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Case Hardening

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Andrew Johnston05/12/2019 17:04:24
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Nitriding is all about forming nitrides. Nitrogen is quite soluble in iron, and iron nitrides give better corrosion resistance with modest increase in hardness. But they decompose at fairly low temperatures. Other metals such as aluminium, chromium, molybdenum, vanadium and tungsten form nitrides that are very hard and much more temperature resistant. Some of these metals are present in small amounts in nitriding steels, such as the EN4x series.

So, while nitrogen may well be absorbed during case hardening of unalloyed low carbon steel it will not do much for surface hardness.

Andrew

Michael Gilligan05/12/2019 17:52:31
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That’s interesting, thanks Andrew yes

MichaelG.

.

Incidentally: It’s too many years ago for me to remember what material was used, but I recall when I worked at Kodak we had a batch of small steel levers [camera mechanisms for the use of] which failed rather dramatically at a 90° bend ... it turned out that the nitride hardening had penetrated into micro-cracks in the bend and formed ‘textbook’ stress-raisers.

Howard Lewis05/12/2019 17:57:15
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My understanding of casehardening has always been that by heating a low carbon steel in a carbon rich environment, the carbon is absorbed into the surface of the steel. Thus, locally, the metal becomes a steel with a higher carbon content, and so capable of being heat treated to harden it.

Salt baths used for case hardening are often Cyanide based..

Nitriding is a process where the steel is heated in a Nitrogen rich environment, such as Ammonia, or a Nitrogen rich compound. This does harden the steel, and also improves its fatigue resistance; hence its use for crankshafts, and other components subjected to fluctuating loads.

Tufftriding is a salt bath version of Nitriding, and, again, is popular for Crankshafts.

What is important is remove the compound layer, which is EXTREMELY hard, but very friable.; I have seen it crumble off a crankshaft almost like grit.. If the compound layer is not removed, this will result in surface degrading in an exponential fashion, which does nothing for the shaft or the bearings in which it runs.

In all cases, the depth of case is determined by the time of exposure. Crankshafts were Nitrided for twenty or sixty hours, depending upon the use to which the engine was going to be put.

Howard

ChrisB06/12/2019 13:55:55
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Is it better to drill and tap in the part to be case hardened before or after? I would prefer not to break any taps in the hardened material, but on the other hand I don't know if case hardening the threads is wise..

SillyOldDuffer06/12/2019 14:56:44
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Posted by ChrisB on 06/12/2019 13:55:55:

Is it better to drill and tap in the part to be case hardened before or after? I would prefer not to break any taps in the hardened material, but on the other hand I don't know if case hardening the threads is wise..

Interesting question! As hardening often equals brittleness, I'd think twice about hardening threads that took a shock load. Conversely, hardening threads that are frequently done and undone would reduce wear.

Might not make much difference unless the application is critical, and home workshop stuff rarely is. As case-hardening is only a thin surface treatment, I'd be inclined to harden first and then drill and tap the soft interior as normal. Thread performance would be typical of mild-steel.

The few times I've tapped a through-hardening steel, I drilled and tapped while it was soft to reduce wear on the tap and then hardened. But I didn't think it through! Although nothing has broken so far, maybe my hardened threads aren't be as strong as they should be. Good job I don't make brakes...

Dave

IanT06/12/2019 15:36:41
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As always Chris different parts may require different approaches - dependent on the size & nature of the part, how much is to be case hardened, where the holes are located etc.

I think the key thing is that case hardening is a surface treatment and usually (in the home workshop) relatively thin. So it's different to something that has been through hardened (like high carbon steel that's been heated and quenched). It's more like a work hardened surface - you just need to get under it...

It's also possible to only case harden certain parts of the work. The 'old' way to do this was to wrap the area (not to be treated) in a slurry of fireclay and asbestos string which was allowed to completely dry before heating. There are probably modern versions of this but I've not tried any. Other approaches are to case harden the whole part and then turn down to soft material in those areas which need to be soft (which is a bit like getting under the skin of a casting). Grinding can also be used to remove 'case' of course.

Once you are under the 'case' - any drilling or tapping will be into normal (e.g. soft) material and it's then just the same as untreated material to drill or tap. For holes in small (case hardened) parts - one way to start a drill is to use a Dremel grinding point to remove the case in the required spot which can then be drilled and tapped as normal.

