View Full Version : heat treating kiln

01-28-2008, 07:03 AM
Hi all,

Does anyone have any experience using a small electric ceramic kiln for heat treating steel? Paragon,Quick Fire, makes an inexpensive kiln.. under $250.00.. for enameling and such. I want to heat treat O1 steel to make plane irons.

Pete H
01-28-2008, 09:09 AM
I'd say the main question would be whether it will reach the critical temperature for O-1 -- which is somewhere around 1400-1500 F. I don't know what the glaze point for enamel is, but I recall that some ceramic glazes - majolicas, I think - flow at a relatively low temp, around 1000 F, too low for heat-treating.

I've used a small electrical "muffle furnace" for heat-treating steel (small knife blades) and it was just fine. That one was used (in its proper application) for ashing stuff for chemical analysis.

Here's a link you might find interesting:

Pete in NJ

Alistair Hosie
01-28-2008, 09:37 AM
Hi look under small burn out furnace under Dental in Ebay you will likely find something there Alistair

01-28-2008, 10:36 AM
i don't heat treat, but the temps will be fine...you've just got to set up some type of temperature control

01-28-2008, 10:39 AM
The only possible problem I see is temperature control. If you can get that accurate enough you should be fine.

Forrest Addy
01-28-2008, 11:09 AM
The analog temp controllers used in cheap ceramic kilns are pretty poor for heat treating metals. You need something like this:

http://cgi.ebay.com/PID-Temperature-Controller-SSR-Kiln-Furnace-Fah-Celsius_W0QQitemZ350018275212QQihZ022QQcategoryZ50 926QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

This sale consists of a simple temperature controller and a K type thermo-couple. You'll need a contactor or a solid state relay to handle the current to the kiln.

Here's a solid state relay that will be suitable but you need a low DC control voltage for it. Maybe there's a source on the controller:

http://cgi.ebay.com/SSR-Solid-State-Relay-40A-Output-24V-380VAC_W0QQitemZ350017485131QQihZ022QQcategoryZ363 38QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Here's a suitable AC contactor:

http://cgi.ebay.com/SQUARE-D-8501CO6V04-GEN-PURPOSE-POWER-RELAY-SPST-30A-AC_W0QQitemZ250208503924QQihZ015QQcategoryZ97184QQ ssPageNameZWDVWQQrdZ1QQcmdZViewItem

Look into "process controller" "temperature controller" ect in eBay, Google, etc. This topic gets technical in a hurry so have patience if you don't immediately understand the words and gadgets used to control kiln temperatures.

Then there is atmosphere control. Most use stainless foil wrap to exclute oxygen from the work surface. A couple of charcoal briquettes tossed in the link before heating will scavange oxygen too and it works for me. But I seldom see this trick used - possilbly because of the carbon monoxide and the Poof! of igniting gas when you open the furnace door. Watch you don't singe your eyebrows.

Alistair Hosie
01-28-2008, 11:17 AM
Heres a little beauty, as said look under dental furnace there are quite a few cheap ones this one may not be big enough .regards Alistair

http://cgi.ebay.com/Jelenko-Dental-Glazing-Furnace_W0QQitemZ270207136912QQihZ017QQcategoryZ11 7401QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

01-28-2008, 12:28 PM
I use an Olympic front loading kiln with a side hinge and an electronic controller with thermocouple. It works very well for heat treat but needs an extra blanket in the winter if I'm shooting for 1900+ degF. I wouldn't use it to HT HSS but for any common tool stell it works very well and runs on 120V. I don't see the exact model on their site: http://www.greatkilns.com/

(A top load or under hinge door is a real PITA if you're trying to remove something for quenching. For some reason the pottery and glass folks don't seem too worried by that...)

01-28-2008, 12:35 PM
Works fine. I have a Cress, that goes to 2300f, internal size is about 18" cube. It takes 3 hours to get to 1550, and $5 worth of electricity.
I also have a Fire Master, 110v about 7" cube inside, 90 minutes to 1550 and 75 cents.
I put digital controllers on both, ebay.

Warning: There can be a huge temperature gradient in a kiln, from the bottom to the top. Place the sensor at the same level as your parts, or in contact with a part or a metal shelf under your parts.

Heat treating oil from McMaster, scale preventing crust from Brownells, tool wrap swapped for welding from a guy on PM.

And then there is the Rockwell tester.... (Another story.)

01-28-2008, 02:42 PM
I do heat treating all the time for knife and tool blades.
There are a couple of good books in ENCO cat.
Also Centaur Forge (blacksmith's/knife making supplies).

Use a K type thermocouple (higher temp than J type) and a digital controller with <1% full scale accuracy.
Temp. control and even heating is the most important thing. Also having a tempering kiln close by and up to temp (lower temp, even an old oven could do) is important because these steels need to be tempered soon after hardening.

Try to set up a gas port in the oven or SS tool wrap with carbon bearing compound to control atmosphere in the oven or around your project and not lose carbon.

With the right set up you can do some very cool hardening and tempering...even sub zero (cryogenic Liquid N) quench that gets you a couple more RC points and really refines the grain of the material.

