Distortion on 440C heat treat for small parts

[loktinl]

Hello all,

I am in the process of designing a custom collet to grip thin wires for a biomedical engineering project. These wires are around 1/16" thick and the collet is around 1.5"long and 3/8" thick at the largest part. What I am wondering is whether there is any heat treating process that can bring this 440C collet to ~58HRC without distorting the through hole diameter by more than 0.0005". I am assuming this would be dependent on geometry, mill specs, tempering procedure, etc. If there isn't, I guess I'm going to have to resort to wire edm to get the hole to size afterwards. There is no way to grind such a small hole correct?

FYI, the exact tolerance of the through hole has not been set yet. Of course, it would be moot to tolerancing this hole to be +/- 0.0001" if the wires it accepts vary by 0.001". I'm just kind of curious whether drilling the hole before heat treating would be acceptable or do I need to get the hole wired. Moreover, Is grinding necessary for a part this size? What range of distortions can I expect?

I merely have novice understanding to the lore of heat treating, so if there are some old timers who could chime in, I'd greatly appreciate it.

Thank you,
loktinl

 

[redpicker]

Not wanting to admit to being an old-timer, I am hesitant on chiming in...

Distortion is one of those things that happens, or doesn't, depending. Actually, it always happens, but when is isn't so bad that the material is useable after HT, nobody cares and when it is so bad that it cannot be used, it has to be some else's fault. In short, try it and see the distortion you get with your standard HT is acceptable, if it is, you are golden. If it isn't, you need to try something else. How many of these do you have to make?

Something else may be a vacuum HT with a Argon/Nitrogen quench. More expensive that your standard HT, probably, but probably cheaper than trying to grind to size after HT.

Another solution is to make them from HT material and have them nitrided. This will only give high hardenss on the surface, but if you need the high hardness only for wear resistance, this may work.

rp

 

[EdStainless]

Since 440C is really just a high Cr tool steel the severity and uniformity of the quench are large factors in the distortion. It may help to rough machine the parts and then re-anneal the parts (with special care to get uniform cooling), then final machine and heat treat.
We grind the ID of draw dies with smaller holes than that, find someone that makes wire drawing dies and they can finish grind the hole.

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Plymouth Tube

 

[Maui]

Or you could just buy a set of jeweler's / watchmakers collets and build a set of adapters for them to suit your application. This would be a much easier path to follow if it is practical. Levin, Boley, Moseley, etc. are some of the better known brands of watchmakers collets.

Maui

http://www.EngineeringMetallurgy.com

 

[loktinl]

Hey All,

Thanks so much for the constructive input! @ Maui, I will look into the jewelers collets. It could be a quicker solution.

@Redpicker, I am making 45 of these, at 3 different IDs ranging from 1/16" to 0.078". I'm assuming that if I notice no distortion through our in house heat treatment process (heat to 1010-1066 C in foil bag, air cool), that I can't guarantee there to be no distortion if I send it out to a legitimate heat treat shop for vacuum HT and argon/nitrogen quench, correct? My question is, do you think I could cut corners and test out the existence of distortion in house instead of sending it out to the legit HT shop to find out?

@EdStainless, thanks for the small ID grinding tip! I will look into a grind shop that can do that as well as an alternative. From the sounds of it, you probably don't recommend doing in house heat treating if we are trying to get away from grinding?

Thanks a lot again for the tips guys, can't tell you how much I appreciate all your input. And thanks for bearing with me and my lack of understanding in this topic!

loktinl

 

[tbuelna]

Every OD collet that I have seen has an arrangement of alternating radial grooves that allow the collet body sectors to move inward and apply a uniform clamping pressure on the part due to the force created by the tapered OD on the collet. The radial grooves would normally be cut after heat treatment, because if they were cut prior to heat treatment they would produce huge amounts of quench distortions.

I do not see the reason for using Rc58 440C material for a collet. The stresses in the collet are not that great. A much lower hardness cres material should work just fine, and it could be finish machined after heat treat.