Home built Heat Treating Oven

[Frank S]

I don't have any experience with compact heat treatment ovens, I think the smallest one I ever had any dealings with had about a 3 cubic foot interior space and most of the others had huge induction coils with forced air circulation. However after reading Wizard69's very informative post I couldn't help but wonder if there might not be a way to direct the currents of the internal natural convection at least partially throughout the interior space of the oven by carving channels into the sides top or bottom thereby taking some advantage of the thermal heat sink of the fire brick. Large flat surfaces readily absorb while poorly reflect but smaller angled surfaces or groves possibly even those coming to a peak might have a tendency to redirect the heat at angles creating a slight thermal circulation.
I'm sure there are those who might have tried doing this and it may be well worth the research

[anthonyget]

Not wishing to teach my granny to suck eggs, and I hope my comments are relevant. Many years ago, a company called "tor gem" made a solid fuel stove (heater), that had mica windows in the front I used to burn furnacite beads in it, they get very hot, and the result was that over time the mica "delaminated" and went hazy, not important in the stove, I only needed to see that it was glowing nicely, but where you need to see a process, that might be important.
I don't know if this is normal, but if it is you might want to design your door to be able to replace the mica without destroying the build.

Thank you and I appreciate your input. I see all of this as an experiment in many ways so you may well be right. They certainly do get hot. The way i designed it, it is very easy to rebuild the door if all goes wrong. Thanks again, Nick

[CanBeDone]

I see all of this as an experiment in many ways . . .

If that is your approach - have a look at Rick Sparbers furnace, when laid horizontally. To operate it horizontally, Rick would have to fill the groves holding the heating elements with refractory cement, of course, otherwise it would just fall out at temperature, producing a dangerous short. But that would give him also the opportunity to cut the heating element into three, and controlling it with three separate controllers via three separate control thermocouples.
If all three controllers are set for the same temperature, the front and rear controller would pump more energy into the furnace than the centre one, producing a more uniform temperature distribution in the furnace . . .

And once the heating element grooves are filled and electrically isolated from the furnace interior, you can safely put a thick stainless steel plate into the central, "uniform" temperature zone to make it even more uniform. And that is where the MEASURING thermocouple belongs, the couple telling you what temperature the furnace has and at which your knives are heat treated. And don't confuse the MEASURING thermocouple with the CONTROL couples! The control couples belong as close as possible to the heating elements, since they control the temperature of the furnace (zones). The measuring couples (plural, if you are as picky about the real world as I am, in contrast to the stories most people try to sell as true) tell you essentially how long you still have to wait until your knife is within the temperature range where you want it to be.

Welcome to the world of continuous improvements !

[DIYSwede]

To operate it horizontally, Rick would have to fill the groves holding the heating elements with refractory cement, of course, otherwise it would just fall out at temperature, producing a dangerous short.

AFAIK, I've never seen or heard of any other DIY oven maker using this approach of yours. Feel free to put some links up.
Seems like claymakers and glass blowers have survived and been doing pretty well with free, suspended coils
laying in the side wall grooves thru the years? Easy to roll your own, fit, fix, and replace when broken.
Guess the intrepid newbies poking the fiery hot, yellow-white coils with metal rods have already pulled themselves out of our gene pool?

IMHO: To fill the grooves/ Kanthal A-1 coils in a DIY oven with any cement would probably shorten the coils' life, if it would work at all...
Industrial furnaces are a whole different ball park, a whole variety of solid heating rods (and price tags!):
https://www.kanthal.com/globalassets...a041-b-eng.pdf

Then - I wonder what kind of tight temp tolerances you are talking about, and also curious of your very need for those specs?

Personally, I'll be absolutely OK with a no-frills, cheap build, but then I'll only use mine for annealing,
hardening and tempering plain vanilla tool steel and Al, Zn (and perhaps some occasional brass) casting.
The tolerances in my own handling will anyhow be much wider than the temp diffs in my oven.
For resilience I'll add a door "Idiot switch" saving me from even touching an energized coil (also taking the coils' lives a bit further).

I guess Stan is providing some "good enuff" ovens for more discriminating and qualified workshop users than me,
so his HotShot series might even be of interest to other HMT:ers:

Cheers

Johan

[CanBeDone]

AFAIK, I've never seen or heard of any other DIY oven maker using this approach of yours.

