Question: We have brazed 304L stainless steel with BNi-1a at 1170°C for some time now. But recently our vacuum weakens to 3x10-2 Torr when temperatures go above 1100°C, no matter how good it was in the earlier stages of that brazing cycle. Recently, we are seeing a few dark spots on the product too. What might be a possible reason for all this?
Answer: The problems you are encountering may be caused by three different phenomena.
1. Volatilization (off-gassing) of contamination from the surface of the components being brazed. If the components have not been properly cleaned before they are assembled for brazing, then much of that surface contamination will volatilize during brazing, which can cause degradation of the vacuum. It is important to thoroughly clean all components before they are assembled for brazing, and those cleaned parts should only be handled with clean gloves prior to brazing. Do not expect the furnace “atmosphere” to be able to clean the parts during the brazing process. Once the parts are assembled, any surface contaminants will be trapped between the tightly fitting surfaces of the joint that you are trying to braze, and it will be impossible to effectively clean the inside of those joints. Weak joints will result.
2. Contamination on the inside walls of the vacuum furnace. Many materials can evaporate from the assemblies during brazing: surface lubricants; low-melting metallic elements such as zinc or cadmium; or copper coatings, etc. All of these materials will condense on the inner walls of the vacuum furnace, and these contaminants can volatilize (off-gas) when you heat the furnace. What happens is that they will re-evaporate from the walls of the furnace, building up some partial pressures in the chamber. This can cause the vacuum level to be degraded, which then shows up on the vacuum gauges as a loss of vacuum in the chamber. The materials that off-gas from the furnace walls could contaminate the surface of parts being brazed, resulting in discoloration (or spots) on the surface.
3. Leak-up rate of a vacuum furnace. It is vitally important to determine the leak-up rate of a vacuum furnace because all vacuum furnaces will leak air into the chambers from the outside atmosphere of the factory during vacuum operations, and the pressure inside the furnace will start to go back up toward atmospheric pressure. That is why it is a called the leak-up rate of the vacuum chamber. It should be measured on a daily or weekly basis. The vacuum-furnace manufacturer should be able to help users determine how to measure this since they manufactured the furnace and should know how to detect the leaks in their furnace when it is operating. The location of any furnace leaks can be found by using a helium leak detector. Good vacuum furnaces are made to maintain good leak tightness, but measuring a furnace’s leak-up rate is an essential part of good furnace control.