Low Temp Salt Set-up

J. Hoffman

Dealer - Purveyor
I'm setting up a low temp salt bath for quenching and tempering. I have the tank and salts, but I'm not familiar with using PID's to regulate the temp. I'll be running it off of propane. Can someone please point me to directions for setting up the burner and temp control. I know I will need a PID, thermocouple, and some kind of solenoid for controlling the gas flow, but I don't know the types or kinds that I need. Any help is appreciated.
Thanks
Jess Hoffman
 
Would either of you guys care to explain the use of these salt tanks? I've heard people talk about salts and heat treating a little, but don't really know what's going on.
 
It's like food cooks quicker in boiling water than in the hot air of an oven. The molten salts transfers heat more efficiently than than a kiln will and will have more control than a forge. Being an open system, unlike a kiln, one can more easily austenizing the thinner parts of a blade while leaving the spine pearletic, as was brought up by Ed on a recent discussion of how to differentally heat treat a dagger. The down side is that you have to play carefully with them, especially the high temperature salt pots, so that they don't blow up in your face. Something that you'll definitely want to learn how to operate before building one.

Doug
 
I agree with Doug.... Salts are essentially the ultimate in "control" when it comes to heat. As with everything we do, there's a trade-off. In this case it's the fact that you're working with a 1,000+ degree hot liquid, and burns from a "sticky", hot liquid are FAR worse then touching a hot piece of steel. :)

Beyond the heat control factory, one of the biggest advantages I find with a high temp salt tank is that I can take a blade to pretty much finished (ie: folder blade), and since no oxides form on the steel (only a light coating of salts), most of the time all that's required after heat treating is hand finishing. (if you choose to do that).

The low temp salts are often used to create a Bainite structure in the steel....slightly softer, but much tougher then Martensite.

In the end, either low or high temp salts are just another kind of heat source....but as Doug described, the conduction heating provided by the steel contacting a liquid is much more efficient then the convection heating of air.
 
Thanks for the info Ed. After reading some info from Kevin, he doesn't recommend salt quenching for most of the steels I was going to use it for. I mostly use 10XX and W2, he stated the salt won't quench fast enough, so this project may be on hold until I can get a high temp salt set-up first. Are low temp salts ok for tempering stainless? I would assume so, but not sure. I also like the idea of being able to blue with them.
 
I don't have low temp tanks....mainly for the same reasons Kevin indicates. In some ways I would think that low temp salts for stainless MIGHT be better then the typical "oven" tempering. You'd certainly have less scale/crud on the blade when it was finished. :) I think the big trick would be maintaining an even temp if propane was the fuel source.
 
The low temp salts really shine when it comes to quenching hypereutectoid deep hardening steels (think 52100, o1...)
You quench directly into the salts at shy above Ms (let's say 420 °F) and let just equalize the piece...then you pull it out from the salts and have the chance to straighten it with gloved hands easily. Then let it cool in still air without worry for cracks. It is marquenching, and you get martensite. Instead, if you keep the steel in the salts for long hours you make bainite.
And yes it is the most reliable heat medium for tempering, after the steel has reached room temperature or Mf. The blade will be always at the same temperature regardless its variations in thickness, a nice pro either for carbon and stainless.

For marquenching you need steels that are both deep hardening and hypereutectoids (they have Ms in the proper tempering range); Hypoeutectoids have Ms at too high temperatures to be used for marquenching techinques.

The temperature control will have you free for other activities, but i guess you could stabilize the temp with a simple needle valve in a venturi burner setup. Once the salts are stable in temp you can add the blade and the salts will go down and then up to your setpoint. With a PID when you add the blade the control will start adjusting...overshooting maybe, and bounching a bit. If you already have the salts and tank i'd try to see if you are able to control the temps without PID.
 
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for basic stainless, lo temp salt may work. but at the temps you are talking about 300 to 400 degrees, you could use vegetable oil and get the same result without the expense and danger.
 
Of course 300-400F vegetable oil will work fine for tempering, but for quenching it will get you plenty of pearlite, upper bainite and, of course, most of your eyebrows and hair singed off. I would strongly recommend against heating anything but properly formulated martempering oils to 300F- 400F for quenching. And then it will only really work well with alloy steels designed for deeper hardening.
 
I think that the big question to ask yourself when you're thinking about employing a low temperature salt pot is what is it that you want to do with it. Yes you can use it for marquenching (also know as martempering in some texts) to beat the nose of the cooling curve and then allow the temperature of the steel to descend trough the Ms point to the Mf point to give tempered martensite. You can also use it for austempering to hold the temperature of the steel to just above the Ms point long enough, it could be many hours, for the steel to convert to lower bainite. That could be followed with selective austenization and quenching to produce a blade with a hard edge and a spine that is both tougher and harder than the usual pearlite spine. Then there's where you can split the difference and quench to steel to a point between the Ms and Mf point to convert a percentage of the steel to tempered martensite and then hold it at that temperature long enough for the balance to convert to bainite. A little experiment that I did one time for grins and giggles that gave me a 52100 blade that creaked and popped when I tried to bend it to 90° with a breaker bar. It almost made it to. Snapped just shy of 90°

I didn't use a low temperature salt pot for that though. I used peanut oil. Didn't get a flair up but I made sure I had the lid to the turkey fryer and a fire extinguisher handy. Also made sure that the garden hose was shut off and nowhere near the shed just to make sure I didn't panic if a fire flaired up and I tried to grab it to put the fire out.

Addendum: I seem to have missed the point that I was trying to make which is just because you can do neat things with a low temperature salt pot doesn't mean that they are worth doing. You can approximate martempering by quenching in oil for 6-7 seconds and then allowing to air cool the rest of the way through Ms to Mf. Does a blade with a bainite spine give better performance than one that is fully hardened? What are you trying to accomplish with a soft spine in the first place? My little experiment with a mixed structure blade was interesting but little more. The blade that I made at the same time and hardened "the regular way" was a lot stronger. I couldn't even start to bend it with a breaker bar; I had to use a 4 lb mall to break it. Whenever you add a new piece of equipment or a new process you need to ask yourself first where you are going with it.

Doug
 
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