The temperature limit for brazing is typically at least 25ºC (50ºF) above the liquidus temperature of the braze alloy, ensuring the alloy is molten and can flow adequately to fill gaps and bond with the base metal. This temperature is crucial for achieving a strong joint, especially in scenarios involving large gaps or thin materials. The brazing process should maintain this temperature for a duration sufficient to ensure uniform heating across all parts, typically ranging from 5 to 10 minutes, depending on the load size. After brazing, the parts should be cooled to at least 25ºC (50ºF) below the solidus temperature of the braze alloy before any quenching to prevent the alloy from being dislodged from the joint.
For specific materials like austenitic stainless steel with high carbon content and no stabilizing elements like Ti or Nb, brazing within the sensitization temperature range (500-850°C) should be avoided to prevent the precipitation of chromium carbides, which can reduce corrosion resistance. In the case of martensitic stainless steel, the brazing temperature must either match the quenching temperature to combine brazing with heat treatment or be lower than the tempering temperature to avoid softening the base material.
Furnace brazing, which provides an oxygen-reduced environment, is recommended for optimal results. The furnace conditions should maintain a dew point of ≤ -40°C, oxygen levels of < 100 ppm, and an inert gas atmosphere, typically nitrogen. This controlled environment ensures the braze alloy flows smoothly and bonds effectively with the base metal.
After the brazing process, the parts are allowed to cool slowly to about 980°C (1800°F) to allow the filler metal to solidify, followed by rapid cooling through gas quenching, usually at a pressure of around 2 bar. This cooling process is critical to maintaining the integrity of the brazed joint and the overall strength of the assembly.
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