High-temp salt pot build - Some (informed) questions

[dustfurn]

I'm looking to upgrade my heat-treating capabilities. Currently, I can successfully heat-treat (12" max) simple carbon steels in my forge, using a thermocouple and manual temperature adjustment (about +/- 50 deg).

I'd like to build a high-temp salt pot setup similar to the one shown in this thread: http://www.bladeforums.com/forums/showthread.php?t=647313, but long enough to accomodate sword-length blades.

I have a few questions, but here's my current plan. Please let me know if you see anything in my proposal that is obviously crazy (or even just sub-optimal)!

- 4" ID, 4.5" OD, 36" long 316L welded pipe: (http://www.mcmaster.com/#stainless-steel-pipe/=44tbsf).

I'm going to have this professionally welded to a stainless base. Planning on coating the outside of the pipe with ITC-213 for added durability. Anything I should let the welder know, specifically?

Any comments on this? Any reason to avoid the welded pipe and go with seamless? is the ~1/4" pipe wall thickness really necessary, or could i *safely* go with the 0.12" thickness?
Anyone know of anywhere I could get this any cheaper (but the same quality)?

- A T-Rex venturi burner (http://www.hybridburners.com/pics/trex-903.jpg)
- This PID controller: http://cgi.ebay.com/UNIVERSAL-PID-T...ultDomain_0?hash=item2ea3ed7104#ht_1690wt_990
- This solenoid valve: http://cgi.ebay.com/1-4-Electric-So...ultDomain_0?hash=item439a935b30#ht_2063wt_765

Any comments here? I've never put together a PID/Valve system before, So this is the part of the project that I am the fuzziest on. Do the controller & valve that I linked above look adequate? Any better recommendations? Any tutorials or comments on putting together a PID/Valve venturi burner?

- Building the furnace out of a hot-water heater shell, 1" wool + satanite + ITC-100.

- High-temp salts from Darren Ellis.

- I need a plug for the salt-pot. Is a straight 1" bar good enough, or does it need to be tappered?

I've tried to do as much reading on the subject as I can, but if you have any relevant links, please send them my way. I really appreciate any & all help you folks can give.

Also, I am well aware of the safety risks involved here, so please keep the "You'll burn your face off!" comments to a minimum. ;]

Thanks,
Dustin

 

[Stacy E. Apelt - Bladesmith]

Keep reading.
The pipe needs to be the best and most robust you can get. 312/316, seamless, and at least 1/4 thick. Schedule 80 is a minimum. No need to coat it with ITC-213. You want it to heat up, not be insulated.
That solenoid is for air and light liquid fuel. A propane solenoid or fuel gas solenoid is different. Again, get the right part for the job.
The welding needs to be done by someone who is familiar with high temperature welds.
You don't want to run a venturi burner with a PID and solenoid, go with a blown burner. For a 36" salt pot, you will need two or more burners.
Darren Ellis sells complete welded tubes that are the right thing for the job. He also sells tubes for you to weld. If you aren't sure about it, buy the pre-welded one.

Why the pickyness on parts and such????

A salt pot is a high temperature container of lava. A sword length pot is a volcano full of lava. Things can go wrong with pots and the results can be tragic. If you are not 100%,...no, make that 110%...., sure about everything involved with one, don't build one at all. If skimping on materials cost is an issue, don't build it.

I would suggest building a controlled 36" HT forge with PID control, a two-stage blown burner setup, and a muffle. It will allow very tight regulation ( as close as +/- 2 degrees) and can HT swords or knives. I would build one of those first, and when you have all then bugs worked out and can operate the forge flawlessly, consider building the same thing in a vertical form with a salt tube instead of a muffle.

Finally, talk to someone near you who uses salt pots and get some first hand experience with them if at all possible. Brain surgery and salt pots are not the best things to be self taught.

Stacy

 

[dustfurn]

Hi Stacy,

Thanks for the advice.

Keep reading.

I'd love to! This is (as I stated) one of the reasons I am posting this thread. Can you supply any links or reading material?

That solenoid is for air and light liquid fuel. A propane solenoid or fuel gas solenoid is different. Again, get the right part for the job.

...

If skimping on materials cost is an issue, don't build it.

