It is not uncommon in vacuum furnaces to observe what may best be described as “silver-colored flakes” or cracked surfaces (Figs. 1-2), which appear on graphite-lined vacuum furnaces. These flakes or cracks develop over time (years), or within a relatively short time span (2-24 months), and get progressively worse. This “silver coating” often appears to be selective with respect to where it deposits or bonds to various surfaces. It bonds to some elements (Fig. 3) but not all and can also be seen on some but not necessarily all hearth rails (Fig. 4).
This often occurs in vacuum furnaces that run primarily stainless steel at high temperatures and low partial-pressure atmospheres, examples of which include annealing of 300-series castings or machined parts and hardening of some martensitic and precipitation stainless steels (e.g., 17-4 PH condition A). The phenomenon is often most prevalent after burnout cycles where higher temperatures are used.
So, what we are seeing?
The vaporization (i.e., volatilization of elements from the surface) of nickel, chromium, molybdenum and cobalt contained in certain stainless steels at high temperature and low partial pressure can result in deposits on and interaction with the surface of graphite material. All forms of graphite (e.g., carbon board, carbon felt, carbon board with CFC hot facing or graphoil) are affected. Vaporization combined with repeated heating (thermal expansion) and cooling (thermal contraction) results in the cracking (aka peeling, rolling or fragmentation) of these impregnated surfaces.
An increase in partial pressure (sometimes to as high as 3-4 torr) is necessary to prevent vaporization and the subsequent chemical reactions on the surface of the graphite. However, this may not always be practical if part surfaces are to be kept “bright.” In the case of brazing, unrestricted braze flow and good fillets are desired.
1. Herring, Daniel H., Vacuum Heat Treatment, BNP Media Group, 2012
2. Herring, Daniel H., “Using Partial Pressure in Vacuum Furnaces,” Industrial Heating, November 2005
3. James Grann, Ipsen, private correspondence