There are lots of exotic tool steels that open up a whole new business area or hobby.

Have fun!

01-28-2008, 05:20 PM
Also having a tempering kiln close by and up to temp (lower temp, even an old oven could do) is important because these steels need to be tempered soon after hardening.

Is this really true?

Few years ago I called Starrett about this and they told me that it's not that important, and that they themselves sometimes temper the parts a day later.

I recall reading from other sources that tempering should be done immediately after hardnening.

01-28-2008, 07:52 PM

For the steels found in most HM shops, tempering today, tomorrow, next week are all the same. Only when you are trying for the absolute best in strength and toughness does the time between quench and tempering become important.

01-28-2008, 09:57 PM
I personally temper as soon as things are stable after hardening, this does not mean it is correct!
The reason I do this is that steel in a hardened state is in fact under stress. Although the material could stay in this state for a long time without degradation it is vulnerable to damage from shock or physical impact.

The most interesting thing about this is that metal in this state is crystalline.
Under stress there may...I repeat may. be micro fractures that affect final tempering. These stresses could be caused by temperature swings or physical shocks by dropping or stressing in a vice or with a tool.

If you are interested, I could reference engineering papers about how various alloys respond to heat treat and temper.

Starrett offers a limited variety of tool steel. W1, O1. A2 These steels are by necessity very forgiving in heat treatment. I personally have heated in a coal forge and quenched in Hyd. Oil, then tempered with a Oxy Ace torch by bluing the back of the blade and leaving the edge straw. Then plunging the whole thing in water!!! This done with Starrett O1 tool stock.

No problem! however a careful look at the blade shows stress at the differential temper lines when tested by destructive test (hammer).

This is no problem in a Japanese blade or knife, chisel, plane blade.

In a cutting tool used at high speed and temp with a unique rake or edge design this may not be good enough.

At any rate its a lot of fun to experiment test hypothesis and make tools.


01-28-2008, 10:03 PM
Thanks for the clarification guys.

01-28-2008, 10:26 PM

Thanks for all the information and links.. lots of good suggestions and information to digest.

It is funny how one project leads to another.. and another. I started out making a laminated wood hand plane and bought the iron from Ron Hock. The plane works great but I started thinking... hmmm. It is the thinking part that gets me into trouble.

01-28-2008, 11:18 PM
I recall reading from other sources that tempering should be done immediately after hardnening.
"Immediately after" doesn't mean time, it means process. That is, don't do anything else between the harden and temper processes, like try the part out just to see how your hardening regimen worked. Temper it before you try to use it.

So far as mere time goes, it doesn't matter to a part at room temperature. An hour, a day, a year - no noteworthy difference. There are time-dependent process at work - mainly formation of precipitates - when hardening and tempering, but they're all glacially slow at room temperatures in the standard engineering steels.

01-29-2008, 03:02 AM
i don't heat treat, but the temps will be fine...you've just got to set up some type of temperature control

Ask and ye shall receive. (http://auberins.com/index.php?main_page=index&cPath=1) :D


01-29-2008, 03:38 AM
"It is funny how one project leads to another.. and another. I started out making a laminated wood hand plane and bought the iron from Ron Hock. The plane works great but I started thinking... hmmm. It is the thinking part that gets me into trouble."

Been there and done that.

I would recomend a propane forge. Look up one "brick forge" on the net, though if possible I would go a little bit bigger. They will heat a plane blade, though I use the Hybrid burner in a home made enclosure, there are coutless how-tos on the net. The gas supply is either from a torch or BBQ bottle.

I can do a blade every few minutes on the forge, not so much that you need that much, but it is just a faster smaller, more versatile option. Next thing you may want a gouge (just hammer some steel into a half pipe), or to bend a small piece for a glue chisel. I made a hold down. All easy as can be with a little forge.

You might think you have more control in a kiln, and you doubtless could work it out, but what you want for the O1 (easiest stuff to HT in the world) is to go through transformation temp and soak long enough. That can be determined by knowing that it is nearly there when a magnet won't stick. When it is no longer magnetic it needs another 50 degrees and a minute or two. In fact it is better if it is held longer, but you can get around that if need be.

When non-magnetic I watch for the first visible change in the colour of the steel, then I hold it at that point for a minute or two, then I plunge it directly into olive oil or mineral oil heated to 140. It should be file hard, if it isn't go through the process again, and this time wait for just a tad more colour. I have only rarely had to redo a piece, even at the begining. For plane blades you generally don't want to temper much since you can use quite a bit of the hardness. This gets specific to your particular stove and temp gage. Just keep heating it for an hour at a time, in your oven, and work up from 350. When you get the hardness you want note the "situation"

Again all the blades I did were fine from the begining. While excess heating will oxydize the best out of your blades, several cycles of hardening and tempering are actually good for the heat treat, and some people have developed the habit of doing it on purpose. So keep records and don't worry about getting it right first time out of the gate, though you probably will. Using an open flame will blue the lower port of your steel and leave a ranibow pattern above. Tempering will blend in some gold. I like that result.

