High-temperature Brazing of Stainless Steel in a Continuous Inert Atmosphere Furnace
Rich Seymour - Seymour & AssociatesTraditionally there have been two methods for the furnace brazing of stainless steel assemblies:
1) Metal mesh belt furnace which employs a combustible atmosphere, and metal muffle. Drawbacks of this technology include the fact that nickel containing braze alloys will react with the metal of the furnace, and temperatures are limited by the metal structure of the furnace to about 1150 C maximum limiting the possible brazing systems that can be exploited.
2) Vacuum furnaces which have a potential problem of evaporation of the chromium, and definitely have a problem with uneven heating and warpage of large assemblies, since only radiant (no convection) heating is available and outside edges always heat first.
Using high temperature continuous furnace equipment (called Oxynon®) developed by KYK (Kanto Yakin Kogno Co.) it is now possible to explore a new method of brazing which overcomes these limitations. The primary enabling feature of this equipment is its all graphite and carbon composite construction (no metals). Belt is entirely made of carbon-carbon fiber composite components, as is the muffle, and the heating elements. Inert gasses must be used to prevent burning of the components, but these lend nice advantages to the brazing process. Oxynon equipment can be run with either Nitrogen, Helium, Argon, or any inert gas, but (as will be discussed later) for brazing stainless steel, Argon is the gas of choice.
The furaces are operated at ambient pressure (1 atm) with Oxygen contents of less than 10ppm (put another way, the partial pressure of O2 is less than 10-5 atm.)
Any metal oxides that may be present in the workpiece are heat dissociated rather than reduced as in normal brazing furnaces.
Another advantage of the Oxynon furnace is the fact that the graphite/carbon parts have high thermal conductivty and low thermal mass, which makes heat-up and cool-down quite rapid and efficient compared to a metallic construction. Because less energy is spent heating furnace components, more product load can be handled with the same energy cost. The belt itself is extremely strong and lightweight (about 9 lb per sq meter).
Finally the furnace components are all stable up to 2400C, which opens the door to many new brazing systems, and to working with higher melting and reactive metals and alloys.
A quick comparison of Operating conditions for similar size continuous furnaces between metal and Oxynon condinuous furnaces:
| Metal | Oxynon (C-C) |
| 1150 C max Temp | 2400 C Max Temp |
| belt weight 20-24 Kg/m2 | belt weight 4 - 9 Kg/m2Temp |
| high creep (stretch) | No creep |
| Strength decreases with increasing temp | Strength increases with increasing temp |
| Defined furnace thruput 50 kg/hr | Defined furnace thruput 270 kg/hr |
| atmosphere cost low | atmosphere cost higher |
Key to the technology of the Oxynon when used for brazing stainless steel is the notion that oxides dissociate rather than reduce. In a conventional brazing furnace a metal oxide combines with hydrogen to liberate water and clean metal. In the Oxynon furnace an equilibrium is established which allows the metal oxide to dissociate (when heated) to pure clean metal plus oxygen. The small amounts of oxygen liberated combine with carbon of the muffle to form carbon monoxide. By monitoring and controling CO (carbon monoxide) content of the atmosphere, a very low oxygen partial pressure is maintained which favors the metal oxide dissociation.
A modified Richardson Diagram shows what oxides will dissociate at what temperatures in the range PCO of 10-3 to 10-6 atm (1000ppm to 100ppm CO) which is the operating range of the Oxynon.
Following the 4/3Cr+O2 = 2/3Cr2O3 line, it is seen that at 700C, 20 ppm CO (PCO = 2x10-5) will allow dissociation; while at 800C it can be accomplished at 200 ppm CO (PCO = 2x10-4)For brazing of stainless steels a significant drawback of the Oxynon process is the cost of atmosphere. Nitrogen is relatively cheap and readily available, while Argon is not nearly so widely distributed in bulk and costs about 4-5 times as much as nitrogen.
KYK conducted studies to see if Nitrogen/Argon mixtures would be suitable as atmosphere for the process. Unfortunately, the presence of nitrogen causes surface nitriding of martinsitic stainless steels, and some penetration of nitrides into austenitic stainless steels; therefore Argon (99.95 pure) is the atmosphere of choice.
On the brighter side, due to the high molecular density of Argon, only about half as much flow is required in the furnace as would be needed with nitrogen, so the cost penalty becomes about 50%.
Carbon transfer into workpieces is avoided by never allowing the work to directly contact the graphite belt. Usually graphite carriers coated with a barrier of Boron Nitride are used to prevent this problem. Other (more massive and less desirable) solutions include molybdenum or tungsten carriers.
Concluding; the advantages of Oxynon furnaces processing for brazing stainless include:
Working above 1150C
Heavier loading (higher thruput)
Safer Atmosphere (no safety systems as with Hydrogen)
No issue of metal evaporation (as with vacuum)
Very uniform heating (meaning less deformation and better precision)
Rapid heat-up and cool-down allows better equipment utilization
Can be used with high temp reactive metals (ie. titanium, niobium)
In a production example involving a small heat exchanger braze; the 4 hrs of Oxynon production matched 9 hrs of vacuum production and one Oxynon furnace replaced 5 vacuum furnaces.
In another example with a large 600mm x 600mm x 100mm heat exchanger braze, and impossible task (high warpage) was made possible.
Finally, and of greatest interest, the first Oxynon furnace delivered to the U.S. has been installed at the Pennsylvania State University Center of Innovative Sintered Products (CISP). This four zone furnace is capable of continuous operation at temperatures to 2000 C. For further information contact CISP (814) 865-2121.
Mr. Seymour may be reached at rbseymour@oxynon.com 843-521-0765.