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Case hardening a part with tapped holes.

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IRT18/12/2020 23:09:23
147 forum posts
39 photos

I have made a part that has some M5 tapped holes.

I now need to case harden it. Something I have never done before, but I see a few threads offering advice on here, and plenty of youtube videos.

My question is, what do I need to do about the threads? Is there a potential problem of them becoming brittle and cracking in use?

Simon Collier19/12/2020 00:55:59
454 forum posts
63 photos

You could experiment with a bit of scrap with a couple of tapped holes. Case harden, put in vice and torque up M5 screws tight. Or, you could put screws/grub screws in the holes before treatment so threads are protected from the hardening compound. This would also keep the threads clean.

Jeff Dayman19/12/2020 01:04:44
2221 forum posts
47 photos

I've done as Simon said several time, putting some sacrificial screws in the holes. The threads will be protected from getting plugged up and also from the heat of the hardening, especially if using an A-O torch.

Neil Lickfold19/12/2020 02:57:20
836 forum posts
166 photos

Drill and Tap the holes after the pieces are carburised, but before being hardened.


David George 119/12/2020 07:27:59
1808 forum posts
503 photos

Copper is used to protect parts from case hardening. Screw a short screw made of copper, with a small head, in to the top of the thread before carburising.


not done it yet19/12/2020 07:58:28
6733 forum posts
20 photos

Clearly not applicable in this case - as you have already made your part in mild/low-carbon steel, but I doubt it costs any/much different, in most cases, to make the part in a hardenable steel and heat treat afterwards?

Thoughts from the experts appreciated.🙂

Ramon Wilson19/12/2020 08:22:19
1283 forum posts
367 photos

Sacrificial screws in the holes is one way and certainly would be the method I would use, Neil's comment is also valid but the part will need a good cleaning before working on.

All that's required is to prevent the case hardening compound coming into contact with those areas required unhardened


JohnF19/12/2020 09:32:44
1146 forum posts
189 photos

IRT -- A little more information, what size is the component, What's it for? How do you propose to do the case hardening, with just a torch and some kasenit type compound or will you use a muffle furnace and pack hardening?

In either case as has been said you can insert a screw but its a good idea to lubricate the thread with Black Lead - the stuff used on fire grates or just fill the hole with clay. Clay will harden but can be drilled out later and the thread cleaned up. Difficult to do in a hole but copper plating any area you want left soft is generally used to prevent carburisation.


Martin Kyte19/12/2020 11:08:59
2721 forum posts
48 photos
Posted by Neil Lickfold on 19/12/2020 02:57:20:

Drill and Tap the holes after the pieces are carburised, but before being hardened.


As Neil has said and seemed to be ignored if you drill and tap after carburising you end up with uncarburised threads which will not become hard when heat treated.

regards Martin

IRT19/12/2020 11:28:16
147 forum posts
39 photos

Some good advice as always- thanks.

I have never tried case hardening before. I was thinking I would heat it up and quench in one go.

I didn't think of splitting the process into 2 and tapping before hardening.

Unfortunately the holes are now tapped.

The part I am making is a knurling tool mounting bracket. Machined out of a 3"x 1.5" x 1" bar

The knurling tool is one I have seen discussed on here before; a clamp type knurl by someone called Tom.

Videos of him making it can be found by searching youtube forTom's technigues.

He even has a video showing the hardening process, and he does not seem to worry about the threads.

I will be using propane to heat the parts. Probably in a forge. I suspect I will pack the bits in a tin.

As for the compound, I have not yet decided if I will buy a tub of something, or make my own by following a recipe from someone like Clickspring.

The suggestion to practice first will be followed - thanks.

I think I need to find some copper screws too.


Rik Shaw19/12/2020 11:37:12
1480 forum posts
398 photos

The bloke in the heat treatment plant at the last place I earnt a crust used to stuff fire clay in the tapped holes to stop the threads from hardening.


Bill Davies 219/12/2020 12:15:53
277 forum posts
11 photos

As Rik says. We used fire clay to seal the cast iron pots with parts to be pack carburised. The carbon used looked like the school charcoal sticks for drawing, broken into short lengths. The parts were in the furnaces for hours, so I hope IRT appreciates that the depth of added carbon depends on the length of time it is red hot.

An alternative to pack carburising would be to use Kasenit or similar to give a thin hard surface.


Mike Hurley19/12/2020 12:20:30
305 forum posts
87 photos

Can you still get Kasenit - I thought they had stopped making it?

Bill Davies 219/12/2020 22:01:24
277 forum posts
11 photos

Mike, it appears not:

Kasenit - US alternative is Cherry Red

There has been discussion here on the ME website before, such as

Making your own case hardening compound

And a UK supplier:

Knighton Tools - Beta No. 1

Is it out of beta test yet?

I haven't used any of the substitutes, I have used Kasenit long ago during my apprenticeship, and on occasions subsequently, and have a tin knocking around somewhere but who knows where?

I expect others will be able to discuss the virtues of additives to the base material charcoal, such as sugar and/or various cyanide salts, with a better understanding than I have. In earlier times, leather, horse hoof and other organic materials were used to provide carbon, nitrogen and other elements to diffuse into the material.

Clickspring on Youtube provides some interesting examples:

Clickspring - the first hardened steel

I have accidently flame carburised a pair of tongs made in a gas/air blowtorch, which when I attempted to 'tap' one of the jaws into alignment, it snapped off cleanly with the fine crystalline fracture one would recognise from a broken file. So a gas torch alone may provide enough carbon for some applications.


HOWARDT19/12/2020 22:28:32
900 forum posts
39 photos

Rather than finish the part before hardening, carburise then counterbore the surface where the holes will go to remove the carbon, then harden, finally drill and tap.

John MC20/12/2020 07:12:22
373 forum posts
44 photos

The way I have tackled jobs that require case hardening is to have the part cased, then machine the screw threads and any other feature that do not need hardening, then heat treat. After hardening, the hard surfaces can be machined to size if necessary.

Don't forget that this process can cause the part to "grow" a very small amount.


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