In a blog posted Sept. 13, 2016, Dan Kay answered a question submitted by a reader. That same reader had a follow-up question, and Dan offered a response.
QUESTION: Thank you for suggesting that we use stainless steel brazing fixtures instead of carbon-steel fixtures. We have manufactured 20 fixtures using 304-stainless and will now monitor the life of these new fixtures.
We are manufacturing plate-type, copper-brazed heat exchangers in India. After brazing, our oil coolers are tested and supplied to the customer. However, the customer sees small, copper-brazed particles in the oil. Is the copper leaching out from the braze, and how can we avoid this effect?
ANSWER: Please remember that when you copper-braze components, the brazing temperature required is very high; approximately 1100°C (2025°F). This temperature is high enough to soften (anneal) the stainless steel. Therefore, please be careful when handling the stainless fixtures during and after brazing. If the fixtures have a heavy load placed on them during brazing, they may deform due to their lack of strength when annealed. If this is the case, it would be recommended to use an Inconel 600-series base metal instead since the Inconel materials can handle much higher temperatures than the 300-series stainless steels. This is just a precaution that you may want to consider.
Your second question is about your customer finding copper particles in the oil they run through the heat exchanger that you manufacture for them. Without any additional information about the exact details of your brazing operations, let me say that copper particles typically come from excess copper brazing filler metal (BFM) being applied to the heat exchangers when you braze them. Excess copper can form tiny droplets of pure copper that sit on some of the inner surfaces of the heat-exchanger, and they can be removed from those surfaces by the flowing oil during actual service.
To solve this problem, therefore, it is necessary to know what form of the copper you are using (sheet material or preforms or brazing paste, etc.) and also how much BFM you are applying to each layer of the heat exchanger. People tend to put on too much BFM. But you can only fill the braze joint one time! Any extra BFM may actually contribute to contaminating the system.
After you have answered the questions I have just asked, then you must consider the actual atmosphere in which the brazing is taking place. If it is being brazed in an atmosphere of gas (such as hydrogen, nitrogen, argon or a blend of any of these), then it is very, very important to be sure that gas is very dry – having a dewpoint of -50°C (-60°F) or drier – when that dew point is measured right at the brazing furnace. Otherwise, the excess moisture in the atmosphere can be a source of oxygen that can oxidize the stainless steel, preventing the copper from bonding (wetting) to it.
Thus, the copper BFM could merely “ball up,” forming tiny particles that can contaminate the oil during actual use in service. This same principle applies to vacuum brazing since some air can get into a vacuum furnace and do the same thing. For vacuum brazing, therefore, you must know what the leak-up rate is for your vacuum furnace.
I do not think that you are seeing any “leaching” of copper from the brazed joints in service. Instead, I believe the copper particles are directly related to your brazing operations.