Tempering Salts

[ANDY CAWLEYParticipant @andycawley24921]

Does anybody know where I can get a small quantity , say 2 or 3 kilos of tempering salts. Googling does not seem to throw up a likely supplier.

[ANDY CAWLEYParticipant @andycawley24921]

[SpringbokParticipant @springbok]

Hi when you say tempering do you mean case hardening and 2 to 3 kilos is not exactly a small quantity. there is a chap on fleabay sells small quantities of case hardening compound.. If it is infact C.H there was a thread recently on this.

Bob

[SpringbokParticipant @springbok]

Hi Andy

You got me interested in this and found this forum

**LINK**

Bob

[ANDY CAWLEYParticipant @andycawley24921]

Hi Springbok,

yes that is the sort of thing I'm looking for. In the commercial heat treating world they use a bath of molten salts held at a tempering temperature into which the item to be tempered is immersed. It is a much more controllable method than the oxide colour method used by amateurs such as our selves.

[ANDY CAWLEYParticipant @andycawley24921]

A bit more googling found **LINK**.

[Ian S CParticipant @iansc]

Salt bath heat treatment is a rather unlikely thing to be tried in a home workshop, case hardening with something like Kasnite, yes, or hardening and tempering, either with a gas torch, a forge, or a kiln. Salt bath work is for the professional shop. Ian S C

[David LittlewoodParticipant @davidlittlewood51847]

Andy,

Interesting link. Just take care with some of the salts in the list though: NaOH and KOH are both extremely caustic, and will cause irreversible eye damage in seconds, NaNO3 and KNO3 are powerful oxidising agents, and if spilled on wood or other inflammables when hot may start a hard-to-extinguish fire (supplpies its own oxygen so conventional extinguishers may not work), BaCl2 is highly toxic and is a scheduled poison, may be hard to obtain.

David

[ANDY CAWLEYParticipant @andycawley24921]

David, thanks for your caveats. I remember enough of my A Level Chemistry to be aware of some of the dangers. Since the range of temperature that tempering is done is 200C to 320C I’m not looking for a fearsome combination. Mixture 5, half and half sodium nitrate and sodium nitrite, looks like the one. With a melting point of 144C and a working range of 150/500C. Weed killer and food preservative sound like a really technical concotion!

What I am curious about is, will the letting down of a powerful oxidant like sodium nitrate with a stable sodium nitrite reduce some of it’s dangers?

I was rather tickled by Ian S C’s assertion that salt bath work is for the professional workshop. Most of what we do could be so classified.

I do however appreciate every ones concern. I actually picked up on the idea from Workshop Practice Series Number 1 in which Tubal Cain ( the real one) makes it all look perfectly feasible. Ive got a Belling Hot plate and a cast iron glue pot.

What could possibly go wrong?

[Ian S CParticipant @iansc]

Andy, I was just thinking back to my school days, in the school workshop we had a large salt bath, used for case hardening/nitriding steel parts. It was a cyanide bath, and we were not allowed near it. I was imagining you cooking up cyanide. What ever you use, make sure you have good ventilation, we want to keep you on the forum to tell us of your exployts. Ian S C

[ANDY CAWLEYParticipant @andycawley24921]

Gosh, Dick, sounds like a really cool(as my grand children would say) school. Cyanide eh? Now thats the way to teach.

It seems these days kids have to don safety glasses even if they use a compass to draw a circle, that is, of course, if they are allowed to handle something with a point on.

Harrumph rant over!!!

[PatParticipant @pat]

Hi Andy There are alternatives to salt for the lower teperature tempering of previouly hardened parts . For example for light straw simply use olive oil. It starts gently smoking at 190C to 200C and the smell is OK but there is a fire risk if heated to its boiling point arround 300C. Other oils can be used SAE30 oil could also be used but the fumes are not good.

As these oils vary a bit it is advisable to measure the temperature  of the actual oil you have.

Here is a link to some other oils and their smoking temperatures. **LINK**

And here is a better link to kitchen edible oil smoke points

http://en.wikipedia.org/wiki/Corn_oil#cite_note-chef-5

 

Regards – Pat

Edited By Pat on 18/07/2012 15:39:24

Edited By Pat on 18/07/2012 15:43:09

Edited By Pat on 18/07/2012 15:58:20

[ANDY CAWLEYParticipant @andycawley24921]

Pat thanks for that.

Somehow a container of molten salt at 200C seems less hazardous than a vat of boiling oil and less smelly as well When cold I guess there is less spill risk with the salt.

[David LittlewoodParticipant @davidlittlewood51847]

Posted by ANDY CAWLEY on 18/07/2012 14:50:50:

Mixture 5, half and half sodium nitrate and sodium nitrite, looks like the one. With a melting point of 144C and a working range of 150/500C. Weed killer and food preservative sound like a really technical concotion!

What I am curious about is, will the letting down of a powerful oxidant like sodium nitrate with a stable sodium nitrite reduce some of it’s dangers?

I wouldn't assume it! Nitrites have also been suspect of being carcinogenic, though their use in food preservation suggests they can't be seriously so. And, is nitrate used as a weedkiller? I know sodium chlorate is, I thought nitrate was more likely to be a fertiliser.

Good to see you are using the Tubal Cain book as a reference, he was a very good writer.

David

[Sub MandrelParticipant @submandrel]

Andy,

Don't expect sodium nitrate to be significantly less risky when 'diluted' with nitrite. Going back to my mis-spent school days there were lots of 'interesting' experiments with sodium nitrate, including in molten form. <counts fingers> 10! Looks like I was lucky.

