Nitronic 50 HS stress relief? 2

[Dyskolos]

Hello,

The company I work for is manufacturing (on contract; we do not own the design) several components from Nitronic 50 HS (120 ksi yield). To ease our machining operations, it has been proposed that a stress relief (900F, 1 hr/inch of thickness) be performed partway through the machining. I don't have any experience with stress-relieving heavily cold-worked materials, so I am concerned about the following points, based on my current (admittedly limited) understanding of metallurgy.

Potential concerns:
1. Potential loss of yield/tensile strength due to the stress relief
2. Sensitization from carbide precipitation

Can anyone provide some guidance as to how valid these concerns are, or point me towards some reference material to digest? The application is for the Oil/Gas industry, if that helps.

Thank you

 

[EdStainless]

If you stress relive you will change the properties.
I am fairly sure that the spec does not allow any thermal treatment after the material has been cold worked to the correct properties.
If this is 120ksi min Yield then it is likely 140-160ksi UTS.
What trouble are you having?

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P.E. Metallurgy, Plymouth Tube

 

[Dyskolos]

Ed,
Thank you for confirming my suspicion that this is likely not an acceptable practice, at least not without prior customer approval.
You are correct; the spec does not specifically allow for stress-relief. It does not explicitly prohibit it, but I figured there was a reason for the lack of stress-relief information available for this material, either in the spec or online.

Is the sensitization an issue as well?

To answer your other questions:
Yes, this is 140 ksi min UTS.
Issues are with distortion during/after machining' likely due to asymmetrically relieving internal stresses. The parts are long and thin, and tend to look like they're practicing their cursive after a long milling operation. We are able to reduce the distortion to an extent by modifying our machining methods, and can straighten them if they get too bad, but it is time-consuming to do so.

 

[MagBen]

If not explicitly prohibited, you may go ahead to do stress relief annealing, but i would use a bit lower temp, say 850F. This alloy has a high work-hardening rate, it needs <15% cold work to get 120ksi yield, the material is not so heavily cold worked. Stress relief at relatively low temp can decrease the strength a bit, but not too much, probably less than 5 points.

As for sensitization, i donot think 850F is high enough to have a noticeable effect, especially given that this alloy has Nb that will tie up with carbon.

 

[EdStainless]

We have used long soaks at 800F (~1hr) for stress relief. We could see the charge in properties but it wasn't much.
Could you clamp the finished parts (either to each other or to a fixture) and SR after machining?

The tubing spec for similar alloys says that properties shall be achieved through cold work only.

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P.E. Metallurgy, Plymouth Tube

 

[shu60]

Just joined today but found this thread of interest. Hope it's not too late but this info could be helpful to others down the road.
Nitronic 50 (N50), has a machinability index that falls between 316 and Nickel base alloys. It does take special care to minimize distortion and does work harden rapidly. Deep cuts and slow feeds are suggested but the added cold/warm worked structure can make it even more challenging.
Low temperature stress relieving may be helpful. I found data on cold reduced sheet stock for different stress relief temps:

15% cold rolled, no stress relief-146.5 ksi YS: 900F stress relief-156.6 ksi YS: 1250F stress relief-152.0 ksi YS
No stress relief time stated but probably 1 or 2 hours.
I'm sure "peak" residual machining stresses are reduced some, but there appears to be secondary hardening occurring even after 1250F stress relief.
Re sensitization: This alloy is especially resistant due to its high Cr, low C and stabilizing elements Cb + V.
If you had limited success at the lower temps, perhaps 1250F could be evaluated.