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Welcome the the forum Paul!
Distortion is a real challenge when dealing with heat treating springs. I like to use oil hardening steels for springs. This is steel with a carbon content of 1 to 1 1/2 percent. Oil hardening steel is good for me as it accepts a slower cooling time than water hardening steel which usually means less tendency to distort and yet can still hold a nice deep looking polish. There are also air hardening steels that are great for eliminating distortion but have proved difficult for me to get a decent polish out of such as that deep black polished look that so many of the high end watches have.
Thin steel pieces such as springs when cooled abruptly will tend to distort. For long straight or relatively straight but uneven pieces I make a sheath out of iron binding wire which is longer than the part to be hardened. I just wrap the wire around different sized watchmakers screwdriver handles to get the necessary shape depending on what size I’m after and then drop the piece into the sheath and heat the sheath directly. This keeps the flame off of the part which is really important if you don’t want it to warp (best way is to used a heat treating oven but…). The entire sheath can then be dunked when the required heat has transferred to and through the part. For parts such as the one you show in the pic I have a bunch of different sized copper boxes that I’ve made. Most are just round boxes that I made out of copper round stock with simple copper caps. The part can be heated and cooled the same way as with the iron sheath. If the part is really thin and sensitive I put iron filings into the box with the part to help distribute the heat more evenly. Another thing that works well is just wrapping a bunch of turns of the binding wire around the part and then go through the same heating and quenching process as above. This works very well if the part has some strength but is risky when working with thin springs as applying the wire can bend the part and removing the wire can break it.
It’s also important to know the characteristics of whatever metal you’re working with. Some require more heat than others to harden. So testing the metal first is a good idea before making parts from it.
Well, hope this helps Paul,