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 (which is carbon) could result in large carbon-pickup by 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 — this is discussed at length in another article I’ve written previously; and (2) if carbon infiltrates the stainless it is possible that the iron in the stainless steel can form low-melting eutectics with carbon, thus literally causing some portions of the base-metal assembly to sag or melt during brazing! Be careful of this!
Therefore, to prevent both of these problems from occurring, it is always wise to coat the C/C fixture with a stop-off material or lay a thin ceramic-fiber blanket on the C/C fixture, in order to eliminate the possibility of any contact between the fixture and the base-metal being brazed.
An example of a special ceramic insert that is laid in place on top of the C/C fixture legs is shown in Fig. 9, which helps to ensure non-contact between the fixture and the metals being brazed. This is very important that separation between those two be maintained.