Hardening clock pinions in EN8 steel

[John HaineParticipant @johnhaine32865]

The blurb on flame hardening I linked to says that the steel should only be heated to just the austenizing temperature and then immediately quenched. I wonder if the reason the pinions were brittle was that they were taken to too high a temperature for too long so the hardening extended right through rather than just the surface?

[lfoggyParticipant @lfoggy]

Posted by Mark Rand on 06/06/2022 10:30:45:

Given the amount of time involved in making the pinions, I'd be tempted to buy a small length of EN24T rather than using EN8 just because I had it.

I now use my wife's pottery kiln for tool steel hardening because it's got a good programmable PID controller and temperature measurement, but that's just because it's there and she lets me use it (if I ask very nicely).

The hardening temperature for EN24 is 830-850°C and this is just a little over the melting temperature of table salt (801°C), so one can use that as the heating medium. Doing so and moving the part directly into an oil quench will also eliminate scaling. The downside is that the fumes from the salt have a tendency to make nearby items go rusty. I used this method to harden all the gears and shafts that I made to rebuild the apron gearbox on my Hardinge HLV

Many thanks to all.

I have just ordered 1m of 10mm diameter EN24T round bar to play with. The Malcolm Wild book suggests that you don't harden pinions cut from EN24 any further once you have machined them. You are saying you can harden the steel further but I do not have accurate enough heating equipment in my shop to do that and the melted salt sounds too scary for me !

Edited By lfoggy on 06/06/2022 14:34:28

Edited By lfoggy on 06/06/2022 14:36:47

[lfoggyParticipant @lfoggy]

Posted by SillyOldDuffer on 06/06/2022 10:06:16

The part definitely claimed to benefit is the pivot; reduced wear plus low-friction after polishing.

Dave

I have now switched from pivots to small ball races, which work amazingly well and result in very low friction movements. When I was still using pivots, I used polished tungsten carbide pins glued into the ends of the arbors with anaerobic adhesive. These seem to wear really well. I recently dismantled the first clock I made with tungsten carbide pivots (made in 1997) and there was no discernible wear on the pivots at all. Having said that, 25 years is not long in horology….

[blowlampParticipant @blowlamp]

I would say it's more important to concentrate on getting a good, smooth finish on wheel & pinion teeth rather than hardening them.

The carbide pivots seem like a great idea, verified by your observation of no wear occuring after 25 years of use.

Are you using stainless steel races, maybe with shields to prevent dirt ingress?

Martin.

[Sam StonesParticipant @samstones42903]

Thank you, Roy.

As you indicate, ‘blue steel’ (already heat-treated to the desired hardness, toughness, and ready for further shaping), is sold in small diameters. I seem to recall reading nevertheless, that slightly larger diameters (e.g. <10mm) of pre-toughed steel with very similar elemental content and performance characteristics being available for related use. However, although I can now find no such evidence while eager for a speedy exoneration, I feel sure someone (MichaelG perhaps?) will delve into this.

Having a longstanding preference for silver steel, the difficulty I have with heat-treating steel is escaping from the simple image of carbon atoms ‘drifting’ or ‘racing’ out of solution during quenching. Couple this image with something to do with the placement of carbon in the atomic structure of ‘body-centred cubed’ and its return to ‘face-centred cubed’, nicely disclosed at the ‘just visible’ point of recoalescence*, and you’ll get where I’m coming from. I’m again sure someone will come to my rescue as I bury myself even deeper.

*Only observed in a darkened room just before the red glow of the heated steel (cooling in air), disappears completely.

Admitting to occasionally confusing pivots with pinions through paying less attention, apart from its surface finish/quality (and any ability to improve same) the idea of tungsten carbide for pivots makes sense.

This latter variation may tend to aggravate the desires of professional clock repairers (and others), should the carbide pivot break in situ. Would they have to drill from the back or replace the whole item?

Some professional clock repairers also dislike the use of glue of which I’m guilty. At the time of need, Araldite was so convenient and, I imagine, so is Loctite.

Minimising friction/corrosion beyond that of steel, the availability of ceramic ball bearings offers a good alternative.

Sam

From a very chilly Melbourne

[roy entwistleParticipant @royentwistle24699]

Sam I don't harden pivots or pinions. The oldest home made Long Case that I have ( @ 55 years ) shows no signs of wear except some slight marking on the pallets. I do burnish pivots and pivot holes. And I.ve no objection to using Locktite. I feel sure some of the old masters would have used it if it had been available.

Roy

Edited By roy entwistle on 07/06/2022 09:37:14

[Anonymous]

In theory EN8 comes in a wide range of variants. I have two types in stock, EN8 (080A40) and EN8DM (212A42). The first is straight EN8. The second is an amalgam of D (tweaked composition for improved flame and induction hardening) and M (freecutting). The D variant has a slightly higher manganese content and possibly some molybdenum. When flame or induction hardening it should be possible to achieve a hardness of 50-55Rc. The M variant contains a small amount of sulphur, and in one datasheet I found a small amount of lead.

Both my stockl sizes are around 1" diameter. To clean up the stock I took a 50 thou DOC, 4 thou/rev feed at 1200rpm. The EN8DM felt smoother than the EN8. This was borne ouit by measured Ra values at three points (in microns):

EN8: 3.62 3.74 3.35

EN8DM: 2.55 2.73 2.64

I have a set of hardness files:

I assume that both bars were supplied in the annealed state. In both cases the 40Rc file easily cut the material, so the hardness as supplied is somewhat less than 40Rc.

Each sample was heated for an hour in an electric furnace at 830°C and then quenched in brine. Using the files hardness was as follows:

EN8: ~45Rc

EN8DM: ~50Rc

Each sample was then tempered for an hour at 650°C and quenched in brine. Using the files the hardness in both cases was reduced to less than 40Rc.

I draw the following tentative conclusions; EN8 is not hardenable in the same way as silver steel or gauge plate. Heat treatment does improve hardness. The D version seems to give higher hardness, as expected. Both flame and induction hardening are surface hardening techniques. While EN8 cannot be hardened as would a cutting tool, it does seem possible to increase the Rc value. That should increase the wear resistance, which is an advertised advantage of using EN8.

Andrew

[Martin KyteParticipant @martinkyte99762]

Regarding use of tungsten carbide rod for pivots the advantages against turned pivots are to my mind and in order of importance.

1. Simplicity. So long as you can drill a concentric hole in the end of your arbour all that is required is to cement the rod in place.

2. The pivot will be smooth. No need to use a pivot file to remove turning marks and a subsequent burnisher to further refine the surface.

3. Very small diameter pivots can be produced.

4. The pivot is hard.

If you are really going to take advantage of this methodology then the use of endstones to control end float is desireable.

regards Martin

[lfoggyParticipant @lfoggy]

If you are really going to take advantage of this methodology then the use of endstones to control end float is desireable.

regards Martin

Yes, I ground the ends of the tungsten carbide pivots to a conical point then used 'endstones', also made of tungsten carbide, to limit movement. This prevents the shoulders of the arbours rubbing against the clock plates.

Many thanks to all for the replies. For my current project I have decided on trying EN24 steel for the pinions. Pivots will run in micro ball bearings and the pallet faces on the escapement will be tungsten carbide.

Will post pictures when the clock is done, hopefully by Christmas time….

Regards

Lukas

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