Question: We are currently using steel dead weights to hold down the top layer of the heat exchangers we are brazing in a vacuum furnace. The added weight does extend the cycle time a bit, and we’re wondering if the C/C spring-loading fixtures we have heard about would be good for us to use. What are the advantages and disadvantages of doing that?
Answer: The use of carbon-fiber-reinforced carbon (C/C) fixtures in the brazing world is gaining a lot of momentum these days and for good reason. The C/C fixtures are very light in weight by comparison to the heavier steel fixturing, and the C/C fixtures hold their shape and spring properties very well over extended times and temperatures.
Figure 1 is an actual example from one of the C/C manufacturers (there are several good manufacturers that you can find and talk to). This shows what they were able to do to significantly reduce the weight of fixturing used in one of their client's brazing operations, which saved that client a lot of time and money for their brazing operation.
As you can see in Fig. 1, the client was using about 20 kg (almost 50 pounds) of dead weight on top of the assembly in order to keep it in compression during brazing. All of that added dead weight absorbed a lot of heat to get it up to the brazing temperature along with the regular production load. This caused the client to become concerned about the efficiency and operating costs of their production furnace. They experimented with, and selected, C/C fixturing for their production to replace the dead weights. They immediately saw a surge in their furnace’s productivity. Two options they explored were the use of C/C springs in coil form or in leaf-spring form. Their final selection was to use the coil springs.
Always remember that there is no “free ride” so to speak as far as costs are concerned. Every pound (kg) you put into your furnace costs you money to heat it up to brazing temperature. The heavier the weight, the more money you are “throwing away” just to keep a heavy weight on top of the load. Instead, the use of C/C fixturing is saving many brazing shops a lot of money these days.
When you use C/C fixtures, please be aware that several concerns must be carefully considered and taken into account.
- C/C fixturing is fragile. You cannot allow your shop personnel to roughly handle, drop or throw C/C fixtures because they may break.
- Because C/C fixturing is made from graphite (carbon), they should never be used in any brazing atmosphere containing hydrogen. The hydrogen will react with the carbon in the C/C fixturing to form gaseous hydrocarbons, which will cause the fixturing to slowly disappear! The gaseous hydrocarbons generated may also cause the atmosphere in the furnace to become more carburizing, which is not good for many base metals, especially low-carbon stainless steels.
- Don’t allow the metal parts being brazed to come in direct contact with the C/C fixturing because it may be possible for those metals to then pick up carbon from the fixturing. This could cause carburization of those base metals and possibly result in the formation of low-melting iron-carbon eutectics in any ferrous materials sitting on those C/C fixtures. Keep the fixtures covered, either with a ceramic cloth or thin sheets of solid ceramic. Alternatively, coat the surface of the C/C fixtures with a brazing stop-off.