Can a sheet metal 'pre-temper' fix warping during quench?

[twistedneck]

I have done a lot of forming on a sheet metal part, and its ready for heat treat. I need to minimize warping during the quench and temper.

To discover and fix potential warping, ideally I'd normalize and straighten the warm part several times before final hardening heat treat. Well, you can't normalize sheet metal w/o scaling / decarb issues, so i thought of this..

Stress relieve the part first by tempring just above Ms temp (710F for 4130) for a couple of hours, then look for warping as the built up stresses relax. It wont cause the bad scale formation, and it could give me a preview of warping. I could then make the necessary straightening while it cools from temp, then re-stress relieve at 725F again.. repeat until i have no warping.

What are some other ways to stop warping? I use a molten salt quench and I do a version of Austempering to avoid the warping from Martensite.

 

[swall]

Very interesting proposition. However, it would take several iterations at least, and you probably would still get some warpage after quench just due to the residual stress differences that would arise from martensite forming in different areas at different points in the quench. But, if you are really doing austempering (not clear what you meant by "a version of austempering") you should ,at least in theory, produce a distortion free part if you go into the process with all of the forming stresses releived.

 

[twistedneck]

swall, I'm going to try it tonight! One hour soak at 750F, then i can see what's going on.

A few threads ago we discussed the 'upquenching' method of austemper.. to introduce a duplex structure of martensite.

By dipping below Ms a bit, then coming back up above Ms and holding in molten salt i get the benefits of even transformation and the huge toughness increases, Kic increases, crack propagation rate slows, (and yield srenght increaes via Hall Petch) that a duplex structure of Martensite and lower bainite can give!!