Regards,

IanT

JasonB06/12/2019 15:59:59
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Chris you can go through the case hardening process but leave the part to cool naturally. Then do any additional drilling and tapping before finally heating up to red heat again and quenching only then will it be hardened.

Lainchy06/12/2019 16:00:44
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On my S50 build guide, by Tubal Cain, it details case hardening of the eccentric. He details heating and carburising the eccentric, then machining, and then once its all machined, then heat and quench.

I'm guessing that simply applying the case hardening power doesn't case harden, until it's finally heated and quenched?? So, you should be ok to use the powder then tap ?

David George 107/12/2019 07:40:13
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I just thought this may be helpful to this thread.

I have done selective case hardening for jobs in batch work and have made screws and shafts which required soft areas like threads. These areas were roughed out prior to carburising, leaving metal on, and then finnished to size removing the carburised metal, then after hardening the soft area can be machined. The second way was to turn to size and then copper plate the object this dosnt allow the part to carburise, then I removed the copper to the areas that need case hardening in the normal way. The other thing we did was to screw a short copper screw into a threaded hole to prevent the carburising to affect the thread and the small area under the head of the hole like aircraft hydraulic parts which needed case hardening but not threads in tapped holes.

David

ChrisB07/12/2019 12:39:56
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I'll leave the holes and threads for after hardening as suggested, seems the most sensible thing to do.

I'm still not sure how the process works tho, is it heating to red heat and plunging into the charcoal powder? Or place the parts in charcoal and let them heat away for say 30min?

not done it yet07/12/2019 12:50:10
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Posted by ChrisB on 07/12/2019 12:39:56:

I'll leave the holes and threads for after hardening as suggested, seems the most sensible thing to do.

I'm still not sure how the process works tho, is it heating to red heat and plunging into the charcoal powder? Or place the parts in charcoal and let them heat away for say 30min?


Method(s) were given quite adequately by JohnF in the third reply to this thread. Here it is again. Take note and all will be clear.

QUOTE:

Chris, Kasenit has long gone but there are some other alternative compounds available form some of our usual suppliers if you dig around.

However it depends on what you want to harden, how big, how many, how thick a case to do you want/need ? If you want to go ahead and without getting into too much detail you need a suitable heat source that can sustain the temperature at 750 deg C for a lengthy period - a small furnace is best. A container to pack the work in, an old tin will work but a thicker walled one is better, you need to be able to seal the top while its "cooking"

Yes BBQ charcoal will work but better if you can get bone charcoal or as Clive suggests some leather charcoal or mix of both. Carbon penetration is generally assumed to be 3-5 thou per hour depending on the compound and temperature.

If your items are small and you go for the kasenit type compound you can use a large gas torch and a makeshift hearth, the case is thin but will offer a fair degree of wear resistance and protection from dings.

Hope this helps a little

John

For the kasenit process holding it at red hart for several minutes will do the job, the longer the better but 2 or 3 mins will usually suffice

UNQUOTE.

JasonB07/12/2019 13:10:08
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Really depends on what you are using Chris, as Kasenit is no longer available you will have to do what the brand you have says.

For Beta you don't do the same as kasenit so not much point in repeating it here. You heat the part and then plunge it into the powder and stir it about, repeat the heat/plunge cycle if you want a deeper case. You then reheat the part and quench, it is only when quenched from red hot that the part will take on a hardened outer layer.

Cherry Red uses the same method as Beta either overering the part in powder of plunging it into the powder.

Not tried a home brew method so can't comment on that.

Edited By JasonB on 07/12/2019 13:12:40

ChrisB08/12/2019 10:29:25
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Thanks NDIY for pointing me to John's post, good explanation. Don't know of any local shop which sells case hardening material so I'll have a look online maybe I'll find a shop which deliver overseas.

Thanks Jason for the explanation, as I don't have a kiln to heat the parts for hours I'll opt for the heat/dip/quench process.

CherryRed is in the US from what I gather, so it will have to be Beta 1. In the meantime I'll try to experiment with some charcoal and graphite powder and see if it makes any change on a piece of scrap.

Edited By ChrisB on 08/12/2019 10:29:41

JohnF08/12/2019 11:25:37
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ChrisB if you put case hardening in the forum search there are many earlier posts on this subject and if you look at the 2 oldest I gave some more info on CH methods etc

John

ChrisB08/12/2019 14:50:50
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Thanks John, read through your posts - there's good information which saved me from wasting time and energy.

