Salt bath ?

M

mtnsct

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Jul 31, 2006
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After reading the posts from K. Cashen and the other members in regard to heat treating and marquenching with salts, I'm pretty much sold on the idea.

BUT... what's the down side i.e health hazzards from vapors, burns, nuclear detinations... the basic stuff!?

I am looking at starting out with purchasing a marquenching setup (low temp. salt bath) first since I already have a 22.5" EvenHeat kiln. I figure I can heat treat 5 blades (O1, 1/8") or more at one time and pull individual blades out of the kiln and stick them in the quenching salts.

Will this work?

Do you think there will be too much fluctuation in temp. from opening and closing the door after each blade is extracted?

Last blade/s to be pulled out sitting too long in the kiln?

Any thoughts are much appreciated.
 
I don't consider there to be many disadvantages to using salt compared with other methods. Just a different process with it's own characteristics.
I use salts in a well-ventilated area. Though I don't believe the vapors are particularly toxic.
I have several salt 'pots' for different things. Two high temp units, a long upright low-temp unit and a large horizontal low-temp unit. All run on natural gas.

I would suggest you get used to the idea of going with a high-temp salt unit because there are some nice benefits including the natural lack of scaling of your part and the superior evenness of heat delivered to your parts, etc.
I've used these things for probably 10 years and have never been burned (which is more than I can say for forging).
Obviously you want to keep water or oil out of your salt bath when it's at temperature because these things cause sputtering and popping as they vaporize (in high-temp salt) and that can get messy and potentially dangerous. I always wear a full face shield and welding gloves when using the hot salts--usually a welding jacket too.
I would recommend you read the material on Don Fogg's site concerning this. There's a tutorial on making salt pots that's pretty good.
 

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Glad to see this subject come up. Thanks for the links for info, J.C. I'll do some research before askin' a bunch of questions.

Well, o.k., maybe one. How corrosive is this solution on the tanks. I know salt is bad for corrosion[I live in the north & did auto body work for years.] Or is this a different salt solution?
 
Deweyknives said:
... How corrosive is this solution on the tanks. I know salt is bad for corrosion[I live in the north & did auto body work for years.] Or is this a different salt solution?
Click to expand...

This isn't salt as in table salt. I use a product called NuSal for high temp and Thermoquench for low temp. I'm sure you could find the exact chemistry online if you're interested. These are chemical 'salts'. They come as a granule 'salt'-like material. When you heat it it goes liquid and when it cools from there it's a solid white opaque block. Heat it again and it goes back to liquid, etc.
I use stainless steel tubes for upright apparatus. You can use mild steel, but that will degrade faster and it's hard to know their condition so you're always wondering.
My low temperature horizontal rigs use mild-steel tanks intended for bluing. In this arrangement you can see into the tank when the salt is liquid and inspect it's condition, but the lower temperatures don't degrade the material like the high-temperature.
 
JC,

Could you or someone else using salt pots explain the basic process and time frame from start to finish for h.t. and quench. It seems like it's a fairly quick process. Is working with air quench steels any different from oil quench?

Thanks again for the help.
 
The nice thing about salt is that it opens up access to more sophisticated HT regimens. A high-temp/low-temp saltpot combination is well suited for marquenching and austempering (marquenching being the more relevant to knives, resulting in a somewhat more stress free martensite compared with traditional heat-and-quench hardening)

Austempering is how you're going to develop Bainite, which has been discussed a lot the last few years. In my opinion, Bainite is not optimum for knives, but is quite suitable for things such as the small sword or rapier).

If you have three salt rigs (One high- and two low-temp units) you can do Isothermal Quenching & Tempering, which results in a structure of Martensite and Bainite.

For more on all the fun things you can do, you should start with a copy of "Metallurgy Fundamentals" by Brandt, Goodheart-Willcox and learn to read Isothermal Transformation charts (it's not as tough as it sounds). You will then have use for an Atlas of Isothermal Transformation, which is a very handy guide that will unlock a lot of mystery concerning the behavior of different steels in HT. Also, by all means read some of Kevin Cashen's excellent posts on the subject on this forum.

Having said all this, here is how a simple marquenching (AKA martempering) goes in my shop.

I set the digital controller on the high-temp unit to 1545F, and to 550F on the low temp unit. I fire them up and they're ready to go in about 20 to 30 minutes.
Meanwhile I hang my clean, degreased blades on wires.
I suspend the blades into the high-temp salts for a few minutes (probably about 5 minutes). One at a time, I remove them from the high-temperature salt and dunk them in the low temp salt for about 5 or 6 seconds then pull them out drop them into oil.
That's about it. You wipe off the oil then wash the salt residue off with water. The finish of the steel coming out of the salt is essentially what it was when you put it in. There's no scale. Temper in the oven like usual. Very fast, very dependable.

