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j. walters

Heat Treating Salt Recipies

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Hi I'm pretty new at all this tempering stuff. I've played around with both oil and water as a quenching medium. I understand oil is more viscous and doesn't cool as quickly as water thus preventing cracks. I have also read a lot about brine solutions being used to quench, but, nobody talked about the science behind it they just gave recipes.

 

Would the salt somehow slow the cooling? Reducing stress cracks?

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I searched and found this PDF on my computer. If anyone is interested in hosting this for public download I will be glad to send it to you.

 

 

Seth

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It's nice to hear from you again, Dee. Just wanted to say hi.

 

Doug :)

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I've never heard of heat treating with salt. Can anyone fill me in on the basics? Or is this used for making a brine solution?

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Not a brine. Molten chemical bath. Also not salt as in table salt, but in the chemical sense of a metallic atom combined with a nonmetallic atom. Ferric chloride is a salt in this sense. Acid plus base = salt.

 

As for how it works, the high-temperature blend is put in a stainless tube and heated by either gas or electricity until it reaches the target temperature for the alloy you want to harden, we'll use 1475 degrees F for fun. If you have good controls you can maintain this liquid bath at that temperature for hours. Why bother? Because it's the fastest method of heating (except possibly for induction) and more importantly it totally prevents oxidation and decarburization and seriously reduces warpage. You can basically harden and temper a finished blade without worrying about scale or decarb.

 

The low-temperature blend goes liquid at a much lower temperature and is used for quenching and tempering. Being a liquid it removes heat fast, with the advantage of being able to hold at heat for extended periods without oxidation or decarb, just like the high-temp salts. This means you can really mess with the crystal structure, making things like a fully bainite blade (Bainite is the toughest form of tempered steel, good for swords) or just hold a warped blade at just below Ms start (the temperature at which austenite starts to transform into martensite, or what we call hardening) so you can take out the warp while it's still hot and soft, but it will fully harden once it cools.

 

It's probably the best way to do production heat-treating of blades, but it has several serious hazards as well. The high temperature stuff can blow out of the tube if there is any moisture at all on the blade when it's put in, or if a drop of condensation falls into the tube while it's hot, if you heat it up from cold and solid without getting the top half liquid before the bottom half it can act like a mortar, shooting the solid top through the roof of your shop. It's basically like having a tube of slightly unstable molten lava in the shop. The low-temp stuff doesn't like moisture either, but since one of the usual main ingredients of it is potassium nitrate or sodium nitrate, aka saltpeter, if you drip it onto organic materials when you pull the blade out it creates a highly flammable and sometimes explosive substance.

 

In other words, they're great, but not to be trifled with if you don't know what you're doing. ;)

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I've been recently contemplating on building a high temp salt bath myself. BaCl is out of question for me, so when searching for a good formula, it just came to my mind to use borax.

I have no experience with this, but no one seems to use it as a heat treating salt. Why is that?

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Borax is very viscous at Carbon Steel HT temperatures. I tried it once in a mini salt pot and it was just too thick to use for O1. I tried low-sodium salt next (Class 5 HT salt, but from my local supermarket) and it was much better.

 

Borax might be usable at higher temperatures, but I'm not sure. In the forge, it seems to go properly runny at about welding temperature and I think that might be too hot.

 

It does have a tendency to dissolve refractories too, which can be something of a nuisance.

 

I'm sure there are other good reasons Borax is not used, but the viscosity was enough for me.

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I've seen Borax generally used to keep steel from oxidizing when forge welding, but that's all.

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Borax is very viscous at Carbon Steel HT temperatures. I tried it once in a mini salt pot and it was just too thick to use for O1. I tried low-sodium salt next (Class 5 HT salt, but from my local supermarket) and it was much better.

 

Borax might be usable at higher temperatures, but I'm not sure. In the forge, it seems to go properly runny at about welding temperature and I think that might be too hot.

 

It does have a tendency to dissolve refractories too, which can be something of a nuisance.

 

I'm sure there are other good reasons Borax is not used, but the viscosity was enough for me.

 

Yepp, viscosity seems to be an issue. I think I will go with a NaCl-CaCl (or similar) mix, may be just as obtainable as borax. Thank you all for the input!

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