A question about the construction of a molten salt furnace

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rallyx99

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Hello All!

This is my first post on this forum, and before I get on to my question, I want to take a second and thank all the BladeForums contributors. I've learned so, so much over the years from reading your posts. Thank You!

So, I've been making knives for seven or eight years now. When I started, I would put my blades in the forge, wait for them to reach the "right" color, pull them out and quench them. Then I'd throw them in my oven (sometimes along with dinner ;) ) and temper them. As I began learning more about metallurgy, I understood that I needed to upgrade my heat treating setup, so I built a PID controlled electric furnace. I'm working with larger knives now (mostly shortswords and long seaxes) and with harder to heat treat alloys and am planning on building a molten salt setup. I've done a lot of reading--including Kevin Cashen's sticky on the subject--but still have SO many questions!

First and foremost, I want to be safe. Most of my questions on the construction of salt pots have to do with safety. I was planning on buying a stainless steel pipe around ~30" long by 4"-6" wide, TiG welding it to a piece of plate (same alloy) and building a long, verticle, tube-like electric furnace out of refractory bricks, a steel frame and skin, hand wound resistance wire and a PID controller setup around it. Does this sound reasonable? One of my main questions is what alloy I should choose for the tube? Somebody once told me that 316L was the best choice, but I don't have any idea. Would 304 or 304L work? How about plain old 316? ...I really don't want to choose the wrong material and have it rupture on me because of heat stress... (BTW, I want to build high and low temp setups and a salt quench setup. And someday I want to build a carburizing setup lol)

Another concern is keeping my salts clean and neutral. First off, I have no idea how to do this (let alone how to test them), and secondly I am concerned that I wouldn't be able to maintain something that long and skinny...

Any thoughts, design critiques and suggestions you could offer would be greatly appreciated.


Cheers,

Tim Harbert
 
There is some great info by Kevin Cashen on saltpots in the stickys. Not a lot on there construction though. I don't have salt pots, but, have witnessed them in action and plan to eventually make a high temp pot for doing high carbon. As there have been no answers to your post. Here is what I have gathered. You can use standard or schedule 40 pipe but thicker schedule 80 would be better as it would last longer. 316l is more noble than 316, but 304 will work. 316 would last longer. It is a case of maintaining and inspecting your equipment no matter which stainless you use. There should never be any pressure involved. Heat and corrosion is the problem. I would bevel the pipe where it joins the plate and do a full penetration weld at this joint to avoid having any gap at this location. Propane burners are usually used because electric elements and salt don't make for long life. You would need to keep the elements from shorting to the pipe and any salt that got on them would cause problems. It would work but, would probably heat up slower and have more problems. Burners controlled by a thermocouple and PID work fine because you have a lot more thermal mass with the salt than with air in an oven and therefore it does not fluctuate or have radiation problems near as much. You only need the burner to swirl around the base 1/2 of the pot because solids and liquids transfers heat so much better and the whole column of salt will have the same temp. You need to have a tapered rod to place in the pot when cooling so it can vent from the bottom as it heats. The salt naturally attracts moisture and expands as it heats. The hole left by the rod when reheating allows for these. Water is your dangerous enemy. You must be sure anything that goes in the molten salt is completely dry. A tiny bit of water makes a lot of steam, especially at around 1500f! Preheat everything before it goes into the salt. Guy who showed me how do do it, laid blade and rod on top of the pot for a while before submersion. Be careful and pay attention as with everything we do. I believe it is the best method to harden non stainless as there is no worries about decarb and the heat is so even. I also need to learn more about maintaining the actual salts too.
 
rallyx99 said:
Hello All!

This is my first post on this forum, and before I get on to my question, I want to take a second and thank all the BladeForums contributors. I've learned so, so much over the years from reading your posts. Thank You!

So, I've been making knives for seven or eight years now. When I started, I would put my blades in the forge, wait for them to reach the "right" color, pull them out and quench them. Then I'd throw them in my oven (sometimes along with dinner ;) ) and temper them. As I began learning more about metallurgy, I understood that I needed to upgrade my heat treating setup, so I built a PID controlled electric furnace. I'm working with larger knives now (mostly shortswords and long seaxes) and with harder to heat treat alloys and am planning on building a molten salt setup. I've done a lot of reading--including Kevin Cashen's sticky on the subject--but still have SO many questions!

