Vacuum Furnace Problems

Welcome to CR4 Vacuum Head.

Is there any contaminants in the vacuum chamber?

Thanks!

We just cleaned the intire furnace. I removed a lot of loose deposits that turned out to be Mangan. Most likely from the brazing that we also do in the furnace. But there was no improvement towards the stainless parts.

Have you ever successfully treated this exact alloy before? Are these parts from a different manufacturing run than those successfully treated? Are they from china? Are the flame jets hitting these parts directly?

We have treated these parts that we make out of stainless steel coils many times before and in the other furnace they look fine. We havent made any changes that we are aware of, its like this discolouration apear out of nowhere and than disapear again. It's about every second or third time it happens.

are the parts cooled before air is allowed to contact them? It may be that a lower final temperature before you break the vacuum would help

We cool it down to 80 degrees C. before we open up. Has not been a problem before.

5x10-² mbar is not a very low pressure, so perhaps there is leakage in the vacuum chamber which is resulting in what appears to be oxidation. Check out http://www.wallcolmonoy.com/techservices/brazing/oxidation.html and http://www.finishing.com/289/74.shtml

I have run a leak up rate in cold condition and it is smaller than 1*10-3 mBar for half an our.

Something is outgassing. Something has changed. Are you using new or different shelves to support the workpieces? Or a different cleaning method? Do you flush with nitrogen or argon before heating? Nitrogen is cheap...perhaps flood the chamber with nitrogen before heating, see if that changes. Could something have spattered onto the firebrick? How do you clean the workpieces...is there some residue from a different cleaning solution? Are you using new gloves, ...those marks look a bit like finger marks to me, and sometimes new gloves leave near invisible residues.

The funny thing is that we have two identical ovens. They are operated by the same personal, our parts are washed in one machine before the heat treatment. The two ovens run the same program. Evacuation, warm up with nitrogen flushing, heat treating with partial pressure, nitrogen gas is let in and cooling down to 80 degrees C. But your right it looks like some kind of grease or oil. But why is it only in one furnace?

Are the discoloured parts in any particular part of the oven? The back, or near the door?

When you clean and polish the coloured components, and re-heat treat them, do they change colour again?

This looks a bit like contaminated compressed air. Check your filters and nozzles.

Yes, we mostly see it on those parts that faces downwards in the left back side of the basket / oven. It looks as if air has brushed over them in this area. We havent tried to reheat the parts. I found a little leak by a valve on the N2 tube. I fixed it and will now try to run a new charge.

Was there any part repair or replacements done on the oven. particularly the heating elements.

Yes, we did a maintanence before time where the oven was claened, but this didnt help. Afterwards I replaced all the heat elements in the bottom (6 pcs) this event out the Amps on the tree phases, so they are almost the same. But it didnt do anything for the discolouration.

I was just getting at if a replacement part is doing this.

If I recall correctly the problem showed up without us doing anything. But I cant say it 100% sure. But what could cause these dark blue coloures????

I recall a post that Milo did on the blue colorizations. It was a post recently in say the past (2) months.

He may have an insight on this.

We once had a problem with an analyzer that we use to measure product quality, and after several days of trouble shooting we changed our nitrogen sample gas.. the problem vanished.. we had been supplied with a N2 gas that was less than 99,5% n2.. thus it had a high content of o2, which was causing havoc with our analyzer.. check to see if this is not your problem also.

Ok. We will analyze the qaulity of our N2 next week. But the two furnaces are connected to the same tank. We only have problems with one of the furnace.

How are you leak checking? This really looks like some contaminent is getting in.

I tryed leak detecting with Helium but could'nt find any leaks at all. The leakrate after a burnout (almost no outgassing) was under 1*10-3 mBar for 1/2 hour (can't measure under this pressure) which is normally OK. I am now trying a hot leakrate test by 1090 C.

I too have lot of hands-on experience with High Vacuum(semiconductor/ discrete components/ Lamps manufacturing - sputtering, CVD, etc). This due due to contamination as many have identified. Some tips that may be helpful:

Clean in inside chamber thoroughly.

All persons working on this must use hand gloves.

The equipment is in clean dry air-conditioned room.

When you have closed the chamber and started roughing ensure that vacuum jar is a bit hot. This will ensure that during high vacuum no ppm level particles left inside.

The chamber should clod just before you are going in put N2.

(hope your N2 purity is as per specs.)

There are many inexpensive methods to detect ppm level leaks.

All the best!

The two furnaces are placed in a production area, so it is not clean and not airconditioned. We always atempt to keep the furnace as warm as possible under unloading and loading normally around 80 C. Quickly closing the door again and starting a new program. How do you detect ppm level leaks? I use a leakdetection spray and I found a little leak around a tube with N2 gas. Can this cause these blue coloures??

Yes, leak at N2 pipe can cause contamination. The dark thing is oxidation. Leaks in high vacuum technologies are two types. Gross leak and Fine leak. If you have both leaks then you need to stop gross leak first. The best way to identify presence of gross leak is drawing pump down time graph. You should keep records when everything just fine so that, when there is a problem you can just identify in time and rectify. If you are sure that there is no gross leak then you can spray any liquid (ex. Acetone/ IPA) that expands several times thus dropping high vacuum. But this will be for a second or so. Hence you have to keep watching high vacuum gauge when going to spray for fine leak detection. All the best.

OK. I read about N2 leaks on www.airproducts.com. So called retrodiffusion causes impurities too travel from the surrounding air into a high pressure line. Maybe this was our problem.

You could check if combustion fumes are equal also. Please also check if position where the pieces are placed, can be influenced by inner currents of fumes at any time of the proces till 80ºC. Are both furnaces the same shape? About the repairing material, was it the same as the one before?

How do I check these fumes? The basket is placed on carbon trays with ceramic rollers about 10 cm above the heat elements, wich I also have replaced with new ones. Both furnaces are the same shape, yes. I fixed a leak in the N2 connection but I have not seen the last parts that came out of the oven. I will see it monday morning. Hopefully this was the problem. Thanks for all the help so far.

I believe that Javier (and someone else before him) was referring to a gas-fired furnace.

Since you mentioned replacing the elements, I assume that this is an electrically heated furnace. (I've only seen a few Ibsens, and they were all electric). In that case there will be no fumes, except a small amount of outgassing the first time the elements are used.

OK. Anyway I think I've solved the problem. I repared a leak in the N2 line and now the parts are shiny again. Hope they stay this way. Thanks for all the answeres guys.

Maybe the problem rooted from the old Ipsen VFC 324, whose temperature was not consistent when heating.