Yes, my goal is to get the right parts for the job & I do not want to skimp on material costs, which is why I posted before starting to order parts and start the build willy-nilly. That being said, I don't see anything wrong with ordering pipe of the correct type myself and having a professional weld it up. From what I can see, this would be cheaper than ordering the equivalent part from Darren Ellis. Am I missing something?

I would suggest building a controlled 36" HT forge with PID control, a two-stage blown burner setup, and a muffle.

This is definitely something that I'm considering. Just curious, why are you so insistent on using blown-burners? Several folks I've corresponded with claim that modern high-performance venturi burners can be just as efficient as blown burners. Is this not true? Are there other considerations besides efficiency, performance and the burn atmosphere? I'd love to hear from the "other-side".

Thanks for your time,
Dustin

 

[Ed Caffrey]

My input is as follows:

I agree with Stacy on the pipe used for the tank! As someone who has had a salt tank rupture, I can tell you its not pretty went a tank lets go. Use at least schedule 80, and if at all possible, seamless. A thought on the size to use is this... Use a pipe that is ONLY as large in inside diameter as the widest knife you ever intend to put in it. The smaller you can keep the thermal mass of the salts, the easier it will be to maintain even temps. Often times I have had individuals call me saying that their salt tanks take huge swings up and down in temp....without exception each of them were using 4"+ ID tanks.....thats just way too much thermal mass...and totally unnecessary. It will just make things more difficult as far as temp consistency goes. Unless you must have 36" of length, and are going use it, its going to create a lot of issues for you...with that length of tank, two burners are a must, and three would be much safer (although much more difficult to setup and operate with a PID) The thing you have to keep in mind is that these "salts" expand greatly as they melt, and if you melt the bottom before the top, you can easily create a motar in your shop. Use the shortest, smallest ID tank you can to get the job done.

As far a burners go, you do not want a burner that puts out an intense, high velocity flame/heat. Something "mild" will work much better, and will reduce temp swings. Your goal with a salt pot is to slowly melt and bring the salts up to temp, and then (the main thing) is to maintain that temp as accurately as possible. I personally use a venturi burner, which runs at approx. 1.5 psi, because I did not like the "harshness" of a blown burner.(I did use a blown setup at first but switched to venturi)

As far as a PID goes, I recommend this one: http://cgi.ebay.com/PID-SSR-TEMPERA...emQQptZLH_DefaultDomain_0?hash=item518d70f943

With a dual line readout, you visually see your set point...with a single line readout you will not, and eventually forget where is was set, and it will be set incorrectly for what your wanting.

After saying all of that, and thinking as I wrote it, my advice is that if you've not dealt with salt tanks, I would highly encourage you to build a small model, that will work with a single burner first, and use it for a while to learn all the ins and outs before you go so big....it might just save you a lot of heartache and a shop disaster.
Unless your familiar with the operation and use of salt tanks, there are a number of things that you would not normally consider, that could get you into trouble...especially trying to start out as large as you mentioned.

 

[SteelSlaver]

4" Schedule 40 has a .237 nominal wall and schedule 80 has .337. I doubt you will find much welded seam pipe anymore in these schedules. If it is welded seam the weld should be of the same material and full penetration. I would trust that weld almost as much as seamless and more than most welds you will get on the base. You maybe able to get it cheaper from an industrial pipe supply or by asking them who is doing pipe work and check them out. Refineries, chemical plants, food processing plants and the contractors who serve them. The base weld needs to be high quality and full penetration and because no one can weld 36 inches down inside a 4 inch pipe the pipe will have to be beveled and then welded out. The people in this industry will also know how to make the weld.

Another thing about salt pots is water. Water expands 1500 times when it makes steam at 212 degrees and at 1500 degrees this would be increased by several factors. If you get even a fraction of a drop of water inside that column of molten salt YOU WILL HAVE A VIOLENT EXPLOSION and molten salt will fly all over. Even a piece of cold steel that has been laying on a shelf in a slightly humid area may have enough moister on/in it to cause this effect. Extreme care must be used in the operation of these. I was at Nicks when he used his. I notices he used his just outside the shop, was careful to let the blades heat a bit before he stuck them in. He also had a long tapered rod that he place in the salts so that when the cooled it was inside and then he tapped it lose and removed it so when the salt heated it had a way to expand.

