process control and produces the cleanest parts, free of any oxidation or scaling. It is the preferred brazing environment for brazing aerospace components, hardening medical devices and other applications that require the absolute highest part quality.
In a vacuum system, parts are heated in a fully enclosed, stainless steel chamber, which can be pumped down to 10 (-6) Torr. Special fixturing can be designed for automatic part loading and unloading, and quartz viewports can provide access for infrared temperature sensing of each individual part. This type of system is often used for brazing steel or nickel alloys with steel. An alternative method is to use a vacuum furnace, which is often the best choice for brazing parts of unusual shapes or “orphans” from other heating processes. The difference is that the induction furnace will heat the entire part rather than just a narrowly focused joint area.
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