The shielding gas commonly used for brazing is argon, which is favored due to its inert properties that prevent reactions with the metals being brazed. Argon is particularly suitable as it does not react with any of the metals being heat-treated or brazed in vacuum furnaces. This makes it an excellent choice for maintaining a controlled atmosphere devoid of oxidants, oxygen, and water, which are detrimental to the brazing process.
Argon as Shielding Gas: Argon is an inert gas, meaning it does not readily undergo chemical reactions under normal conditions. This property is crucial in brazing, where the goal is to prevent oxidation and maintain the integrity of the metals being joined. The use of argon helps in creating a neutral environment that does not interfere with the brazing process. It is often used in vacuum furnaces for partial-pressure brazing applications, rapid cooling, and as a gaseous atmosphere to enhance heat conduction between components.
Purity Requirements: The purity of argon used in brazing is critical. Higher purity levels are typically required to ensure the effectiveness of the shielding. However, if the surfaces of stainless steel are coated with copper or nickel, the requirement for argon purity can be somewhat relaxed. This is because these coatings can help protect the base metal from oxidation, reducing the need for extremely high-purity argon.
Additional Considerations: To further enhance the brazing process, especially on stainless steel surfaces, BF3 gas flux or self-fluxing solders containing lithium or boron can be used. These materials aid in the removal of the oxide film that naturally forms on stainless steel, facilitating better braze flow and joint integrity. The vacuum requirements for vacuum brazing of stainless steel are also influenced by the brazing temperature; higher temperatures can allow for a lower vacuum degree.
Alternative Gases: While argon is the preferred shielding gas, alternatives such as nitrogen, helium, and even hydrogen under controlled conditions can be used. Nitrogen is often chosen for cost-effectiveness, and it must have a dew point of less than 51°C to be effective. Helium and chlorine are also options, but their use is less common. Hydrogen can be used from the perspective of preventing oxidation, but its use is risky due to its flammability and potential for causing explosions.
Safety and Precautions: Despite its benefits, argon can be dangerous if not handled properly. It is heavier than air and can displace oxygen in confined spaces, leading to asphyxiation. Therefore, proper ventilation and monitoring are essential when using argon as a shielding gas in brazing operations.
In summary, argon is the preferred shielding gas for brazing due to its inert nature, which helps maintain a controlled, oxidation-free environment. The purity of argon and the specific conditions under which it is used are critical to the success of the brazing process. Alternative gases can be considered based on specific needs and safety considerations, but argon remains the standard choice in many brazing applications.
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