Reply To: Hardness of steel rod…

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Bob Tascione
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Thanks David!
My reason for an oil quench hardening is to try and carburize the outer layer a bit for polishing purposes. Figure if I can case harden it a little that I should be able to hit the upper Rc range for S7 and possibly beyond and come up with a better finish while retaining S7s toughness. In most cases if the base metal is .5% or above in carbon content then carburizing will have little to no effect but I think going to higher temp. and soak times can sometimes change this. My reasoning is most likely all wrong but figure it’s worth a try and I enjoy messing with things like that. I’ve done pack hardening with low carbon steels used in old timepieces with success and have enjoyed the process. Once in a while stuff actually works out but mostly end up proving myself wrong. I wish I had an oven to leave the part in to test different duration’s and temps where the effects on penetration depths (if any) could be tested when packed in the carbon environment (charcoal and ground up cow bone wrapped in stainless foil.) I’m really just after about .001 or .002 penetration and hoping this might work using a flame. Sodium Cyanide and a gas or vacuum environment works better but that’s not happening in my shop. I’d probably kill myself. There are a lot of reasons for this to fail (S7 having some element that would prevent carbon infusion for one) and little that I can see that would give good results. I know that under ‘perfect’ conditions a depth of around 1/16 inch is possible but that would be for low carbon steel (like .2% or lower carbon content) and well controlled carborizing process. But what the heck…just need a couple of .001″. Also I have to wonder why this would prove to be any better than just making the parts out of a tough low carbon steel in the first place and then case hardening it. My hope is that there ‘IS’ some resistance to infusion that prevents a penetration beyond .002 or .003. The pivots are now around .021 dia. and I would like to cut that dia. by 1/3 or so. So with a dia. of .013 to .014 and a penetration of .002 I’d be looking at a ratio of tough material to hard outer layer that may just work. With regular low carbon steels it’s difficult for me to accomplish this shallow depth using a flame and repeatability would be a big issue. If S7 repeatedly resists penetration to about the same depth then that might help to control the depth to a shallow limit. I could be fairly certain that future parts made using S7 would be within the desired tolerance since the depth of penetration would be determined more by the characteristics of S7 rather than slight variations in my heat treating process. That’s a big if. Only one way I can think of to find out considering my limited tooling resources. If the case hardening works then I can try to figure out a way to run some comparison tests of the old way against S7. If the old way holds it’s own against S7 then pretty much proved myself wrong again, but if S7 comes out ahead then it possibly solves a pesky problem I’m having with my little project.
Not much S7 stock left here and difficult getting it shipped to my location. Would like to experiment with larger diameters but limited to what’s on hand for now. Will try to do this tonight or Sat. morning if I can get some playtime! If this works I’ll post the results up here in case there’s some interest since carburizing steel can play an important role in true horological restoration work . I didn’t intend to hijack Toms thread though. :D