If you are stress relieving it to remove the stresses from the forging, you should have done it prior to machining - to avoid sensitization, you'd probably want to run it up pretty high (austenitize it and rapidly cool it through the sensitization range) but now that you've machined it, distortion during the heat treatment will throw your tolerances out.
Anyone have any thoughts on subcritical annealing without sensitizing? I've heard talk of vibratory stress relief but have nothing to go on.
Its going to be a vacuum chamber with thick walls ,that has to stay fairly stable to keep the measuring gear on the outside in line with what is inside
If I did no heattreatment whatsoever,
what do you think are the chances of it moving over a period of years ?
SMF1964
I was just thinking of roughing it then annealing it at 1100 C with a rapid cool
Is that wise
Thanks
And what is this sensitization you mentioned
If it's at ambient then there is little chance of it distorting in service.
Residual stresses resulting from working or welding will be locked in until you start machining. Then as you remove material the stresses will try to balance themselves by reditribution - hence SMF1964's comments. Similarly if you stress relieve or solution heat treat this may result in distrotion as these stresses redistribute.
Rough maching then solution heat treatment then final machining is probably going to be OK but make sure that you have enough material left to allow for any distortion that might occur. I hope that others would agree.
Sensitisation is the formation (precipitation) of complex carbides in a certain temperature range that will occur on cooling from solution heat treatment if the cooling rate isn't fast enough. Although it will occur on the way up as well, this isn;t an issue as at solution heat treatment temperature the carbides are taken into solution.
Sub critical stress relief can be carried out - we've done it at 750 and theres something like a 70% reduction in stresses if I remember rightly - but this temperature is slap bang in the sensitisation range.
cryo won't help a stainless.
If this will see service at ambiant temp I would use vibratory stess relief. Basically you shake the part and it relaxes like it would in service. But it happens in hours, not decades.
If this will be run warm, but not too hot, you might want to just give it a few thermal cycles to 50F above the service temp. The repeated cycling will help stability.
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Corrosion never sleeps, but it can be managed.
If I do no heattreatment at all
And then this vacuum chamber is baked out regularly at approx 200c
Is there a chance of some distortion over a period of time ?
My guess is yes
Im thinking it would be safe to rough machine then stress relive then finish machine
From memory 1100C then rapid cool
If you are only working at 200C I would still give it a few cycles to 250C before final machining. There will be some residual stress from the quench after anneal and cycling it at the low temp will simulate operation and let it distort before you care about it.
= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
