Effects of Case Hardening on Threads

[Crashmaster]

Was wondering if anyone could give me an idea what the effects of case hardening will have on a roll form taped hole (10-32). Can I expect a significant increase in strength? Are the threads more or less likely to chip or strip? Thanks guys.

[Bent]

We make a lot of part with M12 threads in and they get case hardened. After hardening the hole will easy strip a bolt of the thread. IMO you get "a significant increase in strength"

Bent

[cam1]

Hmmm:It's been a few years since metallurgy class....
hardening is a process used to prevent a surface from being indented or penetrated. It's accomplished by increasing the carbon content (through precipitation), and then heat treating and quenching, followed by tempering to remove residual stresses. The observation I would make is that the purpose of the process would be to prevent the threads from damage, on an assembly that is taken apart and rebuilt often. As far as increasing the yield point of the material (making it tougher), you'd want to temper the material after the hardening.

regards

[Geof]

Are you sure the process will be Case Hardening not through hardening?

Are you dealing with a low carbon steel or a medium to high carbon steel?

Here is an excerpt from Wikipedia about Case Hardening:


Case hardening involved packing the iron in a substance high in carbon, and heating it in that environment to encourage carbon to migrate into the surface of the iron. This formed a thin layer of higher carbon steel on the surface, with the carbon content gradually decreasing further from the surface. The resulting product had much of the toughness of the softer iron core, with the hardness and wear resistance of the outer steel.


Here is the link for the full article:

http://en.wikipedia.org/wiki/Case_hardening

This link is a very good, succinct description explaining the differences betrween Case Hardening and through hardening.

http://www.precisionheattreat.com/difference.html


I do not see how it could be possible to case harden an internal thread.

[HuFlungDung]

I think the method my heat treater uses is gas carburizing which creates a nice unhealthy carbon monoxide rich atmosphere in the soaking furnace and this will permit the whole part to absorb carbon. So it would be a circulation issue, would the gas easily pass through the hole, or would it not? The depth of the case is a function of soak time at red heat, but if the CO cannot get to the bottom of the hole, not much will happen for case depth along the threads.

10-32 is quite a fine thread. The results of successful carburization would almost make those threads "too-high carbon" and they might be fragile due to this. On the other hand, if they are at all slow on getting the part from the carburizing furnace into the quench, the surface will begin to decarburize and this helps counteract the over hardening effect. I can imagine the balance is very difficult to achieve on features this delicate, so a through-hardening steel would give better control in heat treat.

[RICHARD ZASTROW]

A roll-formed thread is already stronger than a similar size/pitch cut thread. The "point" at the tip of the thread will act as a heatsink and probably show a bit of damage if red hot temps reach into the hole. Also, consider using an oversized tap to counteract hole shrinkage. I'd make a trial piece to see what will happen to your thread.

Dick Z

[ErnieD]

From what i remember case hardened or carburized threads are bad news, We always drew back the od threads in a lead bath. Depending on the application we would carburize and slow cool the part, drill and tap all the holes and then harden and draw. This would leave the threads soft. You could also selectively copper plate the threads and the carbon will not penetrate. This can be done with copper sulphate. Threads that are case hardened, especially fine threads will sometimes be hardened through and will be subject to breakage. Also, threads that are to be treated this way have to have an adjustment for size as the surface gets larger. For tapped holes use oversize taps made for the purpose.
For OD threads cut slightly under size. These sizes have to be developed thru experimentation. I have been away from this sort of thing for many years. Had you posed this question to me 25 years ago I could have given you definitive answers for different sizes.

ErnieD

[Crashmaster]

Thanks for all the responses. Just to lay it all out there, we are making custom parts that has to holes taped out to 10-32. Our customer was having problems with the threads stripping, so we switched to roll form taps. That seemed to somewhat help but they are still having issues. We are using a mild steel for the part and switching materials is not an option. So, any ideas?

[Geof]

Longer thread engagement. In other words use longer bolts.

Or use larger bolts.

If threads are stripping and all tolerances are being met, something is under-designed for what it is being asked to do.

[PixMan]

Have you really examined the percentage of thread that your thread roll tap is giving you, based upon the hole size? Most charts will default to 74% thread for cut taps, but only 65% for roll form taps.

If you are using a high-quality tap (Balax Thredfloer, for instance), you should be able to use a drill a tiny bit smaller and get more thread height. Be careful, you can't go too small or you'll have a lot of broken taps. Be sure to use a high-lubricity tapping fluid if you go tighter, it helps tremendously.

You should be checking the size of your pre-tap holes using gage pins to be sure of what the before & after is.

Any chance of thread milling? I've done that on internal threads in 6Al-4V Titanium alloy down to an M1.6 x 0.35 thread. This method isn't necessarily as strong as a "forged" thread done with a form tap, but you do get the maximum thread profile height.