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Case Hardening

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mgj02/02/2011 02:07:07
1017 forum posts
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Well yes and no.
 
Point 1 is of course that one wants a wear resistant outer skin, so it wants to be as hard as possible. It is not there to carry a load - thesubstrate does that.
 
Metallurgucally as we know, its not (generally) possible to have something very hard, and not have it brittle.
 
So that means one has to introduce carbon into the outer layers (there are several ways), of a relatively low carbon substrate, (which in general terms will not respond to heat treatment) because basically high carbon = brittle. And then you quench which locks the outer layer in a hard state, and ones choice of substrate (often mild) provides the unbrittle load bearing material, which will take shock loads etc, and should be very resistant to the intrusion of cracks (even more so if the guy who machined it used a nose radius on his tooling.)
 
So far so good - but what if one wants to toughen the base material - ie machine it while soft, case harden, and then regain mechanical properties by toughening. It can be done by controlled heating and grain imporvement, but it is NOT tempering, and its not done at the same temperatures, and the alloys used are chosen for their toughenabilty or heat treatability, and temperatures which will not interfere with the quenched case.
 
So I ask the question again - why temper the outer case, when the whole object is to get it as hard as possible. You have the iron carbide there, you quench to lock it in the marensitic form, to provide the wear resistant properties, why is why one has case hardened - so why disperse it?
 
 
Isn't nitriding a toughening process? More than a case hardening process?
 
I also accept that there are specific instances involving highly specialised alloys, but in general terms, I think my point is correct.
 
 
 

 

 

 

Edited By mgj on 02/02/2011 02:09:12

Tendor03/02/2011 05:08:01
39 forum posts
5 photos
 
Folk might like to look here
to see the ingredient of Kasenit. It is Potassium Ferrocyanide.
 
KASENIT CO INC -- KASENIT SURFACE HARDENING COMPOUND NO.1 AND -- 
=============  Composition/Information on Ingredients  =============
> >Ingred Name:POTASSIUM FERROCYANIDE (YELLOW PRUSSIATE OF SODA)
Ian S C03/02/2011 11:14:47
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7468 forum posts
230 photos
Quoting of the can of Kasnit No., 1 surface hardening compound for case hardening iron and steel. Kasenit compounds are highly refined, non poisonous, non inflammable, and non explosive. Ian S C
Martin W03/02/2011 12:14:41
940 forum posts
30 photos
Ian
 
That confirms what the data sheet says, in the link above, that there is effectively no hazard with this stuff. Just a recommendation to use in a well ventilated workshop. Extreme use might give a sore throat or runny eyes but that's about all.
 
Cheers
 
Martin
Ian S C03/02/2011 12:26:13
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7468 forum posts
230 photos
If you are heating with LPG you are more likely to be affected by the gas, long use can cause swelling of thetissues in the lungs, and cause bronchitis like symtoms. Never had any problems with Kasnit, but then again I don't use it much, had my 1lb tin for 20yrs, and its still 1/2 full. Ian S C
Terryd03/02/2011 13:02:30
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1946 forum posts
179 photos
Hi All,
 
Wikipedia has this to say on the subject:
 
"Potassium ferrocyanide itself is only slightly toxic, although adding acid to its aqueous solution releases toxic hydrogen cyanide gas."
 
So just be careful when used near pickling acid perhaps?
 
Regards
 
Terry
Nicholas Farr03/02/2011 18:38:00
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3988 forum posts
1799 photos
Hi, a couple of quotes from my college notes.
 
Nitrogen, like oxygen, can dissolve in the molten pool and can also form a compond with some metals; this will be called a nitride.
The presence of nitrogen inccreases tensile strength but decreases the toughness.
 
Toughness, a term difficult to define with precision, but never the less important in assessing the usefullnes of a metal. It can be regarded as a combination of strength and ductility which may be indicated by the amount of work to cause failure.
 
When I was taught how to case harden a mild steel screwdriver that I made, it was necessary to temper it, because the thinness of the blade end would be hardend throughout the whole section. I have tryed to do them without tempering but on first use they always snap off, so I've always tempered them and they have been sucsessful.
 