I'm looking for Beta1 online but are there more than one manufacturer with the same product name? I found this from Knighton tools which costs £66/kg and this from ebay which costs £55/kg, are they the same product? Also quite expensive considering I have to add shipping costs on top.

Then I found another product Felder Aufstreuhartepulver also from the German ebay for €16/kg. But if I understand google translate correctly, it says Nitrogen hardening - is this what Andrew (above) was referring to?

Edited By ChrisB on 08/12/2019 14:51:57

Roderick Jenkins08/12/2019 17:35:30
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Try EKP supplies

**LINK**

Steviegtr04/01/2020 17:51:13
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I am sure we used to use Potash to case harden.

SillyOldDuffer04/01/2020 19:36:21
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Posted by Steviegtr on 04/01/2020 17:51:13:

I am sure we used to use Potash to case harden.

'Potash' is another of those names used loosely by various trades for different chemicals: any of several minerals containing Potassium, Potassium Oxide, Potassium Hydroxide and Potassium Carbonate. Only the Carbonate would be any good for case hardening. I doubt it was used on it's own, more likely a mixture containing Potassium Ferrocyanide and - perhaps - Potassium Cyanide.

Case Hardening is done in a multitude of different ways, sometimes just to provide a hard wearing surface, other times to produce a decorative hard-wearing surface (pretty colour for jewellery and guns), or - in the good old days - armour plate for battleships!

I guess this thread is about improving the wear resistance of mild-steel by carburising a thin surface layer. This can be done with almost any source of carbon, but it's hard to control results. Kasenit, Cherry-Red and other commercial mixes are formulated to simplify their use in an ordinary workshop. Follow the instructions and there's a good chance a semi-skilled user will get a reasonable hardened layer from simple equipment! It won't be very deep, nor is the process particularly cheap. I suspect (but don't know for sure) that all these product also harden by Nitriding as well. Kasenit seems to have been withdrawn because it contained Potassium Cyanide; deadly if misused. Ferrocyanides are safe.

Alternatively case-hardening can be done by baking mild-steel in charcoal, or by playing a Carbon Rich Acetylene flame over metal heated to just the right temperature. But I don't think the method is easy or reliable. Adding Leather or bone-meal introduces Nitrogen, which I think improves the chance of getting a good result, but it's not consistent. No-one knows how much Nitrogen will be in a particular bit of old leather, so the worker has to judge by eye and smell. Plunging red-hot mild-steel into used engine oil is said to work, but I doubt the results are trustworthy. But there's no reason why an amateur method like that shouldn't produce some improvement. Why not try it?

The grown-ups go for expensive to set-up but cheap to run and methods. They strongly favour reliable results, same hardness and colour every time! Controlled heating in an atmosphere of Butane, Propane, Carbon Monoxide, Ammonia or Acetytene or a mixture of same is good for many mass-production items. When colour and/or deeper skins are needed, steel can be soaked in molten mixtures of Barium, Sodium and Potassium Carbonates, Ferrocyanides, Cyanides, Charcoal and other chemicals. As accidentally mixing air with hydrocarbons is likely to cause an explosion, and because the fumes from these processes may be toxic, it's likely the work will be carried out in a vacuum furnace. The details depend much on the particular steel being hardened and the purpose: decorative gun parts are unlikely to be hardened in the same way as a crank-shaft.

If the goal is a vague desire to improve the wear resistance of ordinary mild-steel I don't see much harm in trying traditional recipes. As an amateur though, when looks or performance matter, I'd cough up and buy a commercial mix. Deliberately developing skills in traditional methods is also worthwhile if that's your interest and you have the time needed to learn the ropes.

Dave

Edited By SillyOldDuffer on 04/01/2020 19:36:41

Steviegtr04/01/2020 20:33:51
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Used to harden my screwdriver tips after sharpening. It was a black dry powder from memory you heated the steel & rolled the part in it. Not sure whether we chilled it after or not. I did a large screwdriver 3 times thinking I was good. On the job I was putting a No 12 woodscrew in & the end snapped off like a carrot.

not done it yet05/01/2020 01:41:59
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I doubt your screwdriver was mild steel. As such the screwdriver would likely better respond to hardening and tempering.

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