It is VERY important to remember that ferrous materials, such as stainless steel, should NEVER be allowed to come into direct contact with the graphite (carbon) fixtures. Also, there should always be an intermediate protective layer on top of the graphite to keep the ferrous material from such direct contact with the graphite during the heating cycle up to brazing temperatures.
Stainless steels are ferrous materials and are often used in a variety of brazed assemblies for the corrosion resistance. If the stainless is allowed to rest directly against the top surface of a graphite fixture, however, the carbon can easily migrate into the stainless, resulting in some unwanted and perhaps unpredictable results (as described below).
First of all, the stainless may lose it's "stain-less" characteristics if the absorbed carbon preferentially combines with the chromium in the stainless to form chromium carbides that migrate to the grain boundaries, thus breaking up the corrosion-resistant chromium-oxide layer on the surface of the stainless steels. The welding industry is well aware of this potential "sensitization" of stainless steels (occurs in the 800-1500°F/425-815°C range), so they will specify that only low-carbon variants of each of these base metals be used (such as 304L, 316L, etc.).
Alternatively, a stabilized grade, such as 321 or 347, is suggested in order to prevent this loss of corrosion resistance. The same thing applies to the brazing industry, since brazing also brings these chromium-bearing materials through the sensitization temperature range. So, allowing stainless steel assemblies to directly contact graphite fixtures could prove harmful due to sensitization.
However, far worse than this sensitization issue is the potential for eutectic melting of the stainless assembly when the iron in the stainless and the carbon in the fixture come together!
A number of brazing shops have experienced this destruction of furnace-brazed stainless parts when no protective layer was placed between the parts and the graphite fixtures on which they were resting. Portions of the stainless components literally "melted" as low-melting iron-carbon eutectics were formed at brazing temperature, causing the assemblies to tilt and even collapse in the brazing furnace.
Therefore, always be very sure that stainless components are kept from contacting graphite fixtures during brazing cycles. This can easily be accomplished via a layer of solid ceramic, a ceramic-fiber paper/cloth or an adequate layer of an appropriate brazing stop-off. Develop procedures to verify that the stainless will not "break through" any of these ceramic layers during placement of the components onto the fixtures and also when the fixtures and components are placed into the brazing furnace.