Neil

[SpringbokParticipant @springbok]

Wow we are getting into CIA and MI5 territory here

Suggest a dilute concoction of citric acid and water can be purchased from Boots or your local home brew store. heat metal to cherry red depending on the metal then quenching. (plunging into the water) leave for a short time to get rid of scale clean off and fettle.

Good luck and Stub count to 10

Bb

[TerrydParticipant @terryd72465]

Hi,

my domestic electric oven reaches just over 300ºC (purple on the tempering scale). I made a calibration chart using a reasonably accurate oven thermometer and ran a series of tests to check colours on steel samples. I can now accurately temper hardened components without any dangerous chemistry and I can soak in the heat for as long as is needed.

The main drawback is that I have to wait until my wife is out .

As regards Cyanide baths in school. In the early 70s I helped out in a boys secondary modern school in a very rough council estate when a metalwork teacher refused to return after a half term holiday. While there, the head investigated some packages that had been stored in the cloakroom for more years than he had been there. On investigation he found a lovely potters wheel which the art teacher was thrilled with. He then opened the smaller package which turned out to contain cyanide tempering salts. Understandably I refused the opportunity to use them.

Regards

Terry

[David LittlewoodParticipant @davidlittlewood51847]

Springbok,

You are confusing salt quenching baths (aqueous solution for dunking red hot metal in) with salt tempering baths (molten salts to raise metal to tempering temperature). The OP was enquiring about the latter.

David

[ANDY CAWLEYParticipant @andycawley24921]

How I love the ability that we all have to mislead ourselves.

David is quite right in one respect, the OP was asking about where I would get a home workshop sized supply of salts that I could melt and hold at a temperature that would temper a piece of steel that had been through hardened by quenching either in an aqueous bath or an oil bath.

Pinched from Wiki :-

“ For metals, tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to a much lower temperature than was used for hardening. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product.” (I underlined the bit about temperature.)

I’m not sure David is right about Springbok’s confusion. To the best of my knowledge Cyanide baths for heat treatment are not aqueous but are molten salts usually at 800/900C and are used for case hardening where the salt increases the carbon content of the steel on the surface.

By way of clarification for some of the community I offer the following explaination:-

Steel is an alloy of Iron and Carbon in its simplest form.

Steel with a carbon content of above 0.40% (En8 in old speak or 1040 American speak) is capable of being hardened by heating to a temperature of around 850C and then quenching to reduce the temperature very rapidly. This makes the steel very hard but brittle. The brittleness can be reduced by tempering (see above).

In simple terms the higher the carbon content ( upto 1.0%) the harder the steel will be. This process is generally known as hardening and tempering..

Steel with a carbon content of less than 0.40% is known as mild steel and as such cannot be through hardened.

Case hardening of mild steel is a process by which mild steel can be given a very hard surface. This is done by increasing the carbon content at the surface of the steel and usually at very high temperatures then quenching. The high carbon bit of the steel at the surface then becomes very hard and the low carbon bit in the middle remains soft thus the component its self is tough in the middle.

The carbon enrichment of the surface is achieved by several methods whereby the steel is intimately surrounded by a compound which, when heated, to the magic temperature releases some of its carbon into the steel.

Summing up we can “through harden” for steel with sufficient carbon content and “case harden” steels with low carbon content.

This is a huge simplification (alloy steels have been ignored for instance) but I hope that it clarifies things a bit for those who are new to engineering and the influence of metallurgy on what we do.

For those on this forum, of which I expect are the majority, who are more knowledgeable than I, I beg your indulgence.

[ANDY CAWLEYParticipant @andycawley24921]

TerryD,

Whilst not as adventurous as dangerous molton salts you suggestion is far more sensible.

I'll give it a go!

Thanks for keeping my feet on the ground!

[David LittlewoodParticipant @davidlittlewood51847]

Posted by ANDY CAWLEY on 19/07/2012 12:30:36:

I’m not sure David is right about Springbok’s confusion. To the best of my knowledge Cyanide baths for heat treatment are not aqueous but are molten salts usually at 800/900C and are used for case hardening where the salt increases the carbon content of the steel on the surface.

Now I think you are getting confused! Terry was the one referring to cyanide case hardening baths; Springbok said:

"Suggest a dilute concoction of citric acid and water can be purchased from Boots or your local home brew store. heat metal to cherry red depending on the metal then quenching. (plunging into the water) leave for a short time to get rid of scale clean off and fettle."

, which is clearly a quenching bath not a case-hardening one.

David

[ANDY CAWLEYParticipant @andycawley24921]

Oh dearie me David, now we are getting really confused. I was referring to your post of 19/07/2012 11:05:47'

.Never mind, this is not confusing:-

Done in the oven, no dangerous molten stuff, wife out at art appreciation, no damage, Austin 7 door now stays closed.

Thank's Terry for injecting common sense into my technorambling!

[PatParticipant @pat]

Hi Andy – Never ever raise any oil to boiling point as this is a fire hazard as the fumes given off will be above the liquid oils flash point. I only posted the oil method of drawing the hardness down as this does not need any special equipment other than a suitable source of heat and a container. Olive oil and many others edible oils smell very good where as mineral oil is not good and is probably poisonous.

The domestic oven is ok but in my experience the temperature need to be watched all the time as the oven themostats are not precise so the temperature wanders up and down quite a bit. I use an industrial glass / mercuary thermometer that is in a protective steel tube. The oils are from the supermarket or the local deli.

Regards – Pat

[David LittlewoodParticipant @davidlittlewood51847]

Posted by ANDY CAWLEY on 19/07/2012 14:25:53:

Oh dearie me David, now we are getting really confused. I was referring to your post of 19/07/2012 11:05:47'

So was I!

David

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