First and foremost, I want to be safe. Most of my questions on the construction of salt pots have to do with safety. I was planning on buying a stainless steel pipe around ~30" long by 4"-6" wide, TiG welding it to a piece of plate (same alloy) and building a long, verticle, tube-like electric furnace out of refractory bricks, a steel frame and skin, hand wound resistance wire and a PID controller setup around it. Does this sound reasonable? One of my main questions is what alloy I should choose for the tube? Somebody once told me that 316L was the best choice, but I don't have any idea. Would 304 or 304L work? How about plain old 316? ...I really don't want to choose the wrong material and have it rupture on me because of heat stress... (BTW, I want to build high and low temp setups and a salt quench setup. And someday I want to build a carburizing setup lol)

Another concern is keeping my salts clean and neutral. First off, I have no idea how to do this (let alone how to test them), and secondly I am concerned that I wouldn't be able to maintain something that long and skinny...

Any thoughts, design critiques and suggestions you could offer would be greatly appreciated.
Click to expand...



The tube needs to be heavy walled. Sch80 at the minimum. 1/4" or thicker is what you want.

Now, I will say this......If you don't know anything about salt pots - DON'T BUILD ONE. I am not being funny, here. They are not for those who don't understand them. You can get a forge or a HT oven and learn the hard way with little more than a bit of wasted steel and a few burned fingers. Salt pots can maim or kill you, as well as set the shop on fire.
This isn't a "read up on it and build one" type item. Either go to the shop of someone who is proficient with salt pots and get trained on using them...or don't use them.

If you want a proper tube, I believe High Temp Tools and Supplies sells welded tubes as well as the salts.
 
4" schedule 40 stainless has near 1/4" (.237) walls and schedule 80 has .337. But, still thicker is better. I have been around hot steel and various other things that can screw you up in a hurry if you are careless. Using salt pots makes me nervous and should. One of those things that have serious repercussions for if you screw up. Once when younger I got a tiny bit of water in a pot of molten lead and blew hot lead all over. Spectacular and I was lucky. Molten salt would be far worse. It is one of those things that require added care and your complete attention while using. I will approach with caution.
 
bladsmth said:
The tube needs to be heavy walled. Sch80 at the minimum. 1/4" or thicker is what you want.

Now, I will say this......If you don't know anything about salt pots - DON'T BUILD ONE. I am not being funny, here. They are not for those who don't understand them. You can get a forge or a HT oven and learn the hard way with little more than a bit of wasted steel and a few burned fingers. Salt pots can maim or kill you, as well as set the shop on fire.
This isn't a "read up on it and build one" type item. Either go to the shop of someone who is proficient with salt pots and get trained on using them...or don't use them.

If you want a proper tube, I believe High Temp Tools and Supplies sells welded tubes as well as the salts.
Click to expand...

Thank you for your advice! I know how inexperience can be a dangerous thing. I've found posts on how the process works, how to build the furnaces, etc., I'm just not sure how to keep the pots clean and neutral, how to test to see if they are clean and neutral and where to source the salts (including the HIGH temp ones), test kits, maintenance supplies/chemicals. Surely there is a source of information--book, website, forum posts, a class I could take, ANYTHING--other than just working with someone who "knows how to use them"??? By the way, I'm taking your advice and am going to go to a fellow knifemaker's house and learn how to use salt pots from him. He has a lot of experience and he is pretty safe, but I want to make sure I do things BY THE BOOK. So, is there a book to go by?
 
316 has overlapping chemistry with 304. 316 has a little bit of manganese added for creep strength at elevated temperatures. 316L the "L" stands for Low carbon which aids in weldability. So if you are going to make a weldment out of stainless steel and use it in a high temperature application 316L would be your stainless of choice.

