A. Pure dry hydrogen will not decarburize the 314 stainless belt. What could cause decarburization is, if the furnace is shut down over the week, say lowering the temperature to 1000°F (540°C), and turning off the belt. The belt will oxidize in the hot zone of the furnace; this will burn out some of the carbon from the surface of the belt.

    When starting up the furnace to 2050°F (1120°C) for copper brazing, some of the oxygen from the oxide will diffuse into the wires and could cause further decarburization. The chemistry for 314 is C 0.25 max, Cr 23.00–26.00, Mn 2.00 max, Ni 19.00–22.00, P 0.045 max, S 0.030 max, Si 1.5–3.0.

    I assume there is some breaking of the 314 belt wires and that is your concern. One of the common causes of wire breakage is liquid metal embrittlement. When the belt is in the hot zone loaded with the assemblies, it is under tension. When a drop of molten copper drops onto the clean belt under tension, the copper penetrates the wire and causes failure. It is hoped, the amount of copper is limited to the exact amount that will fill the brazed joint, so there is no excess to drop on the belt. Many brazing shops have found that a stop-off product applied to the clean belt will help to eliminate or reduce the problem. A green product appears to be the most effective.

Q: My second question is if we were to run a 100% nitrogen atmosphere on 300 series stainless steel, should we expect to experience a shorter salt spray life?

A: In reference to processing 300 series stainless at 2050°F (1120°C) in pure nitrogen, as far as I know, this is not in commercial use. There is experience with varying percentages of nitrogen with hydrogen that should indicate that the salt spray corrosion time would be further reduced.

    The brazing industry has found that the salt spray time on 304L, when copper-brazed in an 80% nitrogen–20% hydrogen atmosphere was very poor. There was improvement with a 50% nitrogen–50% hydrogen atmosphere. When brazing in a 25% nitrogen–75% hydrogen atmosphere, the salt spray resistance to corrosion further improved. In order to pass a 75-hour salt-spray test, it was necessary to use an atmosphere of 100% hydrogen of around –60°F (–50°C) dew point, or similarly dry argon with 10% hydrogen.

    The reason for the variation in corrosion resistance is that the nitrogen in the atmosphere causes chromium nitride to form at the surface of the base metal at the 2050°F (1120°C) brazing temperature. This reduces the chromium and increases the iron available in the remaining surface alloy, thus reducing the corrosion resistance of the remaining surface alloy.

    I hope this information will help to put these problems in prospective.©