Oxidation of stainless steel 2

[tripog]

I am troubleshooting an issue with some things I don't quite understand and would like some input.
set up:
Stainless Steel item is placed under vacuum and heated up to 300 degrees Celsius after multiple argon purge cycles. When part comes out there is oxidation on it, I am being told this can only happen if it is exposed to o2 during the bake. The vacuum is held at 1x10-1 mbar with no rise or drop in pressure during the procedure. Nearly identical setup on another machine performs the operation as intended with zero oxidation, the only difference that I can detect is a little damage to a couple of TiAlN coated parts inside the setup that is producing items with oxidation.

So my question is, can anything other that o2 produce oxidation on stainless steel at 300 degrees Celsius?
o2 and h2o is less than .1 ppm during argon purge.
I am unsure of the stainless steel grade.

 

[EdStainless]

I have seen tables for color vs temp for 1hr hold in air.
google SS heat tint and you should be able to find one.

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P.E. Metallurgy, consulting work welcomed

 

[tripog]

at this point its looking like I wont be able to troubleshoot it tuesday, it may be coming off.

 

[isoca]

Have you done a leak test on the furnace? Having vacuum does not mean that air is not entering it. Sometimes this issue is solved by covering the piece with a stainless steel sheet (free of visible oxide)

 

[EdStainless]

I skipped over that, but yes what about a leak up test.
After the bake out cycle the vacuum pump should be isolated and the vacuum monitored.
It should take hours to see any change in presser.
And these need to be documented and kept on file.
A change in this behavior (faster pressure rise) should trigger maintenance activity.

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P.E. Metallurgy, consulting work welcomed

 

[tripog]

It held vacuum with no change for about 6 hours, they ran the oven at that point. It's baked off during testing and they baked it off again, supposedly they performed a helium leak test with no signs of a leak but now it's claimed that a water cooling line is leaking even though the cooling lines are outside of the oven. I am going in tomorrow am to hopefully troubleshoot and find the issue.

 

[TugboatEng]

Just don't step on any toes. The know-betters already know what is wrong despite not being able to fix the problem and not having any facts.

 

[tripog]

My composite toes were made for stepping. Unfortunately when I arrived they had the oven disconnected and taken somewhat apart. Tomorrow I will be setting up for standalone operation and doing some helium leak testing on it, according to them it went down to -11 on a Pfeiffer adixen unit, the old oven today went to 2.00x10 at -09

 

[3DDave]

I recall a gear failure investigator who was often "helped" by people running everything through the parts washer to remove all evidence of lubrication or just piled all the broken fragments into a box where the initial failure site could be pounded to a pulp.

To make it more interesting the usual response to "what was it doing when it broke" was "We don't know, we just found it this way."

 

[btrueblood]

How is the He leak test being run, are you sniffing the interior at vacuum, and spraying helium around the oven seals? Along all the argon tubes/hoses and joints too? A better method would be to bag the oven and put a full helium atmosphere around it, otherwise the leak test readings are "information only".

 

[EdStainless]

We always sniffed so that we knew where to work on things.
But you have to have a fixed practice, the amount of He you release, over how long, the dwell between moving on to different locations, and so on are all critical. In our furnaces the delay between releasing a puff of He and seeing a reading was about 200sec. You had to be very patient and move very slowly of it meant nothing.
I do not believe that they closed the unit, pulled vacuum, and then isolated the pumps, and over 6 hours there was no leak up.
Either the vacuum pump isolation valve leaks or your pressure gages are not working correctly.

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P.E. Metallurgy, consulting work welcomed

 

[tripog]

Helium leak test on old unit was by sniffing, I was not here for the leak check on the new oven. I'm hoping to complete one tomorrow but as of now the new oven pressure reading has been holding at 1x10-2 mbar for about an hour.

 

[tripog]

Can anyone verify what temperature stainless steels will oxidize in the presence of O2? I'm being told around 290C

 

[isoca]

As mentioned before, stainless steel is always oxidized. Normally in this type of treatment, what is defined are several standards with different colors. What's your limit? totally bright? yellowish?. According to my experience, at atmospheric pressure the yellowing starts at around 400ºC (it`s deppend on alloy grade, humidity, cleanliness. etc)

 

[tripog]

Yesterday I ended up having to talk to their bosses and mine to kick all of their well intentioned staff off the machines as they were doing more harm than good.

Today I setup the oven as a standalone oven, pointed out some facts about their current setup that needs to be fixed for any oven to work. Pulled vacuum on the new oven, ran their recipe and the parts came out as intended. The young engineer there was seemingly pretty upset that the parts came out good but I am heading home first thing in the morning. I don't like leaving the reinstallation in their hands as the work they did installing the old oven was awful but my hands are tied.

 

[EdStainless]

Yes, having to let them do it wrong themselves is the hard part.
At least you know what works.

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P.E. Metallurgy, consulting work welcomed

 

[tripog]

Pretty frustrating having most of the people appreciate the troubleshooting and fix only to have the young engineer argue every point and be upset that the machine functions as designed. But thank you guys for all your information, it helped me explain my point a good amount and helped me learn a little about their process.