John
 
Molybdenum is added to 304 to make 316 to reduce pitting corrosion, not Mn .
Yes , make sure you get info on possible safety problems and take precautions to prevent !! Today's news includes a huge fire in CA .A highway under construction was completely destroyed by fire .Construction workers using a torch in HIGH winds !!
 
mete said:
Molybdenum is added to 304 to make 316 to reduce pitting corrosion, not Mn .
Yes , make sure you get info on possible safety problems and take precautions to prevent !! Today's news includes a huge fire in CA .A highway under construction was completely destroyed by fire .Construction workers using a torch in HIGH winds !!
Click to expand...

That happened really close to me, actually. You would not believe the traffic mess it has caused. Apparently, traffic was backed up for 20 miles at times. Yep, safety is KEY; I don't want to be they guy that burns down the $40million bridge! :p
 
I just finished off building my hight temp salt pot. Mine is very close to Ed Caffrey's-if you go to his site, he has a page about his salt pot. A few things you may want to consider are how you heat it (electric vs gas burner) and the diameter of your pipe. I used 316L for the tube and plate, TIG welded together. I understood the 316L to be the more corrosion resistant and went with it, even though it's pretty expensive. Figured if I was going to build it, I was going to do it right. These are not things to cut corners with...

The larger the diameter of the tube, the more mass you have inside. With more mass, you can have more temperature fluctuations. The very center of your tube is further from the heat source. This can cause some variations in temperature (outside getting hotter while inside is coming to temp, longer at the lower temp while the elements try to bring it back up etc). Ed's advice is to keep the tube to the size of the largest width knife you'd likely make in order to minimize that temperature swing.

As for the heating elements, I haven't used or been around that variety. I don't know the particulars, but I wonder how efficient it would be to try and heat up such a large volume by that method. Take that with a grain of salt since as I said, I've not been around that kind of salt pot. I will say, a single propane burner at the bottom of my vertical forge holding a 24" tube is very capable of maintaining the heat properly and was pretty darned easy to build.

The others have hit well on safety. These can be dangerous and you don't want a mortar going off in your garage... I have a rod that goes in when the salts cool. Mine is also on a cart with wheels that gets rolled outside when it's time to use it (like the table my forge is on). Ensure you have protective gear, too and wear it. My face might not be much to look at, but it'll be even less if I get a big splatter of molten salts sprayed all over it ;). Everything in the shop carries risks, this one maybe more than others. Pay attention, educate yourself, and you won't become a statistic. Salt pots are fantastic for me-waaaay more precision than my eyeballing a blade in the forge. Good luck with your build.

Jeremy
 
Thanks for the good first hand advice.

As I said before, and say again;
High Temperature Salt pots are not for any maker but a very experienced one.
 
gudspelr said:
I just finished off building my hight temp salt pot. Mine is very close to Ed Caffrey's-if you go to his site, he has a page about his salt pot. A few things you may want to consider are how you heat it (electric vs gas burner) and the diameter of your pipe. I used 316L for the tube and plate, TIG welded together. I understood the 316L to be the more corrosion resistant and went with it, even though it's pretty expensive. Figured if I was going to build it, I was going to do it right. These are not things to cut corners with...

The larger the diameter of the tube, the more mass you have inside. With more mass, you can have more temperature fluctuations. The very center of your tube is further from the heat source. This can cause some variations in temperature (outside getting hotter while inside is coming to temp, longer at the lower temp while the elements try to bring it back up etc). Ed's advice is to keep the tube to the size of the largest width knife you'd likely make in order to minimize that temperature swing.

As for the heating elements, I haven't used or been around that variety. I don't know the particulars, but I wonder how efficient it would be to try and heat up such a large volume by that method. Take that with a grain of salt since as I said, I've not been around that kind of salt pot. I will say, a single propane burner at the bottom of my vertical forge holding a 24" tube is very capable of maintaining the heat properly and was pretty darned easy to build.

The others have hit well on safety. These can be dangerous and you don't want a mortar going off in your garage... I have a rod that goes in when the salts cool. Mine is also on a cart with wheels that gets rolled outside when it's time to use it (like the table my forge is on). Ensure you have protective gear, too and wear it. My face might not be much to look at, but it'll be even less if I get a big splatter of molten salts sprayed all over it ;). Everything in the shop carries risks, this one maybe more than others. Pay attention, educate yourself, and you won't become a statistic. Salt pots are fantastic for me-waaaay more precision than my eyeballing a blade in the forge. Good luck with your build.

