Thermo-well or Dunkin?

jhiggins

jhiggins

Joined
Jul 11, 2003
Messages
3,258
HAH! I thought that title would get your attention!

Hey all you salt-pot fellas, I was wondering if we could start yet another thread here about salt-pot heat treating - not how good it is, or how it works, but maybe some of us salt-potter-wannabes can get some insight into what we need to make our own. My pressing question is:

What do you guys who already heat-treat with salt think about the design and application of the tube that contains the salt? I'm primarily looking for whether or not the installation of a thermowell into the bottom of the tube (to house the thermocouple) is worthwhile or if just dunking the thermocouple into the molten bath is preferable?

Also any information you could share about size and length of the tube and surronding forge would be most appreciated.
 
Hi Jeff, If you are seriously considering the hot salt method for heat treating be prepared for some dangers. Maybe I just had to learn the hard way but Ive set myself on fire and/or things around me on more than one occasion. If you are sure you want to do this I will try to help you avoid some disasters.

You dont need a big set-up by that I mean the tube doesnt need to be much wider in diameter than the blade in width. Its a waste of salt and propane to heat the salt. The salt takes time to reach the 1500deg temp and if you have a big pot it takes even longer. For one or two blades I dont even bother cranking it up.

My tube is: stainless steel about 4" OD with a .375" wall thickness and 24" long. It is tig welded to a 1/4" plate for the floor. It sits inside a 2 burner forced air round forge sitting upright. The forge body is lined with refractory about 1" thick.

I think my whole setup is too big and want a smaller one so it will warm up faster and not take up so much floor space. You need to do the work outside or in a nonflammable builing with no flammables close.

The problem is when salt melts it expands and builds air pockets which blow up therfore spraying salt straight into the air. I was smart enough to tilt mine but now its like a mortor shooting molten salt straight at the neighbors house barely missing my classic 67 mustang fastback.

You sure you want a salt pot now? Its like owning your own dragon!

The thermal well is a good idea to save your probe from possible corrosion in time but even the ss well gets thinner with time. My thermal well read cooler than actual temp by as much as a couple hundred deg. Mine was ceramic. A stainless one may work better but may not last. I finally went to a enclosed probe and just dip it instead of leaving it in the salt
 
Bruce and jhiggins:

I am sure you are probably aware of this by now Bruce, but it is almost a necessity to create a vent hole in the salts when using gas or any other aparatus that would get the bottom molten even a second before the top. Otherwise, as you have found, you will make a little volcano :eek: . This is very easy to fix. Get some 1" steel stock (any steel) and forge it into a taper and then clean it up nice and smooth. When you are done with the salts for the day insert the taper into them before they cool, leaving an inch and a half space at the bottom. The next time you go to fire up, give that taper a couple of sharp raps on the end with a hammer and pull it right out, making a nice vent hole so no preasure can build, and this will also help with thermal expansion.

this is one advantage that electric has over gas. The electric heats so evenly and slowly that the outer edges of the salts will go molten from top to bottom and reduce the need for this precaution.

This also applies to low temp salts if the heating unit is inferior. Never, put an electric element just in the bottom of a tube! I, and a group of other makers, witnessed a low temp tube
entirely empty itself becuase some brain decided to make a pot by slipping a watter heater element into the bottom of a tube. The thing was turned on high and left for several hours waiting for it to go liquid, nothing happened... until... When enought pressure had built to blow that top crust it sent 500F salts 10 feet into the air and totally covered an area 16 feet around it. We were darned lucky everybody was working at the other end of the shop at the time.

This is also a drawback of using tubes. Ever overheat a test tube? It is not like overheating a pan of water. Tubes tend to entirely empty themselves from bottom to top, instead of just overflowing a bit. For this reason you sould always keep water or any moisture out of your low temp if you are using a tube. Trust me on this one ;). I could tell you stories all day about that nonsense and the agony it can cause.
 
