Stress Relieving of 304L Stainless brazed with OFHC Copper

[nrummel]

I'm doing some research into Stress relieving of a stainless steel (304L) part that has been vacuum brazed to OFHC copper. My issue is finding the correct constraints (eg... Temp,Heat time,Cool time) in which the stainless can be stress relieved without softening the copper. If anyone has some info on the subject to offer up it would be much appreciated.

 

[metengr]

Why would you require a stress relief after vacuum brazing?

 

[nrummel]

The part that i'm working with has issues with its stainless base becoming warped while in service. One of the things we are looking into is the possibility that there is stress being induced by the machining done post braze and if that's the case they may be causing the issues that we are seeing.

 

[metengr]

What is the condition of OFHC copper? I would think it has been supplied as annealed condition. If this is the case, I would agree with your assessment that post braze machining is causing unwanted residual stresses.

Stress relief after machining could result in similar distortion so you need to review your machining process to reduce residual stresses.

 

[3DDave]

Is it possible to use induction heating to limit the volume of material being treated? Apparently 304L doesn't have a phase change so it's not clear why the distortion is happening after delivery.

http://www.bssa.org.uk/topics.php?article=76

 

[EdStainless]

Or the stress could be from non-uniform cooling from the brazing process.
You could re-heat the entire unit to a temp below the brazing temp and then gently cool it.
However you may be sensitizing the SS in the process.

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P.E. Metallurgy, Plymouth Tube

 

[nrummel]

That's Exactly what I'm looking to do Ed. I know that the temperature needs to be below 200 degrees C(as to avoid softening the copper) but I'm looking more for advice on how long to heat the material and how long to cool, to remove as much of the residual stress as possible

 

[EdStainless]

It isn't softening the Cu that you need to worry about since it is already soft.
You need to worry about re-melting the braze alloy.
What is the solidus temp of your braze?

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P.E. Metallurgy, Plymouth Tube

 

[tbuelna]

AMS 2759/11 recommends a stress relief temp of 1650degF for 300 series cres, which is well above your 200degC limit. Have you considered stress relieving the cres parts after performing as much machining as practical and prior to furnace brazing? A light finish machining operation on the critical surfaces of the cres part after brazing should not produce residual stresses.

There is also the CTE mismatch between cres and copper to consider. Might look at modifying the joint design to minimize the residual stress caused by the CTE difference at the brazed joint interface.