Brazing furnaces today often contain a lot of very heavy fixturing materials in their brazing chambers. This “fixturing” includes all the heavy-metal structures in the furnace hot zone (e.g., grates, trays, racks, baskets, etc.).
Everything put into a hot zone must be heated up to brazing temperature and then brought safely back down to room temperature, hopefully distortion-free. All of this weight in the chamber results in long brazing cycles, and it also makes a significant percentage of the furnace load/mass to be nonproductive.
There is no free ride. It costs the brazing shop a lot of money to heat all that heavy fixturing weight up to brazing temperatures, perhaps several times each day, day after day and week after week. This cost detracts significantly from the brazing shop’s bottom-line profitability. I have actually seen brazing shops where the total weight of the fixturing exceeded 90% of the total weight put into the furnace chamber. Less than 10% of the total load going into the furnace chamber was actually parts to be brazed and sold to a customer. This results in a number of brazing shops unknowingly being in the business of heat treating fixtures rather than being in the brazing business.
I have asked some shop managers, “OK, so let me see if I have this right. Even though you don’t realize it, you’re actually in the business of heat treating fixtures, but you do send some parts through the furnace to be brazed so that you can generate some income to pay for the heat treating. Did I get that right?” That usually makes them think more realistically about the poor productivity of their so-called “brazing furnaces.”
To help alleviate this problem, carbon-fiber-reinforced carbon (C/C) is a material being used more and more for making brazing fixtures. C/C fixtures not only greatly reduce the weight going into the furnace but have the added benefit of excellent thermal stability over the long term.
The inner structure of a C/C fixture consists of carbon fibers such as those shown in Fig. 1. These are embedded in a special carbon-matrix material to form a very strong composite structure. The carbon fibers themselves have very high strength, and the carbon-matrix material offers specialized heat resistance, chemical resistance, low thermal expansion, and high thermal and electrical conductivity.
The length of carbon fibers used (often cut into very short pieces) and their orientation in the carbon matrix can be modified, as needed, to achieve desired properties. Compared to other fixture materials – such as compressed pure graphite, ceramics or metal – C/C material is very strong, lightweight and flexible. It will operate over a wide range of temperatures without any loss in performance while maintaining dimensional stability (no distortion).
C/C materials can safely withstand temperatures over 3500°F (2000°C) in any non-oxidizing environment (vacuum, argon, nitrogen, etc.). They should never be used at high temperatures in a hydrogen atmosphere or in any atmosphere with significant percentages of oxygen.
Furnace Fixture Handling
Fig. 2. This 66-pound (30-kg) grid showed significant distortion after nine months of furnace heating/cooling, with no intermediate straightening attempted during that time
(courtesy of Schunk Graphite).
Proper handling of C/C fixtures by shop personnel is very important, and it does require instruction and training. Carelessly throwing C/C fixtures across the ground or dropping them on the floor could cause damage to the C/C material and shorten the life of such fixtures. Remember that C/C material does not dent or distort as metal fixtures do, and even though it has a high resistance to fracturing due to its flexibility, people should still be trained in how to store, handle and use C/C fixturing.
As with all materials, C/C fixtures will expand and contract during heating, but it should be able to maintain its dimensional flatness better than metal fixtures (due to its ability to resist distortion better than metals). This is shown in Fig. 2 and Fig. 3, where a metal grid was cycled many times in a furnace for many months resulting in significant distortion, whereas a C/C grid used for the same purpose and length of time is still flat and distortion-free.
Fig. 3. This C/C grid weighs 5 pounds (2.5 kg) and was used in the same manner as the metal grid shown in Fig. 2 but with no distortion at all.
(courtesy of Schunk Graphite)
Because many of the metals that you are brazing can react with the carbon-based C/C fixtures on which they are sitting, it is very important that you provide some kind of a separating layer between the metal parts and the C/C fixture. This can be done by placing a thin sheet of ceramic material or a ceramic-fiber cloth between the C/C fixture surface and the bottom of the metal component being brazed.
An example of some ceramic pieces that were fitted to the rungs of a grate are shown in Fig. 4, where it can be seen that the ceramic pieces placed on top of the grid will prevent any metal that is sitting on top of those ceramic pieces from reacting with the fixture below it. Because C/C is more than 99% pure carbon, and pure carbon likes to react strongly with any metals that contain iron (such as stainless steel), any reaction between carbon and steel might easily form low-melting eutectics at temperatures under 2100°F (1150°C).
Fig. 4. Ceramic pieces placed on top surface of furnace grid.
(courtesy of Schunk Graphite’s CarboGard channels)
Partial melting of the base metals sitting on that C/C fixture will result from direct contact. You need to know this and take precautions to ensure that metal does not come into direct contact with the C/C fixtures during heating in the furnace. A couple of other metals that can react strongly with carbon are titanium and chromium.
C/C Fixturing Designs
Fig. 5. Careful design of the fixturing is required so that very even force will be applied to the heat exchanger. As the temperature rises, the metal heat exchanger expands and the C/C springs compress.
(courtesy of Across USA)
Figure 5 shows an example of a C/C brazing-fixture design. It is designed to hold the part being brazed between top and bottom plates made of C/C. The coil springs are also made of C/C and are used to apply equal pressure during the brazing process. C/C springs are made of unidirectional carbon-fiber composites and should be able to maintain correct pressure on the surface for hundreds of cycles, even at brazing temperatures up to about 2350°F (1300°C). An example of two kinds of C/C springs are shown in Fig. 6 (coil spring) and Fig. 7 (flat spring).
C/C Springs vs. Dead Weights for Compression
It is fairly common for braze shops to place a number of metal blocks (dead weights) on top of parts to compress the load beneath them, which keeps the metal parts in close contact with each other throughout the brazing cycle. This can often add hundreds of pounds to the furnace load being processed.
By using lightweight C/C spring loading of parts instead of dead-weight fixturing, a huge amount of nonproductive weight can be removed from the furnace. And, because the C/C fixturing itself heats up about three times faster than metals (due to the excellent thermal conductivity of C/C fixtures), the greater heating efficiency of the furnace load can significantly improve the throughput of each furnace.
C/C fixturing has many advantages when compared to metal fixturing being used for the same purpose. Among these are:
- Much lower gross weight of fixturing (grates, baskets, trays, racks, etc.)
- Significant improvement in furnace productivity (cycles run per day)
- Large reduction in cost of energy per part brazed
- Less need for fixture maintenance/replacement caused by fatigue or distortion
As C/C technology continues to develop, it is expected that more and more users will see C/C material as a fairly simple way to increase productivity and profitability in their brazing shops, swiftly offsetting the initial higher cost of C/C fixtures compared to typical metal fixtures. The costs of C/C fixturing are steadily coming down as more and more shops buy them and use them and find out the many advantages of switching to this “new” technology.
Acknowledgements: I wish to give special thanks to Lloyd Nagamine of Across USA Graphite Fixturing Company (Carson, Calif.) for his invaluable assistance in helping with the information used in the writing of this article.