Many brazing shops use graphite fixtures on which to set parts that are to be brazed. Graphite fixtures have excellent thermal stability, enabling them to be used again and again through many brazing cycles. This high thermal stability is often coupled with low cost compared with some metals used for making fixtures (such as the high nickel/chromium or moly- alloy type fixtures).
If you are using graphite fixtures, or intend to consider doing so, it is VERY important to remember that graphite is carbon, and pure carbon likes to react chemically/metallurgically with metals containing iron (such as steel), to form low-melting eutectic compositions at temperatures just under 2100°F/1150°C, as shown in the iron-carbon (Fe-C) phase diagram illustrated in Fig. 1. This temperature is often lower than some of the brazing temperatures being used in many brazing shops today!
Therefore, 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, and that 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 any brazing cycles, since most of your vacuum brazing cycles will involve temperatures that could result in this undesirable reaction if the stainless and graphite fixture are allowed to be in direct contact with each other.
Stainless steels are ferrous materials, and are often used in a variety of brazed assemblies because of its corrosion resistance. However, if the stainless is allowed to rest directly against the top surface of a graphite fixture, the carbon can easily migrate into the stainless, resulting in some unwanted, and perhaps un-predictable 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 chrome-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), and so they will specify that only low-carbon variants of each of these base metals be used (such as 304L, 316L, etc.), or that a stabilized grade such as 321 or 347 be used, in order to prevent this loss of corrosion resistance. The same thing applies to the brazing industry, since brazing brings these chromium-bearing materials through the sensitization temperature range also. So, first of all, allowing stainless steel assemblies to directly contact graphite fixtures could prove harmful from this sensitization point of view.
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, or a ceramic-fiber paper/cloth, or via an adequate layer of an appropriate brazing stop-off painted onto the surface of the graphite.
NOTE: Develop procedures, too, 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.
CHALLENGE QUESTION TO READERS:
How many of you are now using the carbon-fiber reinforced carbon (C/C) fixtures? Have any of you encountered any issues with parts sitting directly onto those fixtures? Please let me know.
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Dan Kay – Tel: (860) 651-5595 – Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for 40-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. Dan can be reached via e-mail at dan@kaybrazing.com, and his website can be visited at: http://www.kaybrazing.com/
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