Trying to understand the best way to bend and heat treat a flat spring...

[mxv]

Hi All,

I could really use some help figuring out how to properly call out a flat spring I am designing. The spring is basically a 3" long x .125" thick strip with a relief section in the center that is about .0625" thick there and bent at about 15 degrees. I want to make this part out of carbon or stainless steel, and machine it flat then bend it to the desired shape, then heat treat it so it has a spring temper. the idea is this part will act as a spring, to get a better idea of what i am talking about, here is a link to a frame lock knife that has the kind of spring i am talking about built into one side of the frame:

https://www.bladehq.com/item--Kizer-Caligine-Frame-Lock-Knife--30394

My question is, if I heat treat it after plastically deforming it to the desired shape, will it relax back into shape or be more prone to fatigue? I was having a tough time finding information online that made me feel 100% sure it would work. Also is there anything special I need to call out in my manufacturing drawing besides "bend as shown in drawing then heat treat to (specific hardness or tensile strength)"?

Thank you in advance!

 

[EdStainless]

Most flat springs are formed after they are heat treated, and the forming is followed by a low temp stress relief.
The stiffness of the spring is controlled by the material (all steels and stainless will be nearly the same) and it's thickness.
The strength of the material will determine how far you can bend it before it doesn't spring back and what it fatigue life will be.
Why would you want the center thinner? Do you need the ends thicker for some mechanical reason? It would make more sense to use a piece of 0.062" flat stock.
You could use a stainless. If you use austenitic stainless (200 or 300) it would be cold rolled to get strength. The spec is ASTM A666. You could also use a PH stainless that you age harden such as 17-4PH, and use one of the lower aging temperatures for higher strength.
For steels it is common to use high carbon grades such as 1080. You would quench and temper the steel and then form it to shape.

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P.E. Metallurgy

 

[mxv]

Thanks for your replay EdStainless!

I the reason for making it one part instead of a outer frame with flat stock underneath for the spring was to reduce part count and cutting operations. I figured since the one part already had to be machined I might as well make it the spring as well. The main frame was supposed to be thick for aesthetics and to keep the product from being flimsy. The thin center is to make it easier to compress the spring. Otherwise, I don't think a person would have enough hand strength to easily compress the spring.

The thing I wasn't sure about with the fatigue is what localized properties I would get from bending forming the metal in the heat treated state. I imagine the forming has to work harden it some and make it more brittle. Sounds like you're saying the stress relief after would reduce the effects of the localized work hardening?

 

[swall]

Don't monkey around machining stock to the thickness needed. Just get some flat spring wire and go from there. Form the spring, give it a low temperature stress relief (like 450F or so)and you're done.

 

[Compositepro]

In my opinion, a part with the shape and function of your would best be made by stamping and forging.