On the question of thermocouple enclosures, on a simple 2' tube it may not be as citical but you will want the freedom to move the thermocoupler around to get different readings and adjust things.

I must admit that I do not understand the "dunking" concept at all:confused: How does your controller accurately regulate the salt temps for you if it does not have continous data input?
I have different length thermocouplers and keep them in the salts to give feedback to the controller, which regulates gas flow, or electricity to maintina the set temp.

I use a standard inconel sheathed thermocouple (Omega KQin-*) and as long as I am vigilant to wash them off after use they will last a few years. They do eventually burn out, but at $28 a pop I consider them a resonable operating expense.

I can also offer thas advice, based on LOTS of bad experience, about constructing additions to tubes... keep it very simple. The more weld seams you have the more open to disaster you are. The salts will attack weld joints and leak there first. The problem is precipitated carbon in the weld zone- you don't want any. If you use 316L tubing use 316L (stress the "L") rod to weld it. The L signifies that it is low in carbon. Of course with my luck I have even had problems with the 316L rod, so I have now taken to using 99% nickle rod! Let the carbon mess with that! Hah! :p But now I am fighting getting a tight seamless weld:grumpy: That nickle is not as easy to make a perfect weld with :(
 
Thanks, Bruce. I am well aware of the inherent dangers with salt pots and the expansion that takes place. Aiming it towards the neighbor's house is a fabulouos idea. :cool:

Kevin... When you were using the electric kiln for high-temp, did you experience a severe electric bill at the end of the month? I'm so tempted to go with Tim Z's setup, but I am hesitant to shag myself heavily with a high electrical bill. Maybe I should build a diesel generator to power it, LOL! :p
 
I know a maker who solved the hot water heater elmenent inside the low temp pot problem after a fashion. He just pours the salt out in a pan and stores it in an ammo can in chunks after it hardens. He then melts it in a pot over a propane burner and pours it back in the pot with the element. The element is just used to keep the liquid salt at temp rather than melting it. Seems kind of round-about to me, but it works. His low temp pot is an upside down scuba tank with the bottom cut off, the water heater elements happen to have the same threads as the tank.
 
I've gotta say I feel like some folks make these out to be some sort of monster....when in reality they are a very excellent tool that we can implement in our shops.

The danger aspect is overblown by many. I mean, use a belt grinder?!?!?! You could grind your finger off!!! A chop saw?!?! You might cut your arm in two!!! A milling machine?!?!?! An end mill could blow up in your face!!!!

Of course they are dangerous, all the tools we use are. You have to respect them and make sure you use every precaution you can...if you do that, they are really quite easy to work with.

My salts are controlled by a Watlow digital controller that uses an inconnel sheathed thermocouple to read the temperature. I leave the thermocouple in the salts when in use (or the controller would have no input) and take it out when the salts are not running.

I also insert a tapered rod into my salt at the end of each heat-treating session.

The tube itself is 18" of 316L that I had professionally tig-welded (Kelly Couples aka octihunter).

Mine runs very smoothly. It has the salt melted in 15 minutes and holds temperature within +/- 3F. As long as the blades are clean and dry, it is an absolute breeze to use.

My only problem with it happened with my first tube. I had a guy weld me up a tube in 312 and the weld was horrible. Better than I could probably do myself, but not good enough to contain a small tube of lava in my shop. The weld failed after only about a month of use, and it leaked molten salt into the burning forge. This sent nasty vapors throughout the shop, that rusted whatever they chose.

So get some 316L and have it tig-welded by somebody that can really do it up right!

Oh, the forge body is 17.5-18" diameter, with 2" of kao-wool painted with ITC-100. The bottom of the forge is simply lined with kitty litter.

The one thing I want to add is some sort of spark ignition to it for running at lower temperatures.

Nick
 
I said I have to learn the hard way. I see Nick has learned a lesson too. The first salt pot in almost anybodys shop is likely to be trouble. All Im saying is to do your homework before building one. We are playing with fire and now hot molten salt. I havent even mentioned the seriosness when water in the salt happens. If there is only one drop of water on the blade it pops when it touches the hot salt. A wet blade causes a shower of hot salt.

