View Full Version : Tempering salts

07-18-2012, 03:44 AM
Does anybody know where, in the UK, I can get a small quantity , say 2 or 3 kilos of tempering salts. Googling does not seem to throw up a likely supplier.

I am not talking case hardening here but using molten salts held at temperature to back temper a quench hardened steel. Lead can also be used but as it melts at 327C it's a bit on the high side for most tempering jobs.

The salt bath method is more controllable than the usual oxide colour method used by us amateurs

07-18-2012, 06:23 AM
A bit more googling found this http://www.substech.com/dokuwiki/doku.php?id=salt_bath_heat_treatment

07-18-2012, 10:29 AM
There is a lot more to this than just buying salt. I have a friend who runs a salt bath in his shop, and its expensive, dangerous, and highly maintenance intensive.
Everything must be made of stainless steel or titanium. And, even then corrosion is a serious, and ongoing issue. Relatively expensive stainless thermocouples have a finite lifespan in this environment, as do the salt pots themselves, and any tools or tongs you use to place things in the bath- he forges his own titanium tooling for this.
The salt temp must be accurately regulated, he uses electronic controllers for this.
And any moisture, even a drop of sweat off your nose, will cause mini-eruptions of 600-900 degree molten salt. Which has a detrimental effect on human flesh, I can assure you.

Generally speaking, setting up and running a molten salt bath is only worth it if you have an ongoing income stream from the products you are heat treating in it. It works, and for some processes (he does some proprietary stuff that is absolutely unique in the entire world) its the best solution. But its kind of a black art, will require a fair amount of experimentation, and is not cheap.

Forrest Addy
07-18-2012, 11:19 AM
Lead too high a temperature? How about solder? Do I recall the melting point of 37 Pb/65 Sn solder as in the 430 F range. Solder is a little more friendly to equipment compared to salt where some of the most popular are powerful oxidizers.

Mike Hunter
07-18-2012, 12:26 PM
What about Potassium Nitrate (Saltpeter), I use that here int he shop, melts about 350 Deg F, and is good up to about 1000 Deg F.

I use it in the shop.



07-18-2012, 02:10 PM
Further googling has come up with this site. http://www.substech.com/dokuwiki/doku.php?id=salt_bath_heat_treatment

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?

Some how or other a container of molten salt at 200C seems less hazardous than one full of boiling oil!

07-18-2012, 02:17 PM
What about Potassium Nitrate (Saltpeter), I use that here int he shop, melts about 350 Deg F, and is good up to about 1000 Deg F.

I use Houghton commercial quenching/tempering salt, and according to the MSDS sheet, it's mostly potassium nitrate. It's not very caustic -- I made a quench tube from 304 stainless, but a lot of knifemakers use mild steel and it holds up OK. If you have access to nitre blueing salts, it's almost exactly the same thing.

These low temperature quenching salts are moderately dangerous -- it's molten salt, but it's no more dangerous than Brownell's Oxynate 7 or Bluing Salts, which are almost the same stuff.

Now, a lot of high-end knifemakers use high temperature salt pots, as done with commercial heat treat, which is what Ries is referring to. That's pretty damn scary -- volcano in a stainless tube. Highly corrosive, and since the salts are at ~ 1500F, any liquid, including sweat or a drop of water left on the workpiece, explodes into a ball of steam..

Mark Rand
07-24-2012, 09:32 PM
I've uses messrs lpchemicals (http://www.lpchemicals.com) in the UK for assorted nasties in the workshop. I believe they've got a 50 minimum order, but I'm not sure. I've used common table salt to harden EN24/4340 (MP 802C) It's rather nasty for corroding every iron based item in the building, but excellent for hardening without adding any scale to the items. I used cold and heated oils for quenching depending on the desired results.

test run (http://www.test-net.com/hardening-test/)

Final run (http://www.test-net.com/hardening-final/)

07-24-2012, 10:39 PM
There seems to be some confusion here. Salt baths at close to 1500F are not for tempering. Tempering aka "drawing" is done at much lower temperatures after hardening. The temps bandied about here are at the austenitizing level for most ferrous metals. After quenching , tempering is usually done at 500-700 F with subsequent quench by air, water or oil, depending on the alloy and hardness desired.

Mark Rand
07-28-2012, 03:26 PM
I was just giving my use as an example. The main reason for the post was to mention a supplier that I've used for chemicals that'll deal with smaller fry than the likes of W. Canning & Co. will.

07-28-2012, 07:44 PM
Where abouts in the uk are you?
I have a deep fat fryer for my tempering bath. SWMBO bought a new one when I (started) refitting the kitchen, tho there was nothing wrong with the old one.

07-28-2012, 07:48 PM
I have a deep fat fryer for my tempering bath.

That's a great way to heat the tempering salts. I made the mistake of building in a hot-water heating element, but that doesn't have enough wattage to heat the tempering salts fast enough (takes a couple of hours).

07-29-2012, 05:41 AM
Our kitchen oven goes to a genuine 280 (checked with a calibrated thermocouple from work) which allows me to do 90% of my tempering in the kitchen. For temps above this I can use he furnaces at work.