Sodium Nitrite for case hardening?

In the latest edition of ME (4467) there is a letter in the 'Postbag' section, entitled "Patents worth Pondering" (by Andre Rousseau) in which he mentions a German WW2 invention to case harden metals using a "simple" process using a sodium nitrite solution. Apparently the details are in a book Andre mentions but that wasn't too helpful in practice (as I don't have a copy)..

I've been thinking about case-hardening some small parts recently but currently have no Kasenit or similar, so would need to purchase them. Does anyone know any more detail about this process and whether the materials are readily available (and as safe to use as suggested)?

Regards,

IanT

My thoughts exactly... a bit like "I don't know why people don't run their cars on water, it's simple enough".

Having played with potassium nitrite (amongst other chemicals as a teenager, I'm not convinced sodium nitrite is necessarily safer than Kasenite).

Neil

A few minutes internet searching turned up the text of Harry Sevens book Hitler's Suppressed and Still Secret Weapons Science and Technology on Google books.

The relevant bit seems to be :-

claimed to produce a nitride coating up to 0.5 cm thick with a hardness of up to 1,800 Vickers is as follows. The steel is heated to 600 degrees C and quenched in 30% aluminium sulphate solution which etches the surface. When cold the steel is packed in powdered charcoal and heated to 750 and on reaching this temperature is removed from the charcoal and quenched in a 15% sodium nitrite solution at 35°C. The method is good for drills and silicon steels.

Nothing very remarkable there. Basic carbon pack case hardening without accelerant but preceded by an etching step which, presumably has a similar effect to an accelerant in increasing the speed and depth of carbon penetration. Sodium nitrite quenching is an established practice said to reduce the chances of cracking and distortion as compared to plain water whilst improving steel properties. So, in principle, no different from all the other nostrums in the carbon, additives in the quench versions. Far as I can see all variations do what is said on the tin if you do it right but the structural variation of steel is incredibly complex in detail so there is much to be gained if you get the process control right. In the home shop simple, repeatable and tolerance of ahem "variable" process control is much more important than uber high performance.

Skimming the book Mr Stevens seems a less than reliable source. Basically reporting what he has been told, what sounds right and so on. Daily Mail science at best. The official document quote should not be taken at face value either. Much face covering, position protecting and politics going on as the folk who approved the final text certainly did not understand the technologies and the implications. In my experience getting accurate technical assessments through the military support 'crats is nigh on impossible!

Clive

Definitely sounds like "snake oil" to me. I'll stick to my old tin of Kasenit.

Russell.

Stub, presumably like me you played with nitrate to make bangs, not nitrite? Although they are both oxidants, nitrite is highly hygroscopic, even more than nitrate, so any "mixtures" you made would soon become very wet.

The biggest bang I ever made was actually a lot simpler than the classic chemical mixtures, even the ammonium nitrate/aluminium (poor man's Ammonol?). I took a 50 gallon oil drum, added a pint of water and screwed the cap back on, then lit a bonfire under it. When the ends started to bulge like balloons, I reckoned it might be sensible to retreat to a safer distance. This turned out to be a good instinct. I never found the end of the drum that flew off. And the drum itself ended up about 50m away. Funny thing was, I was living in Northern Ireland at the time, during The Troubles. I made myself scarce before Plod arrived!

Could have cut someone in half. A vivid illustration of the energy that can be stored in compressors, boilers and other pressure vessels and the importance of ensuring they are correctly rated and tested.

Merry

At least my fears that it was using a bath of molten sodium nitrite were unfounded!

Neil

Russell

Carbon pack case hardening can be surprisingly deep given the right steel and plenty of hot time for the carbon to migrate. However I suspect that Mr Stevens was unable to appreciate the difference between 0.5 cm and 0.5 mm. 20 thou or so is easily achieved. We could also quibble over the confusion between case hardening and nitride coating. Sodium nitrite at 35% concentration in water is not going to do much for the second or so period over which the steel surface remains hot enough for interesting things to happen.

But the guy is a writer so one has to pitch ones expectations at the right level.

Clive

That sounds like a "No" to 'is it simple and safe' then Clive. Thank you for digging the detail out.