 

[bruss]

you can get a full penetration weld on the base plate by grinding a bevel on the OD and then purging the inside with argon, it will pull the weld in, an old boiler makers trick (good enough for ASME code heat exchangers too) at work we make salt pots a couple of times a year but with an outside titanium liner in case something goes wrong, it catches the mess and turns an odd colour in the furnace so they know something happened

 

[JCaswell]

I have 3 sword-length salt pots. One high-temp, two "low-temp."
Use stainless pipe, 1/4-inch or better. Welded plate on the bottom. If you don't weld it yourself, a pro-welder will understand what your needs are if you explain the apparatus. It's no big deal.

As mentioned above, a 2-line PID is preferred.
Just make a burner. (Just build it off the burner on Don Fogg's site. Come to think of it you should definitely consult his tutorial on building a salt pot!! http://www.dfoggknives.com/hightemp.htm ) The burner will cost you less than $30 with a fan.
The first sword-length unit I did had two burners. I find found the extra burner unnecessary, and have since noticed a large heat treat outfit in the area running very large baths (36-inch tubes x 14 inches) with one big burner at the bottom.)
I'd suggest you use one burner as described on Fogg's site and position it toward the bottom of the apparatus. When first heating a long tube of cold salt from the bottom, I place a 3/8 inch (heavy) plate loosely over the top of the tube. Sometimes you'll get a little splurt of hot salt as the salt heats from below and there's a cap of cold salt near the top. The plate is just to keep any splatter down. Usually there is none, but now and then there is and I have found a plate over the top is a sufficient precaution. You remove it once the salt is all liquid. A blown burner is preferable IMO. It's more adjustable, simple and cheap. I'm running mine on natural gas. They heat fine, and the gas is practically free compared with propane.

On mine, I used cast refractory top and bottom--about 2 1/2 inches, IIRC. I ran the compressed ceramic fiber wool "sheet" and a layer of 2 inch heavy-weight ceramic fiber on top of that--top and bottom. I lined the sides with the compressed ceramic fiber sheet and covered it with more 2-inch material. One-inch is insufficient IMO, you'll have a really hot apparatus. Keep the heat inside and everything's more efficient and keeps your apparatus from struggling to maintain consistent temperature. Mine will swing about 3 degrees at 1550F. They run very mellow cycling a 1-second blast every 5 seconds or more.

I also have a couple knife-size units. It is good advice to build a knife-length unit first. You'll want one anyway. It's kinda a waste to light up a sword-length unit for a couple little knives.

I used to run mild steel tubes. A lot of big heat treat outfits do. They just change them fairly regularly. After a few years, I changed for heavy SS and never looked back.

Here's a picture of a few of mine:
http://www.bladeforums.com/forums/showthread.php?t=441141

Good Luck!:thumbup:

 

[Kevin R. Cashen]

I almost passed over this thread in my quick perusal of this forum, since I have backed off from giving salt bath advice for a couple of reasons. First is the attitude that heaven forbid we may be pushing a fancy, high-tech tool agenda and destroying the traditions of bladesmithing, and perhaps even the fabric of space time itself. But more importantly between the inherent cheapness of the average bladesmith and our tendency to have to cobble up and half@$$ everything, I don’t need the responsibility. If a guy has a need to quench into rancid lard instead of prying open his cobwebbed wallet for some good quench oil, that same attitude is going to get somebody killed with salt baths. I am not saying this lightly, it is not a boast that a good number of people using salts got started with information from me and it wasn’t far into that trend that I started seeing people mixing up homemade salt recipes, running without controllers and not respecting the forces present in a melting salt tube.

I feel the most important thing you listed in your first post, is the “plug” steel rod. Make it tapered, as it should not plug anythign but create a vent hole. It must be placed in the tube after each session to allow the salts to solidify around it, and then before firing the next time a few sharp raps with a hammer will allow its removal leaving a chimney from the bottom to the top of the salts. Without this vent, just covering the top of the tube is like just putting your hat over a grenade. I had a plate of 3/8” 316L for the bottom of a tube once bubble out to make the whole thing look like a test tube from the pressures involved in a sword length tube with a liquid bottom and solid top. Please make a tapered rod and use it, the vent hole will also relieve pressures on the tube from the solid salts expansion and increase its life.

Be careful with the solenoid valves, most are not made for gas, you want a gas burner valve, I have always used the Redhat series by Asco and have been happy.