I do this with plane blades so only the first few inches harden. There is zero advantage to having the whole blade hard, you are only going to hit it with a hammer so why have that part hard? To wear out the adjuster in a mechanical plane? Once you start making your own tools there is little chance you will ever be short of blades again, and you couldn't even use the whole blade if you wanted to. By the way, you can use your Hock as a test sample to see how well your home made blades are hardening. If you can scratch your blade with the corner edge of the Hock, but not vise versa yours is soft. Use the corner of one blade to mark mid-bevel of the other, that way you do no harm that needs grinding out.

I use 3/16" O1 mostly. Also if you have a metal bandsaw, or stamina with a hacksaw, it is much cheaper to buy really wide O1 rather than material the width of your blades. Mean a little sawing and grinding, but the price for a 4" piece of stock is not that much more than a 2" piece.

You can see how high heat and being able to handle the steel during the process can make your life easier and your results better. I don't personally want to have to get your red hot piece of steel out of the kiln and then have to rebuild all the heat, if something needs attending to. There are many more complex steels useful for various purposes that require kilns that can perfectly run certain cycles, O1 is not such a steel, and smiths working with the open hearth have generated inumerable twists on heat treating that come from working directly with the product. For instance, you need more oil, a lot more oil, to properly heat treat a fully heated blade than one just edge heated. Just another convenience. Or you can use risidual heat to draw the temper.

By the way, if you already have an AO, or PO torch, you can do a fine job with that alone. You will probably need 3 hands, or a torch holder to start with. or just someone to handle the magnet. Some very fine internationally known smiths use a torch, but it is hotter than necesarry so go propane if you can. If you are using AO, think of it as painting on colour and go easy, keep it moving. With a small piece of O1, you can use a propane torch in the open, just to see how well it works, make a small marking gage blade, or screw driver, use some bacon grease for the quench if you keep that.

01-29-2008, 07:01 AM
Again, I'd like to thank everyone for their time and suggestions.

Blacksmith might be on to something.. a forge would be more versatile than a kiln.. oh oh... here we go! We have a local blacksmith club that I've been meaning to join.. of course, I'll need more tools.. hammers, tongs, anvil.. bigger shop.

I do like the colors imparted on steel by using a torch. Does using different oil for quenching impart different colors?

Your Old Dog
01-29-2008, 07:02 AM
I've had great luck using an old gunsmith trick.

I quench in a can of 10 weight non-detergent paraffin base motor oil(any pennsylvania product). If you sink your project straight down into the oil and do not swish it around, the oil forms a hot jacket that self-tempers the steel. It won't likely work where critical applications are necessary but I see no reason why it wouldn't work well enough for simple uses like plane irons. I did my metal engraving chisels that way. If you swish the part around in the oil then you cool the part too fast and you have to do the seperate tempering process.

01-29-2008, 12:34 PM
Get one of the knife making books. They're pretty common. Or, check out the knife making forums.

Having both a forge and a temperature controlled oven to do the tempering are nice. Heat to critical temp in forge, quench as recommended (air, oil, water, goop), and then use your oven to temper to the desired temperature. These days a digital temp toaster oven is pretty darned cheap.

You're not going to harden exotic alloys perfectly that way, but you'll do an awful lot and very quickly with pretty cheap tools. Forges are not that hard to build. I'm in the process with a friend of putting a nice one together with which to make some Damascus steel billets. It'll be handy for heat treating too.



01-29-2008, 03:57 PM
"I do like the colors imparted on steel by using a torch. Does using different oil for quenching impart different colors?"

I'd say no. The colours come during the heating, if you took a nice shinny piece of O1 and simply heated the end and then set it aside and walked away, you would get the colours. You can see this band of colour around a clean weld like a TIG weld.

Two things happen when you quench, one is that the very darkest blue oxide at the end, which has a superficial burned layer of steel, will kinda explode off the end, like a shale. So that end looses it's colour, but it fits in nicely enough, overall. Anyway you are sharpening that end and will be laping the back and the bevel. It's not a problem it's nature's protective layer, but it does mean you get a different colour there than if you didn't quench.

The other thing that would happen if you just set the red hot steel aside is that the heat would continue to spread. That might take the rainbow out beyond the end of the steel depending on the heat transfer and size of piece, so your whole piece might turn a dark colour if it uniformly got to 700 degrees or whatever it is for O1. Of course your steel would be soft.

If one wanted that overall dark look but didn't want to use a kiln, one could just preheat the metal to a uniform blue, then proceed with the heat treating, however, having some bare steel on the blade gives one another source of feedback when tempering.

"If you sink your project straight down into the oil and do not swish it around, the oil forms a hot jacket that self-tempers the steel"

Scientific aspects of heat treating are well above my pay grade. I would guess though, that what is really happening here is just that the material is not hardening fully. One has to use a quenchant that is neither two fast and may crack metal, or too slow and does not fully harden the metal, unless the not fully hardened hadrness is just what one is after. For woodworking tools, peak hardness, just backed off a tad is normally what one is after.