The use of carbon-fiber-reinforced carbon (C/C) composites in the brazing world is gaining a lot of momentum these days and for good reason. Brazing furnaces today often contain a lot of very heavy fixturing materials in their brazing chambers. This “fixture” includes all the heavy-metal structures in the furnace hot zone (e.g., grates, trays, racks, baskets, etc.).
C/C composite is a carbon-carbon composite material reinforced by high strength carbon fiber for improving mechanical strength and thermal shock resistance. C/C is a high strength lightweight material suitable for fixtures. C/C’s density is only 1/5 of steel, so C/C fixtures are much easier to handle. Also, it is about 10 times stronger than steel at 1000°C and there is no thermal deformation. Because of these excellent features, C/C fixtures realize process capacity increase and running cost reduction.
In heat-treating, including brazing, the furnace heating capacity and maximum load weight are two important factors that determine the amount of total load capacity processed in a heat-treat cycle. Using C/C fixtures, a significant increase is achieved in the quantity of parts that could be processed per cycle.
With metal-jig brazing, when the parts are aligned too closely, the shadow of the supplementary weights have a tendency to block radiant heat waves. This, in addition to the heat capacity of the dead weights, compromises the heating uniformity and leads to defective brazed parts. With a C/C fixtures setup, the C/C itself is very compact and does not encounter these problems. Hence, a decrease in the defect rate and an increase in process quantity is achieved.
After long-term service, the metal fixtures tend to deform due to high-temperature deformation, which also has a negative effect on the distortion of the parts. On the other hand, C/C fixtures maintain high strength from room temperature up to 2000°C (3500˚F), thereby assuring a positive effect on part distortion. Also, C/C has a very high deformation resistance, which leads to longer service life.
The production savings is significant, and these saving can offset the cost differential between C/C fixtures and alloy fixtures. The resultant overall cycle time savings (door to door) was 45 minutes, which overall cost saving in production hours when using C/C fixturing is 15%.
For some metals, direct contact between the C/C fixture and the metal being brazed is not acceptable. This is especially the case with stainless steels, in which contact with the C/C fixture could result in large carbon diffuse to the stainless, which can cause two problems:
1) Loss of corrosion resistance if the carbon from the fixture reacts with the chromium in the stainless to form chromium-carbides, which then migrate to the grain boundaries and this, in turn, robs the surface of needed chromium-oxide protection.
2) 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. Eutectic melting can occur with C/C materials at temperatures exceeding 1050°C (1922°F), but it is highly dependent on the alloy(s) being run, thus literally causing some portions of the base-metal assembly to sag or melt during brazing.
Therefore, to prevent both of these problems from occurring; coat the surface of the C/C fixtures with a brazing stop-off or keep the fixtures covered, either with a ceramic cloth or thin sheets of solid ceramic.
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.
The uses for C/C composites as fixtures are almost limitless. It is a great product, just making its way into brazing production, and has a great future. Consider its benefits of strength, durability, light weight, and lack of distortion when you are designing fixtures for your brazing needs. As C/C technology continues to develop, 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.
Dan Kay, Specialized C/C Fixtures for Today’s Brazing Needs, Industrial Heating, August, 2018.
Dan Kay, Carbon-fiber reinforced carbon (C/C) fixturing for brazing?, Kay & Associates brazing consultants, August, 2018.
Bill Warwick, Robert Hill Jr., and Daniel H. Herring, Technical Considerations for the Use of Carbon/Carbon Composite Materials For Fixtures and Grids, Industrial Heating, December, 2013.