I would stick with the “T-Rex” venturi burners. I have been wanting to ditch the blower based burners for some time and this year after helping a higher production manufacturing business set up their heat treating with the “T-Rex”, I made the change myself and they have been wonderful! Fast and even heat ups and solid steady regulation and cycling, running at around seven PSI of LP, and no extra electrical hassles and back fires from the blowers.
Now that I got you to read my post let me say this:

...Also, I am well aware of the safety risks involved here, so please keep the "You'll burn your face off!" comments to a minimum. ;]

Respectfully- NO! And I hope everybody participating in this thread cares enough about your well being to disregard your request as well (it is the reason I decided to post). I have been working with salts longer than many on this forum have been making knives, I introduced many to the concept, I have now consulted for several commercial firms on heat treating (two of which make swords)… and I will never reach the point or level where I don’t need to be reminded how dangerous these things can be. You may find the safety lectures annoying but I am here to say that is because you have yet to reflect on your life while shaking because a 1500F NaCl cannon went off right where your face was just 2 seconds ago, and you can see the black scorched circle on the ceiling that would have been your head! Some say I am an alarmist, I say they have been way too lucky with their salts if they are not. And the idea that we should lighten up with emphasizing the dangers of these things is the main reason why I no longer freely hand out advice about them.

 

[dustfurn]

Thanks to everyone for your replies. Very, very informative. I know it's starting to become a cliche to say this, but: This forum is amazing!

I have decided that I'm going to take Stacy's advice and first attempt a long temperature controlled HT forge, see how that goes, then if I feel up to it attempt a salt-pot rig.

I'm going to stick with a venturi burner (probably the T-Rex). That being said...Do you think I will need 2 burners? Also, would this solenoid be appropriate:

http://www.drillspot.com/products/4..._120_60_110_50_General_Purpose_Solenoid_Valve

If not, what exactly should i be looking for in a solenoid?

Respectfully- NO! And I hope everybody participating in this thread cares enough about your well being to disregard your request as well (it is the reason I decided to post). I have been working with salts longer than many on this forum have been making knives, I introduced many to the concept, I have now consulted for several commercial firms on heat treating (two of which make swords)… and I will never reach the point or level where I don’t need to be reminded how dangerous these things can be. You may find the safety lectures annoying but I am here to say that is because you have yet to reflect on your life while shaking because a 1500F NaCl cannon went off right where your face was just 2 seconds ago, and you can see the black scorched circle on the ceiling that would have been your head! Some say I am an alarmist, I say they have been way too lucky with their salts if they are not. And the idea that we should lighten up with emphasizing the dangers of these things is the main reason why I no longer freely hand out advice about them.

Thanks for the concern, Kevin. By saying:

Also, I am well aware of the safety risks involved here, so please keep the "You'll burn your face off!" comments to a minimum. ;]

It was just my way of saying "I understand the theoretical dangers of high-temp salt pots (I'm not an idiot, and I'm only marginally ignorant) but I know you guys are still going to feel the need to warn me -- that's fine."

Thanks,
Dustin

 

[J.McDonald Knives]

What about using Drill Collar for the pipe? Its about 1/2" thick.

 

[JCaswell]

"You'll Burn Your Face Off!"
Well probably not if you use good sense and add an extra dose of caution to account for the inexplicable "Who woulda thunk"/ freak accident factor.

If you've ever seen burn victims--especially facial burn victims .... well, heat treating a knife just won't seem that important if you get burned.

When I light up salt pots, I stay away from them except for quick checks to make sure things are going right and to actually do the HT. I always wear a leather welding jacket, welding gloves and a full-face shield when I'm around them from the moment they're fired and absolutely recommend the same.
I've been using salts since 1995 and have found it to be a pretty docile medium, however, as Kevin and other point out, there's potential for DISASTER inherent in the process, so be sober in your approach to the whole thing (as I trust you are), proceed with care and I think you'll find the technique offers great advantages.

 

[dustfurn]

JCaswell,

Any tips on where to find the "compressed ceramic fiber sheet" you mention here? I can only seem to find the Inswool blanket material.

On mine, I used cast refractory top and bottom--about 2 1/2 inches, IIRC. I ran the compressed ceramic fiber wool "sheet" and a layer of 2 inch heavy-weight ceramic fiber on top of that--top and bottom. I lined the sides with the compressed ceramic fiber sheet and covered it with more 2-inch material. One-inch is insufficient IMO, you'll have a really hot apparatus. Keep the heat inside and everything's more efficient and keeps your apparatus from struggling to maintain consistent temperature.

