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8:35 pm
November 4, 2010
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One of the reasons I got off the fence and signed up here (not that this doesn't seem like a great forum, but I already spend too much time on these here Internets :)) is that I emailed Frosty a question, and he suggested that I oughta post it here. So here goes.
I'd like to make a little propane fired heat treating furnace, mainly (though not necessarily solely) for regular mortal sized knives and the like, likely using something along the lines of a 20 pound propane tank as the shell. (The diameter is about right, but I might try to find something a little longer.) I'll monitor internal temp with a type K thermocouple, and tune it by hand. Right now I'm mainly concerned with simple carbon and low alloy tool steels like 1045-1095, W1/W2 & O1, so I shouldn't need to get very far above 1700 F for heat treating. What's really important is control, plus a furnace that can deliver a nice, even heat. Hot spots are bad. As propane furnaces go, here's one that a lot of folks seem to like:
http://www.dfoggknives.com/pho.....mForge.htm
The only problem is that I don't want anything that big; space is at a premium, so I need to scale it down quite a bit. (Plus I'm not making swords.) The problem with that is that Don's design depends on large interior volume to help even out the heat. So I'm really not sure how well it'll scale. I'm trying to think of ways to make sure the heat stays nice and even, despite the smaller size. One idea is some kind of baffle/flame diffuser arrangement. Another is firing the burner into a radiant tube, and letting that heat the larger furnace. A third would be some kind of long ribbon burner the whole length of the furnace. (The recent discussion of ribbon burners here and on another forum really interested me, but it sounds like a ribbon burner may be hard to tune back sufficiently for something like this.) Of course it would be possible to combine some or all of these ideas into one: e.g., a ribbon burner under a radiant shield, no so different frmo what you see in a propane grill.
I'd like to hear design ideas, suggestions for what type of burner to use, etc. If possible I'd prefer to stick with a naturally aspirated burner for simplicity. I'm not trying to weld in this thing. It doesn't have to set performance records.
Thanks in advance for your thoughts. Especially if they're, you know....good. 😀
9:35 pm
March 22, 2010
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11" long 8" black stove pipe with cast refractory inside, the ID is about 5". Venturi burner. Soft refractory bricks on the ends make it more efficient. These were bought at a pottery supply place in Seattle. Hope it gives you some ideas.
9:42 pm
November 4, 2010
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Gene, have you ever stuck a thermocouple in that thing? I'm just wondering if it doesn't have a hot spot where the burner impinges on the opposite wall.
9:51 pm
March 18, 2010
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If you just want it for heat treating, have you thought about a salt pot?
The little Johnson heat treat furnace is a semi-muffle design. The burners are under a raised shelf.
Johnson Atmospheric Oven Furnaces for Hardening, Tempering and Drawing
“There are painters who transform the sun into a yellow spot,
but then there are others who, with the help of their art and their intelligence,
transform a yellow spot into the sun.” ~ Pablo Picasso ~
9:58 pm
November 4, 2010
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Heh. Yeah, Grant, I've thought pretty hard salt pots. The main thing is that they skeer me a little, and a salt pot would be pretty serious overkill for what I'm doing right now. Plus I think a kinda cavernous, propane fired thing might give me a little more flexibility in terms of the size of the work, etc. But yeah, I've definitely thought about salt pots.
That Johnson furnace looks interesting. I might need to try to get a closer look at one of those.
10:38 pm
April 21, 2010
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No need to be scared of salt pots. But you do need to know what you are doing and talk with someone that does use them before doing so on your own. I made my salt pots and use it frequently. It is a gigantic help for knife making - I hand finish to a good 320 grit finish before heat treating and after treatment there is NO SCALE.:wavespin:
11:45 pm
March 22, 2010
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No, haven't used a TC in it. although before I retired I maintained some heat treat equipment in the facilties where I worked. (Just thought to add that tidbit.) When firing up the forge there is a hot spot opposite of the burner but once it gets up to heat, oddly,there is a small darker spot opposite of the burner, perhaps because of unburned propane hitting it and cooling that spot???
This forge takes about 10 minutes to get up to heat, although with small pieces, skewers, hooks etc. I stick them in within a couple minutes of lighting the forge.
I've made kaowool lined forges but prefer refractory lined, less stuff in the air, although a kao wool forge can be lined with refractory slip. That stuff eventually crumbles and must be replaced.
