Graphite Fixtures and Eutectic Bonding
Eutectic melting is the process where an alloy of two or more metals, when heated, will completely change from solid to liquid at the same temperature. There are processes in which the eutectic melting is a desired result, such as brazing. There are thousands of eutectic compositions used in brazing applications. They can play a major role when heating a brazing filler metal up to the brazing temperature, as well as when cooling after brazing. During heat-up and melting, the eutectic composition will/should be the first to melt, and because of its narrow melting range (solids and liquids are the same temperature), it will flow into the braze joint.
There are also industrial processes where the eutectic melt is not a desired effect, and it can actually cause major issues within the process. A bonding effect can occur between various fixtures and the processed materials.
Various heat treat processes where graphite is used for fixtures are susceptible to eutectic bonding. The cause for this is the interaction between certain elements through solid state diffusion. In this particular case, carbon atoms can transfer from the graphite fixture to the metal to be treated. This transfer can change the melting point for the metal, and create fusion between the working surfaces, or potentially complete melting of the metal. The near-perfect surfaces on both the metal and the graphite fixtures as well as oxide free metal surfaces can aid the process.
Sintering as well as vacuum processing are examples where eutectic bonding can occur, with adverse results. In these processes, a major consideration is given to selecting the graphite fixture material. Solid state diffusion of certain elements can cause the formation of a lower melting point alloy (eutectic). For example, solid state diffusion between carbon and nickel can begin to occur at temperatures as low as 1165ºC (2130ºF) and cause local melting/bonding. Under the proper conditions, molybdenum will also combine with other elements to cause eutectic bonding. However, the molybdenum reactions with common elements in steel tend to occur at temperatures well above the normal heat treating range.
The process of selecting the proper graphite fixture material is not to be under estimated. Severe eutectic bonding reactions can cause extensive damage to the workloads. The repair costs could reach tens of thousands of dollars, in cases where entire hot zones need to be rebuilt.
The attached photo exemplifies just that. An entire set of graphite fixtures, and the associated stainless-steel parts to be heat treated, ended up being scrapped. The bonding process was so severe that the graphite plates as well as the stainless-steel parts warped, making them unusable.
The logic solution for these issues is to separate the potentially reactive materials with non-reactive materials (Insulators).These are often available in the form of papers, graphite foils, or other solid shapes upon which the work pieces can be placed. Additionally coatings of high purity ceramic materials such as aluminum oxide can be used. These are available in form of paints and sprays to be applied to the surfaces in contact. Please give consideration to all these aspects, in your heat treat processes, to prevent these undesired effects.