Regards Nick.

Edited By Nicholas Farr on 03/02/2011 18:39:10

mgj05/02/2011 18:15:22
1017 forum posts
14 photos
Nick - screwdrivers, yes I can see why one would temper that, and thin sections are difficult and special cases. But in the generalitiy of things....
 
I'd make a screwderiver out of a properly heat treatable steel, and then harden and toughen properly. Just case hardened doesn' make something like that tough, so it will still bend in the thin section. The general point about case hardening is that while it makes the outer layer very hard, and wear resistant (nearly tool steel hard in many examples) it doesn't alter (in principle) the properties of the substrate.
 
Nitriding. Yes, it is a surface hardening treatment, but its generally used in the context of other heat treatments as well, hence my somewhat loose comment, for which I apologise. I was thinking more on the lines of our racing cranks which were heat treated after  machining, for strength, and then nitrided. The reason for nitriding was it retained dimension well and didn't distort.

Edited By mgj on 05/02/2011 18:25:02

Edited By mgj on 05/02/2011 18:27:44

Frank Dolman06/02/2011 00:57:36
106 forum posts

Crankshafts would surely be gas carburised, or carbo-nitrided. A much more
controllable process than dipping in Kasenit.
This changes the whole story, including the question of tempering. Please
Meyrick, tell me I'm right.
Ian S C06/02/2011 08:43:18
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7468 forum posts
230 photos
Most of us have had a go at turning car half shafts, if so you'll have found the results of niriding, often a hard layer, about 1/4" thick, you can get through it with a carbide tool. Kasnit is ok for the fairly casual use we make of it in the workshop for small items(my opinion only). Ian S C
David Clark 106/02/2011 09:22:14
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3357 forum posts
112 photos
10 articles
Hi There
How long did they leave it in to get 1/4in. thick nitriding?
I always thought this was extremely thin depth.
Did you mean 1/4 of a thou?
regards David
 
mgj06/02/2011 09:24:26
1017 forum posts
14 photos
Frank - well yes and no, depending on how you went about nitriding.
 
To be precise, yes nitrogen hardens surfaces, but it can be applied in different ways and at different temperatures, so you can get an improvement in the grain or toughness out of the process - but to be absolutley fair to Nicholas Farr , it is the temperature rather than the nitrogen that has that effect. The nitrogen migrates into the surface layers and alters the crystalline structure and natrue of the alloy.
 
Controlability - I'm not a process engineer, so I'll make a suggestion raher than go out on a limb. Controlling the depth of case in any process is not usualy a problem - (even dunking in Kasenit gives a moderately known quantity!) What made our people specify a particular process and us in DQA police it was other factors - vulnerability to surface cleaniness, ordimensional stability., and whether you want to treat the whole item, or just some of it.
 
However, once with steel, as far as I know, you start adding carbon, you are involved in controlling cooling because carbon alters the crystalline structure of a solid solution. That, generally means changes in dimension (in complex shapes and near finished items?), because some kind of "locking" of the structure is required. Depends on temperatures of course, so one might well choose the process according to the item and how it is made. Nowadays there are so many processes that one ought to be very precise about what is being done before going any further than a pretty wide generalisation, and at that point one wants to talk to specialists.
 
Kasenit, at our level gives a depth of about .003 per dunk, so a couple of dunks followed by a quench does the trick. it is VERY hard, but if you do need to be rid of it, you can eiher he-heat and air cool before machining, and then reheat and quench if need be, or at that sort of thickness get under it in one, given a decent tool.
 

Ian S C06/02/2011 10:00:05
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7468 forum posts
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david, about .250" is right, its tough rather than hard, it's got to take tortion more than anything else. Ian S C
David Clark 106/02/2011 10:09:07
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3357 forum posts
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10 articles
Hi there
The crankshafts we were inviolved with only had a very thin layer.but I beleive it was a specila alloy material.
It was used for Seagull crankshafts.We had to polish out the odd mark where the grinder had not cleaned up the surface but were told not to polish to much as the layer was very thin.
regards david
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