Like Nick I too use a inconnel probe and remove it. The bottom of my pot is cold compared to the top so I have to stir the salts and let the temp stabilize. I should have put the flame inlets lower in the forge body.

My next setup will have a thermostat similar to Nicks.
 
I agree that it would be negligent not to stress that salt baths are very powerful and useful tools, ones that I personally would not do without in my shop now that I have had them.

But it would also be irresponsible not to stress that the things present a higher danger potential in more than a couple of ways. Grinders and chopsaws can give you boo boos; heck a buffer can stick you faster than a rabid convict! But a buffer doesn’t spit 1500-degree corrosive liquids in your face if you forgot to get that last drop of water or oil off the blade. Now you have a heat burn complicated by a chemical burn.

I know people who work for the power company and handle thousands of volts with big blue arcing sprays routinely. They can do it safely because they know what they are doing, they respect it, and they are always careful. But despite the fact that they have never been bit, they would never tell you that it is harmless and if they ever got too comfortable with it they would probably get out of the business.

Most bladesmith’s (myself included) shop practices are like a Three Stooges film festival. To be honest I am amazed that many forge shops haven’t been blown off their foundations yet, considering all the homemade gas forges out there. A careless moment with a forge is good for some laughs from an impromptu hairstyling, but with salt the stakes are getting up there to where we need to be responsible in how we approach it. If I make the salts out to be so dangerous that a guy getting started is careful enough never to get hurt, I can’t see the down side to that.

Low temp salts have burned me worse because of the way they stick and burn. They are nitrite/nitrate based, anything they come in contact with that is flammable is now much more flammable. Try putting out some paper that has been soaked in the stuff (I dip paper in it to make nitre paper and use it as a fool proof campfire lighter). Should you not control the heat on the low temp, they become very “unstable” (kaboom) at around 1200F. And this tube of molten saltpeter gets set right beside a glowing tube of chlorides.

If kilns are used, both make excellent conductors (I have experienced that wonderful tingly feeling)! If gas is used, it is a little more enclosed than a forge so that you can get a nice build up of gas before the ignition, how much will determine whether you still have a solid roof on the shop. I could go on and on with all of the things that could happen if you don’t respect the power of this tool.

One look at my shop would tell you that I am no safety nazi. I wear eye or hearing protection when I feel like it and hate being preached to about it, but I am also no fool and I have worked with salts for more than a few years now and am always aware that if you screw up with them, they can hurt you real bad and burn your shop down around your ears to help you remember it.

All that being said, the benefits outweigh all of the troubles by so much that the choice has always been quite simple for me.
 
Not that I can add any value here, but I love my salt pot. Mostly because of its ability to provide an oxygen free, super even, and fast heat to the blade.

Other than that, nothing really compares to the hypnotic ebb and flow that 1500° molten salt has on a guy like myself. ;)

No doubt, the stuff is wicked dangerous, I swear it is less viscous than water when it is at temp, but it has already been said; you have to give it the respect that it deserves.
 
Perhaps you guys thought I was trying to blow off the danger of salts.

If you read what I wrote, I am not. I understand wanting to make someone realize the potential dangers of salt. But I have heard makers taking it to the point that they are trying to scare people away from salts. I don't think anyone should be scared of them, they should UNDERSTAND and RESPECT them.

My point with the tool comparison is that with any tool in a knifemaker's shop you can get hurt very badly if you don't respect the tool and its full spectrum of possibilities and/or you get complacent with your shop practices.

Salts make for an extremely dangerous tool that is also extremely accurate and open many doors for meticulous heat-treating. I feel the positives far outweigh the negatives for a smith that is cautious and understands what he/she is getting into.