Jeremy
Click to expand...

Thanks Jeremy! I'll check out Ed Caffrey's page. I haven't run into him in a long time and haven't looked at his site in even longer. I'm probably going to go with a propane setup too. What diameter of tube did you choose for your build, and how long does it take to get up to temp? Also, where are you sourcing your salts? I've seen super high temp salts at one of the supply houses (I don't remember which one) that will allow me to do my stainless (CTS-XHP @ 1950F) and CPM 3V (@ 2050F if I remember correctly), but don't really know much about the chemistry of them. I know some salts contain barium which I guess presents disposal problems as it's toxic... Anyway, thanks for pointing me in the right direction!

Cheers,

Tim
 
My forge is about one foot in diameter. It has one gas burner I made near the bottom of the forge, placed so it swirls around the interior. The lid is removeable and I cut a hole in it and welded some plate to make a directional chimney for the heat to escape. It's lined inside with 1" kaowool coated in ITC 100. This includes the floor and lid. The stainless tube is a bit over 2" inside diameter (can't remember exactly off hand, but under 2.5"). It's professionally TIG welded to a 3/8" plate on the bottom. It doesn't take all that long to get things molten, but it's not something I necessarily want to "hurry" along. Remember the mortar thing-way too hot early on melts stuff near the bottom really fast which means expansion...with solids above it... So, heat it up a bit slow. Again, mine is modeled after Ed's and it was very beneficial for me to get to see his running in person. With this setup, once the salts are molten, the burner is rather effective at keeping them close to the target temp.

As for your salts...I'm not going to give you specific suggestions. The reason is, every maker has a purpose for their setup. You have some things in mind for yours. I'd research those specific heat treats and decide if a salt pot is best for those. If so, onward you go. Next step (IMO) is to find and talk with or better yet visit makers who have the same thing you're wanting. I did a lot of asking and research when it came to the salts. Many folks have strong opinions on what to use or not use. But because I haven't had mine running for long, I think it best not to be recommending it to someone else. After I've used them many times with great success, sure. But if I end up with some problems for some reason down the road (which I'd be surprised at, but still...), I don't need to be out advising for it.

I will mention salts have a working temp range (when they melt, when they boil, etc). Generally, your higher temp operating ones melt at higher temps. If you're only doing stainless, ok. If you plan to do carbon steel, you need to plan accordingly because not all of those might work with one given salt. I'm not specifically familiar with the steel's HT you mentioned, but if there are specific ramp times/temps, etc., I'm not sure salts are your best bet. You may be better off looking towards an oven. Just food for thought.

See if you can get a data sheet for the salts you're looking at-it'll tell you a lot of good information. Couple that with sound advice from however many people you can find who've been using what you are looking at with successful results. If you can't find anyone who has used it....frankly....I'd steer clear. There are a lot of people who've been doing this and call me a chicken, but I'm just not willing to be the guinea pig with a new recipe. Maybe with a cookie, but not with molten salts ;).


Jeremy
 
Thanks Jeremy. I guess it's time to call the supply house, get data sheets and talk to the experts. I don't know anybody that is using the high temp set ups I'm thinking about, so I'm going to have to rely on the expertise of the people at the supply house.

I currently run a really nice oven and get great results. I do high volume and have been looking for a way to increase consistency, speed things up and stop using that annoying heat treating foil.

I'll keep you posted.
 
Gotta disagree about the size of the salt vessel... One of the biggest benefits of salt bath heat treating is the fact that the salt heats by conduction, which is extremely fast and consistent... much more so than radiation or convection heat transfer. That's why a blade will be the same temperature even at a needle point, as it will be at the much thicker cross section of the ricasso.

If I had the means to have a ceramic salt pot big enough to hang 20 bowies in it at once, I would! ;) :)

Salt maintenance is sort of a two part equation. For one, keep it as clean as you can in the first place--- only put clean steel in it, don't allow any dirt, oil, Dykem, scale, etc. in the salt.

