Question on vacuum heat treatment

[slyth]

Hello. Currently I am doing a test on a vacuum heat treatment machine and came up with an unsolved problem for the past 2 months. I am out of clue and I’m here to hear everyone's opinion. Below are the steps what I did.

1) Vacuum chamber pumped down to 4.5E-04 Pa (Rotary + Mechanical booster pump for roughing, then to turbo molecular + cryo for main pump)
2) A few SUS304 plates inside the chamber are heated up to 1200℃ for 12 hours (pumps are running)
3) After heating, SUS304 plates are cooled down by; 250kPa Argon gas vent into chamber (pumps off, of course) and leaving it for 5 hours.
4) After coolant, SUS304 plates are taken out to see any changes

My problem lies here. Through this process, there SHOULD NOT BE any changes seen on the SUS304 plate. But, I’m seeing a small change of color on the plates. I cannot upload an image of it though, the color changed from a normal SUS304 color to a darker color (Oxidation?! Corrode??)

Few notes:
1) Chamber went through bakeout before test
2) Helium leak test shows no leak (during vacuum)
3) I do not own a spectrometer
4) Argon gas is 99.9999% (supposed to be)
5) Argon gas is supplied by a clean　reserve tank
6) No one touched the machine while I was away

My question:
1) What made the SUS304 plate changes it color?
2) Say there is a leak after venting with 250kPa Argon (due to stress). A small amount of Argon gas would escape. From there, are there by any chances that air from outside the chamber gets into the chamber? (which I believe, nearly zero though…)

 

[etudiant]

It is not easy to get clean, high quality vacuums.
Maybe the plates outgassed during the vacuum bake phase. Or possibly something got deposited on them/reacted with them during the bake/cooloff phase.
I guess you need to inject the Ar while the chamber is hot, but it does raise concern that some contaminant is mobilized at that point. Are the valves clean?