Do You Use Silicon-Carbide Hearth Plates?

Nickel-base or nickel rich alloys are highly suited for high temperature application, especially carburising ones. This makes them attractive and suitable for high temperature environment; heat treatment furnaces, retorts, muffle, calciners and ducts. What is common practise however is the use SiC bricks or support systems to hold lengthy equipment in place and also, the use SiC pads, slabs or hearth plates in combination with the nickel rich alloy in different high temperature applications. SiC is naturally a choice for such because of its high temperature melting point and refractory properties. When such happens, a reaction between the nickel rich component and the SiC material is always very fast and quite catastrophic.

Nickel-base alloy in direct contact with SiC leads to a solid state bonding of SiC with Ni in the alloy, leads to a reaction that results in the formation of intermetallic silicides and in this case; nickel silicides. Over the temperature range 250°C – 950°C, different composition of nickel-silicides forms. These intermetallics are low temperature eutectics, which transforms into liquid phase at 964°C- eutectic temperature. However, before this reaction can occur, Si must be released from its bonded state in the SiC compound. Protective oxides are destroyed in the high temperature environment and continual eating away of the alloy occurs- sometimes ending as a hole through the component wall thickness; causing great damage to the alloy.

It should be pointed out that SiC is volatile at 500°C- SiC dissociates at around 500°C, thus releasing the Si required for a reaction with nickel in the alloy, hence running facilities with SiC parts in contact with nickel rich alloys may not be a good idea. A suggested way forward if contact cannot be avoided is to use an inert material e.g. alumina sheet, to insulate the components from each other. 

Akin Fajimi
Rolled Alloys - Blackburn, UK