Nicrobraz Technical Articles Library
Nitrogen and its Effect on Braze Joint Quality
We're having problems brazing Multimet* to 304L stainless steel. We're using a BNi-2 brazing filler metal in a vacuum at 10(4) Torr. We made sure our vacuum furnace was clean, but we're still producing poorly brazed and discolored parts. Where are we going wrong?
Some base metals containing nitrogen (e.g., Multimet, 304LN) do not braze well in a vacuum furnace even if the furnace is clean and the vacuum atmosphere is 10(4) Torr. Nitrogen, if present in the base metal, can have a significant effect on joint brazeability. This is particularly true with brazing filler metals containing boron, such as Nicrobraz LM (BNi-2).
Multimet and 304LN are examples of alloys where nitrogen is present as a strengthening agent. The nitrogen level in these alloys may range from 0.020.30% and have the following effects:
1. The nitrogen in the base metal will combine with the boron in the brazing filler metal, and thus deplete the melt depressant from the filler metal. This will raise the liquidus temperature and inhibit filler metal flow. The degree to which this happens depends on the amount of nitrogen in the base metal, mesh size of the brazing filler metal powder, furnace heating rate and the brazing temperature.
2. Nitrogen can also produce an iridescent bluish-gray film on the base metal surface. This film will prevent the brazing filler metal from wetting and flowing across the base metal or into the joint. Formation of these films is not only dependent on the nitrogen content of the base metal, but also on the pumping rate in the vacuum furnace, gas atmosphere flow rates and the amount of parts in the load.
The adverse effects of nitrogen in the base metal can be minimized by the following procedures:
- Run the part to be brazed through a furnace cleaning cycle.
- Adjust heating rate during brazing.
- Nickel-plate the base metal.
Furnace Cleaning Cycle.
The furnace cleaning cycle is similar to a braze cycle except that the part is held at high temperature for ample time to allow sufficient outgassing. The cleaning cycle should work satisfactorily on base metals with low nitrogen contents; however, may not work if the nitrogen content is too high (0.08-0.18%), or if the joint has very tight clearances.
Heating Rate.
There is an axiom that states, "the tolerable amount of contaminants in the furnace atmosphere is directly proportional to the heating rate." Thus, the higher the nitrogen content of a base metal, the higher the rate of heating required to produce a properly brazed joint.
Nickel Plating.
Another way to minimize the nitrogen problem in the base metal is by nickel plating both surfaces of the joint by the electrolytic process. It is important to note that some joints may not be suitable for electrolytic plating (e.g., blind holes) due to the poor throwing power of nickel. In such cases, internal electrodes may be used. Electroless nickel is not suitable since it is a nickel-phosphorus composition which melts at significantly lower temperatures (about 1600° F/870° C). Approx. 0.0005 to 0.001 inch of electroplated nickel should suffice. Plating thickness is dependent on the heating rate, brazing temperature, percentage of nitrogen in the base metal, and type of filler metal.
Difficulties due to nitrogen also arise when using case hardened (nitrided) steels as base metals. Alloys that tend to absorb nitrogen during annealing in dissociated ammonia experience similar problems. In these situations, boron containing brazing filler metals such as Nicrobraz LM should be avoided due to their affinity for nitrogen. Brazing filler metals without boron such as Nicrobraz 30 (BNi-5) and Nicrobraz 50 (BNi-7) may be used instead.
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