An overheated brazed joint is indicated by a red glow on the molten pool, which gives off a white smoke. This occurs when the zinc in the braze metal is burned out due to excessive heat. It is important to avoid overheating the joint as it can result in joint defects, such as the braze filler metal being pulled away from the joint. If joint defects occur, they can often be repaired by re-brazing. However, instead of re-melting the existing joint, it is recommended to apply a small amount of additional braze alloy in the defective area. The re-braze temperature should be lower than the initial braze temperature, especially if wide joint gaps are involved. It is also important to ensure the defective area is clean before applying additional braze alloy and repeating the brazing cycle with modified temperature settings.
Furnace brazing, which is a type of brazing process, offers several advantages over other metal-joining techniques such as induction or torch brazing and welding. In furnace brazing, the base metal of the joint is not melted, resulting in tighter control over tolerances and clean parts and brazed joints without the need for secondary finishing. Furnace brazing also allows for the brazing of dissimilar metals and non-metals. It produces less thermal distortion compared to welding due to the uniform heating of the brazed piece. Complex and multi-part assemblies can be brazed cost-effectively using furnace brazing.
The furnace temperature is gradually increased to the brazing temperature, usually between 500°C and 1200°C, to ensure proper heat distribution and minimize thermal stresses. The brazing temperature is then maintained for a specific amount of time to allow the filler metal to melt, flow, and wet the base metals, creating a strong joint. After the brazing process is complete, the furnace is slowly cooled to room temperature to minimize thermal stress and distortion. The parts are then inspected for quality, and if necessary, post-brazing treatments such as heat treatment, machining, or surface finishing may be performed to achieve the desired properties and appearance.
In summary, an overheated brazed joint is indicated by a red glow on the molten pool and white smoke. It is important to avoid overheating to prevent joint defects. Furnace brazing offers advantages such as tight control over tolerances, clean joints without secondary finishing, and the ability to join dissimilar metals and non-metals. The brazing process involves gradually heating the joint to the brazing temperature, maintaining the temperature for a specific time, and then slowly cooling the parts before performing post-brazing treatments if necessary.
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