By the way, I checked out your website...those are some really nice knives!

Thanks again,
Dustin

 

[JCaswell]

JCaswell,

Any tips on where to find the "compressed ceramic fiber sheet" you mention here? I can only seem to find the Inswool blanket material.



By the way, I checked out your website...those are some really nice knives!

Thanks again,
Dustin

The last time I bought refractory materials it was from Smith-Sharpe:
http://www.ssfbs.com/
They were fast and cost-effective at the time. I would start with them.

The CF sheet or "paper" has a higher insulating rating than the wool.--much higher. It's also quite expensive. You usually use in space-sensitive areas. I like the added barrier it adds when used in conjunction with the heavy-weight 2-inch wool (there are two weights of 2-inch inswool. Get the heavier stuff). The shell of my apparatus doesn't get very hot. I think the lower heat loss contributes greatly to the ease of maintaining steady temp. and the unit's overall efficiency.
I would recommend getting a sack of cast refractory for the bottom too. My tube sits on a 2 1/2 inch pad of refractory cast right into the bottom of the apparatus. On top of that is some 2-inch wool. I also do the top too along with the CF lining.

***Be careful with CF. I hate using the stuff and only use it in the salt pots anymore and that's only because it's the easiest way to insulate the vertical round sides. I don't use it in forges where it can get knocked and release fibers into the air while in use. Ceramic Fiber is listed as a "probable human carcinogen" at the CDC. Use respirator, long sleeves, and shower after handling the stuff.***

Thanks for the kind words regarding my work!

 

[Stacy E. Apelt - Bladesmith]

The main difference between a venturi and a blown burner is that the venturi is set up and balanced initially....and pretty much stays that way. A blow burner can be balanced for different atmospheres.

On a PID controlled venturi burner, it cycles from High to Off. The gas re-ignites each time the solenoid reopens. On a blown, two-stage control, the burners cycle from a setting slightly higher than the target to one slightly lower. The burner never shuts off. it just purrs along ....no huffing and puffing. It sounds like it is breathing.

I posted plans several times in the past for my two-stage controller. Here is a link or two. I still am building a new and improved system. The forge blowers finally came from England.

http://www.bladeforums.com/forums/showthread.php?t=523845
http://www.bladeforums.com/forums/showthread.php?t=635924

On the subject of solenoids, I used to use most any red hat unit by ASCO. I have since learned that the solenoids for LP gas are different that the ones for air, and light liquid fuels. Yes, the wrong ones will work for most simple setups, but the right one will be more reliable , and in the case of a very tightly controlled unit, it matters. A new solenoid for LP costs from $60 to $80. Ones can be found on Ebay for as low as $5 ( I bought one two days ago for that). Just check the PDF on the burners listed in the auctions and see what they are rated for.

Stacy

 

[dustfurn]

I've got my hot water heater cut up and I've got most of my parts ordered at this point.

To reiterate, I've decided to make a HT furnace instead of trying straight away for a salt-pot.

With that in mind, I have a few questions:

-Vertical or horizontal? I'm leaning towards trying a vertical forge as:
o it would be closer to what I'd be making if i were making a salt-pot rig.
o i've heard, anecdotally, that vertical sword HT setups solve the potential "drooping" problem.
o would save floor space.

-Length? I'm thinking of going for a total length of ~45 inches. Does this sound reasonable? The only argument I can think of for going longer than this is that it may allow more area for the heat to evenly diffuse through, resulting in a more even heat.

-Burner positioning? Most pictures I've seen of long HT furnaces show a single burner entering into the lower back section of the furnace, inline (parallel) with the length of the furnace. Here's a link, since my description is probably pretty sub-par: http://jhbladesmith.com/heattreatingforge.htm.
o If i go vertical, this burner position is not really practical.
o In either case (vertical or horizontal) is this position really the best? Intuitively, it seems as though this position would give you a heat gradient, with the tip of the blade receiving the most heat.

Thoughts?

Thanks,
Dustin

 

[65535]

I'd probably try putting the burner towards the bottom at a tangent angle to the ID of the chamber, get a vortex effect which should evenly heat the chamber best.