Residual Stress in a Nitronic 60 Forging

[matkenmd]

I believe I may have a residual stress problem with a forging which becomes apparent only after stress is applied to it and it yields more than anticipated. The material is Nitronic 60 (UNS S21800). Standard practice after forging is solution anneal at 1850°F for a few hours then rapid quench.

I have a couple other theories too, but was hoping I could get some guidance on measuring and/or eliminating the stress without redesigning the part.

Can I heat to part below it's solutionizing temperature for a longer duration and let it furnace cool? Can I put the part on a vibration table used for welding stress relief?

Any clues would be appreciated.

 

[metengr]

matkenmd;

I believe I may have a residual stress problem with a forging which becomes apparent only after stress is applied to it and it yields more than anticipated.

How did you confirm that it yields more than anticipated? Did you conduct actual tensile testing? Strain gage?

 

[matkenmd]

Actual load testing on the part. Prior to and afterward it spent several hours on the CMM. The part is yielding.

Can Nitronic 60 store residual stress? That's question 1. Question 2 is how can I get rid of it?

 

[metengr]

You mention the standard solution treatment followed by quenching, which is typical for Nitronic 60 stainless steel. What exactly was done to the forging or how was it handled after it was received? Is there cold work or mechanical straightening performed prior to final fabrication of the part? How was the forging supported during shipment?

To answer your questions;

1) Yes, residual stress can be introduced in a material after solution treatment by any of the methods I listed above.

2).You can remove the residual stress by thermal treatment - lower temperature stress relief (800 deg F) based on 1 hour per inch of thickness) or during subsequent machining steps as material is removed, residual stress is altered.

 

[matkenmd]

After receiving the part we started cutting our finished geometry. We didn't attempt to work harden by strain or machining stress. The parts are domes of about 18" round so support or shipping isn't a likely source of trouble.

 

[metengr]

matkenmd;
I would conduct a thermal stress relief (as mentioned above) on several forgings, prior to machining, and see if this solves your problem.

 

[HPAlloys]

Have you tried using a 1950F anneal instead? I do not think the 1850 is giving your the solution anneal you are looking for.