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Hot zones are the hearth of every vacuum furnace. When purchasing a new furnace, you might face with the Hamletic doubt about hot zones: graphite-based or all-metal design hot zone? That’s the question!
The graphite wafer is an excellent material. It allows operation at very high temperatures (up to 3000°C based on the vacuum level), has low density, reduced weight and modest thermal capacity. It creates the ideal black body conditions (emissivity about 1) inside the heating chamber for obtaining high uniformity.
All-metal hot zones are used in high demand industries where sensitive materials are processed, such as aerospace, electronics and medical. There are heat treatments that require a particularly clean environment or extreme vacuum levels. There may be different reasons: in some cases the chamber’s graphite could interfere with the process, resulting in unwanted carburation of the pieces treated. In other cases, the load could be particularly sensitive to the presence of residues in the oxygen or hydrogen atmosphere (which could lead to embrittlement of the pieces), and so graphite wafer degassing during the cycle could be damaging. In these circumstances, the user should opt for all-metal heating chambers (shields and resistor).
If you want more information on this topic and a detailed analysis of the differences, take a look at the following 2 articles:
Vacuum furnace hot zone: graphite vs all-metal design [1/2]
Vacuum furnace hot zone: graphite vs all-metal design [2/2]
Thanks for the information. My direct question was related to fixturing, not hot zones for htre furnace.
The answer is complex in that it depends on a number of things as follows:
Graphite is easy to work with and relatively inexpensive.
Molybdenum is difficult to work with and is expensive
CFC is strong but expensive and not as easy as graphite to repair