Atmosphere-Brazing

- Jan 12, 2019-

Since the brazing work requires high temperatures, oxidation of the metal surface occurs in an oxygen-containing atmosphere. This may require the use of an atmospheric environment other than air. The usual atmosphere is

* Air: Simple and economical. Many materials susceptible to oxidation and buildup of scale. Acid cleaning bath or mechanical cleaning can be used to remove the oxidation after work. Flux counteracts the oxidation, but may weaken the joint.
* Combusted fuel gas (low hydrogen, AWS type 1, "exothermic generated atmospheres"): 87% N2, 11–12% CO2, 5-1% CO, 5-1% H2. For silver, copper-phosphorus and copper-zinc filler metals. For brazing copper and brass.
* Combusted fuel gas (decarburizing, AWS type 2, "endothermic generated atmospheres"): 70–71% N2, 5–6% CO2, 9–10% CO, 14–15% H2. For copper, silver, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, nickel alloys, Monel, medium carbon steels.
* Combusted fuel gas (dried, AWS type 3, "endothermic generated atmospheres"): 73–75% N2, 10–11% CO, 15–16% H2. For copper, silver, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, low-nickel alloys, Monel, medium and high carbon steels.
* Combusted fuel gas (dried, decarburizing, AWS type 4): 41–45% N2, 17–19% CO, 38–40% H2. For copper, silver, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, low-nickel alloys, medium and high carbon steels.

* Ammonia (AWS type 5, also called forming gas): Dissociated ammonia (75% hydrogen, 25% nitrogen) can be used for many types of brazing and annealing. Inexpensive. For copper, silver, nickel, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, nickel alloys, Monel, medium and high carbon steels and chromium alloys.

* Nitrogen+hydrogen, cryogenic or purified (AWS type 6A): 70–99% N2, 1–30% H2. For copper, silver, nickel, copper-phosphorus and copper-zinc filler metals.
* Nitrogen+hydrogen+carbon monoxide, cryogenic or purified (AWS type 6B): 70–99% N2, 2–20% H2, 1–10% CO. For copper, silver, nickel, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, low-nickel alloys, medium and high carbon steels.
* Nitrogen, cryogenic or purified (AWS type 6C): Non-oxidizing, economical. At high temperatures can react with some metals, e.g. certain steels, forming nitrides. For copper, silver, nickel, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, low-nickel alloys, Monel, medium and high carbon steels.
* Hydrogen (AWS type 7): Strong deoxidizer, highly thermally conductive. Can be used for copper brazing and annealing steel. May cause hydrogen embrittlement to some alloys. For copper, silver, nickel, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, nickel alloys, Monel, medium and high carbon steels and chromium alloys, cobalt alloys, tungsten alloys, and carbides.
* Inorganic vapors (various volatile fluorides, AWS type 8): Special purpose. Can be mixed with atmospheres AWS 1–5 to replace flux. Used for silver-brazing of brasses.
* Noble gas (usually argon, AWS type 9): Non-oxidizing, more expensive than nitrogen. Inert. Parts must be very clean, gas must be pure. For copper, silver, nickel, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, nickel alloys, Monel, medium and high carbon steels chromium alloys, titanium, zirconium, hafnium.
* Noble gas+hydrogen (AWS type 9A)
* Vacuum: Requires evacuating the work chamber. Expensive. Unsuitable (or requires special care) for metals with high vapor pressure, e.g. silver, zinc, phosphorus, cadmium, and manganese. Used for highest-quality joints, for e.g. aerospace applications.

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