Nick
 
I thank you guys for all your input to this thread so far. I agree that salts are simply incredible tools for absolute confidence in heat treating, but can deal horrifying damage if taken for granted. I have worked around aircraft and machinery my whole life. The minute you get comfortable with something is all the chance it needs to hurt you. In 30 years of my working life I pray that not too many serious accidents have happened. I still have all 20 fingers and toes and Lord knows I have done foolish things, and its a miracle I still have them and my health.

I suppose I'll start out with the gas-powered pots. The thought of my meter spinning at warp speed when I turn on an electric kiln kind of makes my sphincter pucker.
 
No not at all Nick, I wasn't beating on you, your post made me realize that I could be percieved as being too negative, so I felt I needed to clarify why I was all gloom and doom and still rienforce the reasons why salts are so darned useful.
 
Originally posted by jhiggins
...I suppose I'll start out with the gas-powered pots. The thought of my meter spinning at warp speed when I turn on an electric kiln kind of makes my sphincter pucker.
Click to expand...

My meter does break the sound barrier when I am running my full stack, but that is for swords and it is four kiln sections stacked up to 4 feet high. A knife size stack (2 kiln sections) is not nearly as bad. I do think the gas is still cheaper, but whith the electric you then have the kilns to use for annealing and other operations. Decisions, decisions...
 
Originally posted by Sweany
Hey, Mr Higgins. lookie here

ZOE

and maybe this is what you were talking about?

Mstarling
Click to expand...

The one by ZOE is based on the same concept as my gas setup but many deatails are a littel different. I preffer the round tube to square and I still like my probe in the salts. I also like to actually put the tip of my probe against the blade as it approaches temp to make sure that I am where I want to be. You would be suprised at how much of a temperature difference there can when you move the tip 1" closer to the blade in those salts.

This looks like a good thread for folks to share their salt baths here are some images of mine (I wished I had some without me in the picture):

lowtemp.jpg

this is the low temp rig (electric)

hightemp.jpg

The high temp

units.jpg

another view

hoisting.jpg

Since I am both a knifemaker and a swordmaker, my units are stackable so that I can go with 24" or a taller configuration. I have a chain fall to assist in these operations.

anneal2.jpg

these are the controllers that run the rigs quite accurately.

I also have a page at my website that gives the basics on how they are constructed:
Salt Baths
 
"Most bladesmith’s (myself included) shop practices are like a Three Stooges film festival. " -Kevin Cashen

:D Kevin, you have no idea (well actually I guess you do). My saving grace is usually no one actually sees the bone headed things I have done. It's always bone headed in retrospect isn't it! Actually if we stopped and thought about things first we probably wouldn't do half of the silly things we do. Unfortunatley we'd still do the other half anyway.

Nick, I don't think anyone thought you were trying to downplay the dangers of salt pots. We know you better than that and I think your analogy with the inherent dangers of other shop equipment was appropriate. I think the main difference is there aren't many items in the shop that have the capacity of such catastrophic failure if something does go wrong as a salt pot. There are reasons why nitrites/nitrates are one of the primary ingredients of black powder, they are powerful oxidizers. The high temp salts are completely unforgiving of even minute amounts of moisture. (Water expands many thousands of times in volume, if I remember right, almost immediately in contact with molten salt.)
 
Kevin! I see you are using the Omega CN76000 series controller! I bought one of these (not from Omega however) for a bargain price and I am having trouble figuring it out. Do you suppose I could lean on you for a little tech support? The manual for it seems extremely basic on some matters and almost incomprehensible on others. Mine is set up for a 5 volt output, which most people are telling me won't control most solenoid valves (I'd need a switch of some kind in the circuit?)

I'd initially like to just set it up for use as a pyrometer only and then use the controller functions as I understand it better. Will it accept input from a thermocouple and show a temperature reading without any programing? (that is, can I attach a power chord and leads from a thermocouple to it, plug it in and immediately use it to say read the temp in my forge?)
 
I agree with Guy also Nick,,
I'm glad both sides of this is out.
The unknowing minds need to know.:) I'm one of them
that like to play with stuff..:)
 
You must log in or register to reply here.
Back
Top