The salts will need to be rectified periodically to keep them neutral. I use silica and a graphite rod.
 
I posted this video in another thread, but it's more relevant to your thread... :)

[video=youtube;o1zcdSowErA]https://www.youtube.com/watch?v=o1zcdSowErA[/video]
 
Nick, I love your videos... :). I just watched part two in the other thread and it was worth the wait.

I completely agree with wanting a salt pot that could do a bunch of blades at once-it would be awesome. I was just talking to a guy who works at a place where they have a great big one for commercial purposes. For a home setup, I just don't know how to heat it well enough to not get the bigger temp swings? After it falls under the target temp and the burner starts back up, I'm just wondering how much longer it would take to get it warmed back up? It sure would be cool, though..... Maybe if there were a few burners spaced out?


Jeremy
 
That's where a good controller comes in. They have a fuzzy logic and they actually learn the process and so tune themselves to the process. When I turn mine on it will swing high by about 15-20 degrees then undershoot by the same, but the next time is will over/undershoot by a little less, rinse repeat until it holds withing +- 1 or 2 degrees of the set point.

gudspelr said:
Nick, I love your videos... :). I just watched part two in the other thread and it was worth the wait.

I completely agree with wanting a salt pot that could do a bunch of blades at once-it would be awesome. I was just talking to a guy who works at a place where they have a great big one for commercial purposes. For a home setup, I just don't know how to heat it well enough to not get the bigger temp swings? After it falls under the target temp and the burner starts back up, I'm just wondering how much longer it would take to get it warmed back up? It sure would be cool, though..... Maybe if there were a few burners spaced out?


Jeremy
Click to expand...
 
Nick: thanks for the video! It's great to see your setup. Why did you decide to use a blower instead of venturis?

Matthew: do you have a similar setup?

Do any of you guys have a good source of 316L tube? My local place wants $1100 for a 20' piece of 5". I'm not really complaining about the price, I'm complaining about the quantity!

Maybe we could forget the tube altogether and use a square/rectangular tank? Would we need to use stainless, or is there another material that would work? Nickel alloys? Ceramics? Titanium? ...surely they don't always use tubes in industrial applications?...
 
I got my 316L at a machine shop that sells steel. They were happy to cut the length I needed, and it wasn't too expensive either. Well, compared to the prices I had been quoted on 20'-22' lengths. I really don't remember for sure, but wanna say it was about $60.

You can use regular 'ol black iron pipe, but it will fail much sooner than 316L.

The most important thing is that it's welded up properly. I built my salt rig when I barely knew how to turn a welder on, so had a millwright make me a salt vessel. He did a less than stellar job welding it with a stick welder and it failed. HUGE FAIL!!!

Find somebody that really knows their stuff to TIG weld it. Kelly Cupples welded my second salt vessel, and it could not have been done better than what Kelly did. :thumbup:


There are a lot of other things that would work, but most of them would be harder to come by and/or cost more. Quite a few of the commercial units I have seen, have some type of ceramic basin for the salt vessel. The cheapest ones of those that I've seen, was $10,000. :eek:

edited to add* Regarding the burner set-up- is there a reason to use venturi instead of blown? ;) I knew I could make a blown forge that would have the control I needed, especially when combined with the PID controller and solenoid. I've ran it with a Reil style venturi burner just to see if there was any difference, and the only thing it changed was it took longer to melt the salt. At working temps, it ran about the same.
 
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I got like from onlinemetals. The tube, along with some 3/8" plate was over $100. I shopped around some welding shops and ended up getting lucky. A friend knew a guy who did TIG welding. I paid him to do it and it turned out very very nice. The steel and the welding cost me a decent chunk of money, but again-this just wasn't something to be cutting corners with....

Hey, Nick-does your pot's PID work like Matthew's to "learn" when to click on and off? Guessing my cheap PID isn't that high tech... With that kind of setup, I'd think the larger diameter tubes would be that much more workable with very manageable temp swings.


Jeremy
 
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