Question: Our vacuum brazing furnace does not seem to be yielding high-quality, leak-tight joints on our tubular brazed parts. We have a powerful vacuum (about 10-6 torr), so we don’t know why we are getting poor results. What should we be doing to ensure good brazing in our vacuum furnace?
Answer: In my experience, the two most important items for any vacuum brazing operations are:
1. Leak-up rate
This is the rate at which air leaks into the furnace from the outside and the pressure inside the furnace begins to go back up toward atmospheric pressure. A vacuum furnace’s leak-up rate is something that must be understood and measured regularly by anyone using vacuum furnaces for brazing. Common sources of leaks are the door seal of the furnace, then the seals around each of the fittings at all the connections/valves in and around the vacuum furnace. They must all be leak-tight. Leak-up rates are usually specified in microns (millitorr) of pressure leak rate per hour of time. Thus, a good furnace leak-up rate is only about 5 to 10 microns per hour.
2. Cleanliness of the furnace
Regular furnace maintenance requires furnaces to be “burned-out” on a regular basis at approximately 100°F (50°C) above the highest temperature you normally operate your vacuum furnace for regular work, holding it at that temperature for at least one hour or more.
For example, if you typically operate your vacuum furnace at a temperature of about 2000°F (1100°C), then you should run the “clean-up cycle” (burn-out cycle) at a temperature of 2100-2200°F (1150-1200°C) and hold it at that temperature for one to two hours maximum. If the furnace is really dirty, you may need to hold it at that temperature for an even longer time.
Please remember that when a vacuum furnace is run at a very strong (hard) vacuum (i.e., around 10-6 torr), metallic elements from both the base metals being joined and from the brazing filler metal (BFM) are being outgassed, as well as any organics in paste or tape products introduced into the furnace. These elements condense on the cold (water-cooled) inside surfaces of the furnace walls and can re-outgas in subsequent furnace runs, contaminating the materials in those future furnace runs. That’s why it is very important to keep the furnace clean at all times.
Note: Too many people feel that having a strong vacuum is the most important thing to ensure good brazing results. This is not true at all. The most important thing is the furnace leak-up rate, followed closely by the cleanliness of the furnace.
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