Plan B is to pack them into a cast-iron junction box (3" circular type) with some chopped up shoe leather. seal it up with some kind of fire cement (trip to local hardware shop required) and cook it all in my garden incinerator for a few hours next time I have a bonfire (early November is always seems a good time to do this kind of thing!)

Anything better (than leather) to use?

Regards,

IanT

Ian

I'm not too sure about the "chopped up shoe leather" - maybe a bit hit and miss? I notice that there is some case hardening powder listed at a well known on -line auction site HERE which would probably achieve more predictable results?

Regards

Norman

Thanks Norman,

I also found this thread on this forum which gives quite a bit of information on 'home-brew' solutions.

**LINK**

Regards,

IanT

Hello Ian, you found the messages about home brew case hardening, I hardened some gun trigger parts and got a good hard skin which has lasted many years and has not worn just using the leather, I would add some powdered charcoal. about 30% of the volume to the pieces of leather. I found it best to close the box and keep a good ,more than cherry red heat for some time to get the depth of hardening. The box can just have a loose lid not totally sealed but well packed. It will be OK if it is not disturbed while firing. If no air can get to it the surface comes out clean when quenched. Throw the whole thing in some salty water.

Clive

Something that hasn't yet been mentioned is the time required. With a typical case hardening compound such as Kasenit or even just a mixture of charcoal with about 1% of washing soda, the case will be formed at a rate of about 0.008" per hour. So you need to decide how deep a case is needed and then decide on the time the work has to be kept at red heat. It is best to let it cool naturally and then clean and reheat it to "cherry red" and quench to harden the case. Quenching it still in the tin is poorly controlled and may not cool it quickly enough.

Russell.

Something that hasn't yet been mentioned is the time required. With a typical case hardening compound such as Kasenit or even just a mixture of charcoal with about 1% of washing soda, the case will be formed at a rate of about 0.008" per hour. So you need to decide how deep a case is needed and then decide on the time the work has to be kept at red heat. It is best to let it cool naturally and then clean and reheat it to "cherry red" and quench to harden the case. Quenching it still in the tin is poorly controlled and may not cool it quickly enough.

Russell.

To be honest guys, at this stage, I simply want to experiment a bit (before I try it on any more serious work and b***** it up). I also like the idea of coming up with my own (simple/cheap) homebrew solutions whenever possible.

I have been thinking about adding some charcoal to my 'old boots' and have read somewhere that coconut shells make really good charcoal for this purpose (walnut shells were mentioned in the other thread - but I don't like walnuts!). So as SWMBO generates lots of empty coconut shell husks from her bird feeder, these have been quietly accumulating in the Shed. They recently got broken up and now live in an old Tate & Lyle tin (rescued for the purpose) awaiting my first attempt at making "special" charcoal.

It may well be that I will end up buying the proper commercial stuff but in-between I will have lots of fun with bonfires and sticking things in them (just like I did a long time ago). Last of the Summer Wine anyone?

IanT

I have been looking to case harden a couple of pieces of mild steel and have been reading quite a few web sites about the processes involved, this one is quite good and informative

**LINK**

Cyril

Tubal Cain's book "Hardening, tempering & heat treatment" (Workshop Practice Series) has a chapter on case hardening which explains the various processes very well. At about £7 from Amazon it's good value.

Russell

The simplest one I can recall is someone who had a motorbike part which kept failing

So they made a replacement bit out of steel, heated it up then quenched it quickly in-and-out of motor oil and left it to one side to cool off

The part had been fine ever since and no more failures

Definitely 'bling ' reporting if they are quoting 0.5cm thick.

Only the BBC and dressmakers use cm.

The Sodium Nitrite quench would just be the same as a Brine quench, it softens the impact of the cooling and would stop surface cracking.

Clive

The bit for the motorcycle reminds me of my New Triumph Tiger 100, there was a steel strap across the bottom of the tank and it supported the tank to the frame and it snapped. I obtained a new one and left it in the firegrate overnight and repainted it and had no further trouble. All i had done was normalise it. Looking at the old one showed crystalline fracture which I have found to be quite a common failure fault.

Clive