Oxidation of stainless steel
Oxidation of stainless steel
(OP)
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.
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.
RE: Oxidation of stainless steel
Every surface is covered with a thin (molecular) layer of water.
Your vacuum level is only a rough vacuum, not even medium vac levels.
I take it that you are getting a light yellow/gold tint?
You can try heating to 150C and then doing repeat Ar fill cycles.
This will help remove moisture.
And I don't believe that you can measure H2O and don't see it spike during heating.
Every vac oven or furnace that I have ever run would do that.
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RE: Oxidation of stainless steel
H2o is supposedly less than .1ppm
Yes light yellow/gold tint color is correct
Would the 150C ar purge cycles have a better chance at removing moisture than 600c 48hour bake? that was done over the weekend.
Also besides h2o and o2 is there anything else that could cause the discoloration?
RE: Oxidation of stainless steel
Keep it closed and under vac until you are ready to process parts.
In fact it should be closed and under vac all of the time.
The parts should be cleaned with solvent (usually fresh alcohol) and stored in a desiccator if possible.
One mistake is using alcohol or acetone that has been open and has absorbed moisture from the air.
This leaves more water on the surfaces.
We used to do work at 0.1micron pressures (10e3 lower than you) and we measured oxygen and H2O using mass spec.
How you report gas concentrations is tricky. 0.1ppm of what?
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RE: Oxidation of stainless steel
RE: Oxidation of stainless steel
I presume that it is from liquid Ar?
We never wiped out chambers with solvents, just clean dry rags.
Even if you leave just a little isopropanol it is much harder to pump out because it is higher molecular weight.
So you mass spec has its own vacuum pump (differential pumped), correct?
You should have a sample port just outside of the oven in the line to the primary vacuum pump.
Is there some big reason why this is a problem?
The light gold tint is very thin.
You could strip it chemically (or electrolytically) if you just don't like the color.
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RE: Oxidation of stainless steel
They took it upon them selfs to clean the chamber with iso, I recommend clean dry rags as well.
I have not had a chance to use the mass spec yet.
Yes there is a port i can tie into
I can not answer that question
Do you believe the gold tint to be chromium oxide?
RE: Oxidation of stainless steel
But it is still passing light through it so it is very thin.
And the Cr depletion under it is very minimal.
Do you have a corrosion test that you need to meet?
Why on earth are you baking parts at 300C?
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RE: Oxidation of stainless steel
I asked the same question about 300c and was unable to get an answer, the ovens go from 60c up to 600c
RE: Oxidation of stainless steel
No stress relief will happen at that temperature and no metallurgical transformation.
Unless they are curing a coating or adhesive I don't know why they would do it.
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RE: Oxidation of stainless steel
RE: Oxidation of stainless steel
Again, the vac levels are not very good.
Honestly I would expect more like <10microns and you are >75microns.
Your vac isn't helping pull impurities, only repeated backfill and pump cycles can help.
And doing this warm (150C) will help a lot.
You need to test the Ar at point of use.
It had better have dew point <80C and <0.5 ppm O2 (V/V corrected to atmospheric pressure).
I am dubious of the <0.1ppm H2O, this is <-90 DP
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RE: Oxidation of stainless steel
the blue tint is from moisture? how is the blue different from the golden color?
RE: Oxidation of stainless steel
Is this a vacuum sterilization process?
RE: Oxidation of stainless steel
RE: Oxidation of stainless steel
If they leave the ovens open other than just to load or unload it would make things worse.
If it is one oven more than the other I would be concerned about the vacuum lines and the gas supply lines.
A tiny leak in a supply line under pressure will allow O2 and H2O into the line.
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RE: Oxidation of stainless steel
If one takes gold leaf and shines a light through it the light that is transmitted is blue as it blocks the red and green.
Things I would try -
swap the Ar feed at the oven to eliminate the potential there is a contaminated supply line.
create test coupons from a single piece of metal and try identical cycles on the two ovens.
ask if the same people are running the parts. (I've had problems where one technician decided to "make an improvement" that, at the time, ruined parts. Good news was it was so terrible it was evidence for doing the opposite as an actual improvement. Remember this: "No one is in trouble. I don't care what you did, I just need to know what it was. Then we can fix it and move along.")
Whenever everything is the same but the results are different, then not everything is the same.
RE: Oxidation of stainless steel
You run samples of Cu, steel, and 304SS.
All must be bright and clean.
By comparing the colors you can make judgements about the atmosphere.
Old article in HT magazine which I can't locate right now.
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RE: Oxidation of stainless steel
the ar feed has been swapped from the box atmosphere to right from house gas, i'll have to see if a tank is avaliable although the other oven is on the same house gas and even though it too is fed from the box it is producing parts fine, well was, they did something to cause the oven to no longer hold vacuum.
the cycles are identical according to them on both ovens, and the parts run have been from the same lot/batch, I am not sure how strict their tracability is.
the people swapping is a great suggestion, i dont know when they run their parts but it is typically a one batch a day thing.
the boxes the ovens are attached to are a little bit different, as the machines running inside the boxes are different, but properly purged and evacuated I dont see how much difference that could make. On a side note, the previous oven that was in place of the one producing golden and blue parts was apparently always breaking down and having issues.
At what temp does the golden oxide layer become blue? and at what temp / o2/h2o levels does they oxide layers begin to form?
RE: Oxidation of stainless steel
that would be a great article to read if youre able to track it down at somepoint.
RE: Oxidation of stainless steel
google SS heat tint and you should be able to find one.
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P.E. Metallurgy, consulting work welcomed
RE: Oxidation of stainless steel
RE: Oxidation of stainless steel
RE: Oxidation of stainless steel
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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RE: Oxidation of stainless steel
RE: Oxidation of stainless steel
RE: Oxidation of stainless steel
RE: Oxidation of stainless steel
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."
RE: Oxidation of stainless steel
RE: Oxidation of stainless steel
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
RE: Oxidation of stainless steel
RE: Oxidation of stainless steel
RE: Oxidation of stainless steel
RE: Oxidation of stainless steel
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.
RE: Oxidation of stainless steel
At least you know what works.
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P.E. Metallurgy, consulting work welcomed
RE: Oxidation of stainless steel