Austempering goes like this: Heat in high-temp salt for a few minutes. Quench to low temp salt (again, about 550F) and leave in there a little more than one hour. Then "quench" to room temp. That's it.

Isothermal Quenching & Tempering: Heat in high-temp unit for a few minutes, quench to low temp salt unit set at 350F for a few seconds. Then remove and immediately submerge in a separate low-temp unit set at 650F and leave there for about an hour.
What happens here is in the first quench to 350F about half the steel transforms from Austenite to Martensite and about half remains semi-transformed. When moved to the 650F unit, and held there the rest transforms to Bainite.

As you can see, these practices are very easy, relatively safe and convenient, though explanations of what's going on might sound heady.

Marquenching (martempering) on carbon-steel damascus is what I end up doing the most. Other steels often require different temperature settings or even different procedures altogether which is where the Atlas of Isothermal Transformation comes in.
 
Aha! I just noticed there is a 'sticky' by Cashen about Isothermal Transformation Charts at the top of the 'shop-talk' page ....
Very convenient! Check it out.
 
J.C., can you go from an oil quench to a low temp salt for a martemper or is oil to salt not a good idea?
 
Deweyknives said:
J.C., can you go from an oil quench to a low temp salt for a martemper or is oil to salt not a good idea?
Click to expand...

I'm not sure why you'd want to do that if you have both a high and a low-temp unit. You'd go from high, quench to the low and then to room temp. A typical marquench is like a traditional heat-and-quench technique, but you arrest the transformation for a moment at a specified temp range, then go to room temperature.
Still, more to the point, if you put something with oil on it in the hot salt, you'll get a violent reaction as the oil expands/vaporize--something like an explosion. You're heating the residue oil on your part to whatever temperature the salt is pretty much instantly, so it's going to be pretty tumultuous.
Expansion/vaporization of contaminants creates the greatest safety hazard you're likely to encounter using salt. A very small amount of oil residue makes a pretty powerful 'pop' out of the salts and that is accompanied by a nice splatter of screaming-hot salt.
So in short..... DON'T DO IT!!!!:D
 
You can go from hot salt to oil, however, without incident (Just not the other way). The thin salt residue mostly freezes to you part. The salt is water soluble, so you just wipe off the oil and wash with water.
It's this salt residue that protects your finish when coming out of the high-temp bath--essentially no chance of scaling unless your bath is badly contaminated.
 
O.K., I was just wondering if I could start with a low temp tank for marquenching without going the whole nine yards. So when I get at it,I'll have to take the plunge and set up both tanks.Thanks.
 
Deweyknives said:
O.K., I was just wondering if I could start with a low temp tank for marquenching without going the whole nine yards. So when I get at it,I'll have to take the plunge and set up both tanks.Thanks.
Click to expand...

You could, but you'd still have to deal with scale. Finish retention is a very nice perk.
Good luck!
 
Hello,

Thank you so much for the informaion. How is it possible to obtain a martensite edge with a bainite back?

Also, if one has a high temp pot only, can the lower temp step be skipped?

I can't figure this one out..

Gil.
 
This thread is three years old????

You may not understand how complex your question is, but the short answer is;
You will need three salt pots. One to austenitize, one to quench just below the Ms, and one to hold at the lower bainite range for 8 to 12 hours.
It will require a good bit of experience, and a lot of experiments.
Stacy
 
I think he may mean if you use high temp salt do you need to quench in salt or can you use water/oil.

Further to this, could one clay quench using a high temp salt and water/oil quench?

I would hazard a guess that the potential for the clay to have water in it would be cause for disaster.
 
Thank you.

My post was actually two questions.. The first about how to obtain a martensite edge with bainite back was just for curiosity. I am just starting forging, so it would be a long time before I'd attempt or invest in anything like that. It doesn't hurt to know though, and the subject is fascinating.

The second one well, it seems like using a salt pot has some advantages over using a gas forge (which I have), temperature control and no scale.. I was just wondering if people do quench directly from high temp salt pot to oil, without the 5-6 secs in low temp salt. It would seem to me the same as forge to oil.. Now, I didn't think about clay in salt.. Really not the greatest idea I guess. I am sure though something else than clay could be used.. But again, I am a beginner..

Great forums by the way, I just signed up. I have only forged three knives, and already wondering about things way over my head, but like I said, metallurgy is fascinating to me!

Have a great day!

Gil.
 
One disadvantage is that it will still decarburise / carburise for succeptible materials. Also, you cant use as close to net finish as say controlled atmosphere high pressure furnaces.
 
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