Question: How can we prevent the erosion of thin stainless tubes that we need to braze into several through-holes in some thick stainless plates during our furnace brazing operations? The tubes and base plates are both 316L stainless, and the filler-metal paste we use is a nickel-based alloy (BNi-2).
Answer: This problem in not uncommon when joining thin-walled tubing into different (heavier) structures at high temperatures, such as that found when brazing with a nickel-based brazing filler metal (BFM). This erosion is primarily a concern when the BFM is being applied externally (i.e., on the outside of the joint). Externally applied BFM is usually in the form of either paste or a preform ring, and since the mass of this BFM is so much less than the mass of the base plate being joined, this BFM can reach its melting temperature long before the much-heavier base plate comes up to proper brazing temperature.
Similarly, the thin tubing can also get hot fairly quickly, and the molten BFM can start to flow around the thin tubing and also “flash-out” onto the tubing (i.e., an ultra-thin layer of the BFM can begin to migrate over the hot surface of the tube since the molten BFM likes to flow toward the heat/hotter surfaces).
Typically, a lot of BFM is applied to the top of the joint between the tube and the heavy base plate (Fig. 1) in order to have enough BFM to fill the joint in the heavy plate. But, as mentioned above, BFM does not like to flow toward the colder parts of a joint being brazed. Thus, the BFM, when it melts, will tend to sit on the top of the joint, flash-coat the tubing and also begin to aggressively alloy with the tubing – sometimes eating large holes right through the tube wall. This aggressive alloying, also known as “erosion” of the base metal by the molten BFM, is not a desirable occurrence, and steps should be taken in the design of the joint to prevent both the erosion and the “flash-out” that can otherwise occur.
More next time.