When operating vacuum furnaces, situations may arise in which the hot zone and/or cold walls may become contaminated (Fig. Nos. 1 – 2). This can occur from a variety of sources: air leaks, outgassing from residues left on the parts as a result of the manufacturing or cleaning processes, vaporization of sensitive materials (e.g., chromium-bearing materials), process induced contaminations such as carbon in the form of soot or tar, fluxes from brazing pastes, excess braze alloy as well as many other sources. Often times the work being processed is also affected (Fig. 3). The question becomes, how do we attempt to clean up our contaminated vacuum furnaces?
The leak test should be performed as part of routine operation and maintenance of equipment and typically consists of isolating the heating chamber (i.e., no pumping on the chamber) for a period of one (1) hour and measuring the increase (if any) in vacuum level. For single chamber vacuum furnaces, 10 – 20 microns per hour is considered an acceptable leak rate for general heat treatment (Note: specific materials, parts and/or processes may require a significantly lower leak rate). The vacuum level should be recorded before and after the test. Detailed procedures can be found in Reference 1.