There is a CO detector about 4 feet away from the forge, mandatory in my opinion.
just my 2 cents worth
2:30 am
April 12, 2010
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im sure Tom Ferry could shed some helpfull light on this subject. Ive been contimplating building one myself.
3:29 am
November 4, 2010
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Bill Cottrell;3961 wrote: No need to be scared of salt pots. But you do need to know what you are doing and talk with someone that does use them before doing so on your own. I made my salt pots and use it frequently. It is a gigantic help for knife making - I hand finish to a good 320 grit finish before heat treating and after treatment there is NO SCALE.:wavespin:
OK, Bill. I guess the big concern is what needs to be done to prevent steam explosions. What do I need to know?
10:54 pm
April 21, 2010
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I have two burners on my salt pot. I light the top burner first, wait about five minutes and light the bottom one. I have never experienced any problems this way. I keep and eye on the salts while heating up and they appear solid until right at 1400 degrees then a small hole will open in the middle of molten material and the hole just gets bigger until the opening is full size.I do not put a spike in mine like some folks do. I NEVER EVER put a cold piece of metal in molten salts. Always preheat material in forge or whatever (I use a hand held propane torch) past the point of any condensation. The condensation will cause water to form on the steel and water in molten salts causes the salts to basically explode. Tom Ferry nearly lost an eye to this happening when he started using salts. Salt pots are like a lot of other things - engage mind before ignition and use thereof.
Feel free to ask questions - I use salts, Tom Ferry does, and Dave Lisch also as well as a lot of other knife makers. Daves set up is identical to mine since I built both:happy:.
8:38 pm
November 4, 2010
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Thanks, Bill. Do you use the tapered rod that goes into the molten salts as they cool, or do you find that isn't necessary when you heat from the top down? And do you do anything special to prevent the salts from picking up moisture from the air between uses, or is that not really a problem?
By the way, what salt recipe do you use?
11:35 pm
April 21, 2010
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No I don't put a spike in mine at all. The theory behind that is that it will create a hole in the salts when they solidify again after melting. Again theory is that the hole will prevent salt explosions. I have never had any problems with my pots and at times they have sat next to my back shop door for a couple of months in a very high humidity atmosphere. When heating up you will see some hot vapor coming off the salts but that is long gone by the time they actually melt.
I will have to look and see if I can find the type salts I am using - I'm sure I have the info somewhere. I do know Dave Lisch, Tom Ferry, Bob Cramer and I all use the same material.
If you decide to go with salt pots I do urge you to go see some in operation and get some info before trying it on your own.
4:03 am
NWBA Member
March 22, 2010
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Matt,
The salts are the ultimate in the way of controllable and repeatable heat treating. The heat treatment of the knife is the soul of the blade, its what makes it a knife and not a shiny piece of steel. heat treating is not just black magic there is a science to it and many ways to achieve it.
For a forge design any of them will work, although one with a blower can have more control and adjustability. The first thing you will need is any forge that will accomadate your blade length plus a couple inches. Second you will need to purchase a pyrometer so that you know what temp the forge is at. I have built numerous forge designs that will run and maintain a temp of 1500-1550 degrees when adjusted properly. It is important to be able for the forge to maintain the correct austenizing temperature so you don't over heat the thinner sections of the blade. Lastly I generally have more success with keeping a direct flame off the blade as Grant suggested by inserting a steel tube that is full length of the forge and big enough to heat your blade inside of it. These instructions with a little practice can be successful and repeatable.
The salts are the ultimate but as Bill has stated must be given respect. Many mix their own with various types but I do not. Bill's, and my salts came from Heatbath products and I believe they are called Sta-Hard 17, the are non moisture absorbing salts, so they will never absorb any out of the air. They are also a neutral salts as long as they stay under 1800 degrees, so they will not absorb carbon from the steel. Each type of heat treating salts has a specific temp range so you must know what steels you will be using and what austenizing temps you need to achieve. The tapered rod is to just ensure if the bottom melts prior to the top it doesn't build pressure and blow the top off. I run two burners offset from center towards the top so I ensure my top half melts first, so no need for the tapered rod. As bill stated all Steel in your shop at room temp has moisture on it so all things going into the molten salt Must be preheated to eliminate the moisture. Not a big deal but don't get ahead of yourself as I did when mine blew up as a cold blade went in. Hence if I don't answer my cell I am probably heat treating, lol.
