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Discoloration of superalloy part after vacuum heat treatment

We used vacuum furnace to heat treat parts of superalloy (Ni-based materials, such as AMS 5596) at 1600~1800 °F under pretty good vacuum level of 10-4 ~10-5 Torr. Sometimes the parts got discoloration (blue or dark blue). My question: what caused the discoloration and how to prevent it?
Max Gui
Orenda Aerospace Corp - Mississuaga, Canada
2006

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First of two simultaneous responses --

Classic results of oxygen in the system.
First do a burnout cycle on the furnace and then do a leakdown check. There should be instructions for both in your operators manual. If not, contact the manufacturer for that information. Fix any leaks that you find and your problem will go away.
James Watts
- Navarre, Florida
2006

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Second of two simultaneous responses --

Max, my best guess is that the seal on the door was a bit dirty: we've found that even a hair laid across the door seal is enough to get discoloration. Another source of the contaminant that will cause this kind of problem is if the parts aren't cleaned and dry enough. If you still have a small amount of solvent or soap on them, it'll burn on, or evaporate and discolor, or cause some such thing.

Lee Gearhart
metallurgist - E. Aurora, New York
2006