work hardening stainless steel

[darryl]

I'm making a mechanism for which I need a sharp pin to stand upright in a dimple. It has to be free to tilt for maybe 20 degrees in any direction. I don't want to give details of the mechanism, but the question is- if I form a dimple into stainless steel sheet using a properly ground tool, will the stainless work harden? I've done a test with the punch, backing up the stainless on some aluminum, and I can get the dimple shape I want quite easily. The aluminum gives about the right resistance for the dimple to come out the right size.

If I keep whacking the punch and rotating it each time, the shallow cone shape formed in the stainless seems to get smoother, which is what I want. I'm wondering how much harder will the steel get if I keep on smacking the punch- say 50 times or something- or is this not going to impart any significant hardness? Harder is better in this case, for wear resistance and low friction.

I'm thinking to use a sewing needle for the pin because it's already got a smooth, sharp point, and is pretty hard already. I need conduction through the pin, otherwise I could heat form a dimple in a small piece of glass, using the same punch. I've played with this already, and I can get a decent 'glass bearing', but of course it's non-conductive.

[PixMan]

Any of the austenitic (300 series) stainless steels will work harden, EXCEPT 303. It does take more time without heat, but the effect is still there.

It's virtually impossible to predict just how much harder it will get with so many variables. You should just look for the hardness to be gained in a more traditional way so you have repeatability of process. I'd suggest using a precipitation hardening stainless such as 17-4 and heat treat it to the hardness you need. It sounds like you need conductivity, but how would magnetism affect the piece?

[tdmidget]

The "hardening" in work hardening is not in any way similar to heat treatment. It involves properties that affect machinability more than wear resistance. Repeated punching as you described will have very little effect compared to, say, an end mill dwelling.
+ 1 for the 17-4. Hardens rather nicely and easily.

[Evan]

Stainless steel is a poor choice for any sort of sliding bearing surface. It has a very strong tendency to gall against other surfaces even when hardened. A point contact bearing may seem like a low load but when the contact area is considered it can really be very high in terms of PSI.

Consider using a piece of tape measure spring and making a dimple in that. You can anneal it by heating with a torch and slowly air cooling. Then shape the part, heat again and water quench. The bearing may then be attached to the structural surface by whatever means is most suitable.

[darryl]

That's a good idea, Evan. In fact it would be quite easy to just locally heat the metal and punch it hot. It would be cooled so fast as well that the forming and re-hardening would be done in just a few seconds.

I have lots of tape measure spring, and judging from my experiments with arc-depositing copper, then subsequently soldering to it, it should be no problem to make an electrical connection to it.