Hope this helps
3:32 pm
November 4, 2010
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Thanks, Tom. I'm totally sold on the value of salts, and I absolutely agree that a good HT furnace -- regardless of whether it involves salts -- needs to be able to hold right at the appropriate austenitizing temperature for the steel, in order to allow soaking the steel with no danger of overheating. The inability to reliably soak is why I've stuck with a very limited steel selection up to this point. The desire to broaden my horizons is what got me started down this road.
I actually got started on a salt pot a while back, but I stopped in mid-stream because I wasn't confident that I had a handle on the safety aspect, and also because I wasn't sure I was up to rigging it to be self-regulating. I do have most of the materials I'd need to finish the project. How do you manage temperature control? Do you adjust your propane burners by hand? If it's feasible, I would prefer to go that route, at least to start. It'd keep the build relatively simple and also help keep the cost down. I could always add automated controls later.
Your salt mix sounds excellent, in the sense that it doesn't pick up moisture and shouldn't need to be rectified. Those were two of the concerns that I had about salt baths. I looked up Sta-Hard 17, and the working range is right where I would want it to be. Where did you get your salts? I've heard that Heatbath can be difficult to deal with if you're not a corporation buying industrial quantities.
And what are your pots made of? 304L or 316L stainless? What's the hottest that you run them (I'm wondering about structural integrity at high temps), and do you have a sense of how long they last?
4:05 pm
August 29, 2010
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Matt, I don’t have a salt bath HT system but like you I’d like to have it. I’m not an expert but I can tell you controlling temperature in a salt pot by hand would not be practical, too many changing environmental conditions you would never get it under control. However the price of computerized control systems have dropped dramatically, check out this http://www.auberins.com/index.php?ma...041fb92ac85d14 (it’s the PID controllers if the link doesn’t work right) these should keep your system under control and at the temperature you require.
it's been fun, later!
4:29 pm
November 4, 2010
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Thanks, Robert. I actually have a PID that should work. But a PID controlled propane system is going to require solenoid valves, and I think those get pricey. Electric heating is cheaper, and I even have the wire for it, but it has its own problems.
5:01 pm
August 29, 2010
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You’ll get no argument from me about the expense of salt pots. But before you go down the electric heating path you should check out how others have found the system. From what I’ve read corrosion from gasses and spillage take a short time work its magic with the coils, and the danger of molten salts coming in contact with the coils would have to be considered. I’m sure it’s doable but it would have its own problems and expense. I think it requires a minimum of two control valves to operate a gas system, but you need at least one for an emergency shutdown (with blown systems). I’m not sure but I would think a fan speed controller for a blown burner would be required to provide multiple heat levels for maintaining and raising the temp. when adding the work to the pot. I was thinking a ribbon burner would be an excellent choice for a burner, made long enough to spread the heat evenly and not cause hot spots (the salts may spread the heat quick enough that this may not be needed, you would need to ask the experts) please keep posting I’m eager to see how you do.
it's been fun, later!
5:35 pm
November 4, 2010
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Robert Suter;4120 wrote: But before you go down the electric heating path you should check out how others have found the system. From what I’ve read corrosion from gasses and spillage take a short time work its magic with the coils, and the danger of molten salts coming in contact with the coils would have to be considered. I’m sure it’s doable but it would have its own problems and expense.
Absolutely. These are some of the reasons that I backed off the salt pot idea for quite a while.
5:37 pm
NWBA Member
March 22, 2010
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Matt,
I use a pid controller and a solenoid valve, I got mine from Bill who got them off ebay. They are not that expensive if you look around.
Robert is right in that its a good idea to have a ball valve for emergency shut off but if electricity is cut off the solenoid valve will close also.
My tube is 316L and 1/4" thick. when using these salts, its not the salts that eat the tube, it is the scale formed on the outside each time you turn the forge off. I know numerous guys heavy walled mild steel tubing (3/8-1/2") once they shut the propane off they just close the exhuast opening with kao wool or brick. They have been running with the same tube as long as I have with no issues. But the 316L is the best choice.
Heat Bath is a pain and will not deal in small quantities or individuals, send me an email and I will give you a source.
[email]tomferryknives@q.com[/email]
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