17-7 PH Spring Steel forming with heat treatment 3

[SABAS]

I am on a project where we are using 17-7 Condition C (Cold Rolled)
The strips are either 0.012" or 0.008" thick and ~1/4" wide
We need to form these into a radius of proximately 6" radius and 4" radius (in actuality it is not a perfect arc but this gives an idea of the curve required

We made our first jig and then heat formed these to CH900 condition with the hope that they will take on the shape of the curve. The actual curve after removing from the jig was a larger radius (almost double the intended radius). So assuming there is a certain relaxation and non conformance, we made a new jig with a significantly tighter (smaller) radius than we needed in the hopes that after the relaxation, we will be at approximately the desired radius.

Surprisingly, the curve obtained from the second tighter curve jig, relaxed to almost identical curve as the first, less curvy jig. It appears as if there is a limit to the curve that can be applied. The second jig had a radius approximately half the desired curve we want to form the spring to

I guess we can try an even tighter curve to see if that will do it, but thought I would ask others before we go through the effort time and cost to run another experiment. Suggestions or recommendations are highly appreciated

 

[mfgenggear]

what the problem is spring back. form , anneal, form, then heat treat, and reform again if necessary.

 

[EdStainless]

You are trying to make these in the CH condition? You will age after forming, right?
There is no real stress relief at the aging temp.
You probably cannot heat form, you need to cold form them, and yes it will take a lot of force to do that.
The problem is that as you heat the material is becoming stronger, so you need to form before you heat.

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P.E. Metallurgy, Plymouth Tube

 

[dgallup]

You will need to over deform in the soft condition because of spring back to get your desired shape. That will take some experimenting to get right. You may be able to feed the material into an adjustable shoe that will shape it. Take a look at this spring coiling video. Only after you have the right shape can you do the CH900 heat treatment. You should not need any other process and there is no need to hold the parts during heat treatment. We get precision 17-7 springs made this way and they can hold very tight tolerances.

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[tbuelna]

Which way are you forming the stock material? Are you winding the strip with the 1/4" width in the radial direction? Or are you winding the strip with the .008"/.012" thickness in the radial direction?

Manufacturers like Smalley routinely wind 17-7PH stock with the section width/thickness/coil diameters noted, and HT to CH900 while maintaining good dimensional control. For applications like wave springs or spiral wound retaining rings.

 

[SABAS]

Thanks To all the responders.
Cold forming such a shape is cost prohibitive at this stage in terms of cost. I had approached Smiley in the past and although their customer support is excellent, this is outside their normal process enough that they cannot do it

Here is what I am thinking now. Your comments would be appreciated

Have these strip springs in the put in the original jig, ie the jig with the precise curve we desire as an end point rather than the one with an even sharper turn to compensate for relaxation

Next ask my heat treating shop to heat treat to CH900, then while keeping the springs in the jig, proceed to anneal them and then again heat treat to CH900. The idea being to have them as annealed while partially curved so that the partial curve is the new starting point. Then since they are annealed as a starting point, have them take on the additional curve to result at the desired curve or close enough to it.

If the above process works without relaxation of the spring shape. In the future I can have these 17-7 strips made from annealed material and therefore reduce the 3 step heat treating to a single step. Since I am currently paying the minimum lot charge, I can also experiment with the over formed. Any additional comments would be appreciated

 

[EdStainless]

You can't anneal them and get CH900.
You can anneal at 1900F, cool to room temp, and then either heat treat to TH1050 or RH950.
RH950 is stronger, but requires 1750F, -100F for min of 8 hours, then 950F.
TH1050 is 1400F, cool to room temp, then 1050F.

You need to figure out how to cold form these.
TH1050 is 180ksi min UTS (185-200 typical)
RH950 is 210ksi min UTS (220-235 typical)
CH900 depends on how much prior cold work there is in the strip, but 240ksi min UTS with 250-265 is typical.
You may need to recalculate your thickness if you want to use a softer condition.

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P.E. Metallurgy, Plymouth Tube

 

[MagBen]

yes, you will need cold work for martensitic transformation to obtain condition CH900! could it be possible to cold form 90% desired shape at condition C, then aging hardening to CH900, then form to the final shape.

Anyway, this is a pain in the b**t alloy, 15-7PH grade is a little better, Customer 450 is much better alloy to work with

 

[EdStainless]

Now Ben, I love 17-7, because when you anneal it it is austenitic so it welds and forms well. It cold draws nicely as tubes also, which is a bear with 17-4 or 455. It is just a pain to HT.
I would look at rolling these to get the shape. Three small diameter rolls and a length of strip that is a little too long (to allow trimming at the end). Roll to shape and then age in your fixture.

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P.E. Metallurgy, Plymouth Tube