About 50 years ago, when starting to do my master's thesis in physical metallurgy, the local Research Assistant taught me how to build a furnace: take a tube of sintered aluminum oxide, wrap coiled heating element wire around it, and hold it in position by smearing refractory cement around it, filling all the gaps inside and between the coils. For my doctorate at a different university, exactly the same procedure. Ditto, when I went to do research at a third university in a different country. The materials used for thermal insulation were different, however: ceramic wool (Kaowool) on the first two, hollow aluminum oxide spheres on the third.
The most important instruction, however, was on the spacing of the wire: closely spaced near the ends, and much wider near the middle of the alumina tube, to compensate for the extra heat losses that occur at the end of the tube. Even with this, the temperature profile over the length of the tube was pretty pathetic - the zone within which the temperature stayed the same within a degree or two was only a few centimeters long. So, we extended the zone by inserting a piece of metal, utilizing the metal's higher thermal conductivity to extend the uniform zone.

Feel free to put some links up.

Unfortunately, at the time this was done, there was no internet, and nobody thought of putting up videos on how to construct a furnace. Who would have cared, when the Research Assistant knew how to do it, and where to source the materials? That was part of his job, and he was paid to do just that.
What I learned only much later (when I learned why CORTEN steel can be left to rust without ruining it, and what the Japanese did to use it without paying royalties to its American inventor) is that the refractory cement not only held the heating coil in place, it also protected it partially from oxygen, and from mechanical shock, extending its life indefinitely. Only if we allowed the furnace to heat beyond its design temperature (1100^oC) would it fail, even with daily use.

Seems like claymakers and glass blowers have survived and been doing pretty well with free, suspended coils
laying in the side wall grooves thru the years? Easy to roll your own, fit, fix, and replace when broken.

I rather have a furnace that does not need fixing, and to heat treat carbon steel I do not need a furnace that goes hotter than about 1000^oC, or, for stainless steel / HSS, above 1100^oC . But if you want to melt glass or sinter ceramics, you do have to routinely work above 1200^oC in an atmosphere containing oxygen and nitrogen. And that means that your heating elements will not last, unless you go to fancy stuff like silicon soaked silicon carbide rods, which, if I remember correctly, are good for up to 1400^oC .

Then - I wonder what kind of tight temp tolerances you are talking about, and also curious of your very need for those specs?

This depends on the kind of steel anthonyget is going to use for his knifemaking. If he follows the most prominent choice on the internet, that would be SAE 1080. Assuming his SAE 1080 is continuously cast and thus contains about 0.05% aluminum, his choice of quenching temperatures would be from 850^oC to 1100 ^oC or even 1150^oC. If he were Japanese and would want to use tamahagane (an aluminum-free, unalloyed high carbon steel), his upper temperature tolerance would drop to 950^oC. Both of these steels need to be tempered at 200 ^oC. The temperature tolerance applicable here is only a few degrees, if you want to make everything optimal. Going for optimal tolerance is applicable only if anthonyget were to manufacture knives on an industrial scale, as it is about maximizing toughness (=resistance to breakage at the same thickness). If quantities are small, the safest approach to avoid breakage is to slightly increase the blade's thickness, since the force needed to break the blade is proportional to the fourth power of its thickness. Since toughness is not only dependent on tempering temperature, other contributors to toughness like oxygen or sulphur content will overshadow the effects of temperature on toughness, and annealing temperature tolerance is firstly chosen to maintain hardness, and that means 200-250 ^oC or even 200-300^oC - assuming that the quenching temperature, and time at that temperature, has been low enough not to lead to an increase in grain size, and high enough to dissolve all iron carbide into austenite.
If I were in his place, I would use SAE 1060 (most ready source for DIYers: leaf springs from the scrap yard), assume that it does contain the required aluminum, quench from somewhere between 900-1000^oC and skip tempering at all, as the supposed benefits of tempering on toughness are seriously constrained by the steel's inclusion content (mainly in the form of silicon oxides / manganese sulfide mixed phases, and in aluminum bearing steel, calcium aluminum oxides, too. Occasionally, you'll also find copper (from an electric motor that was sneaked into the scrap from which the steel was melted) or tin (from too many beverage cans in the scrap) limiting the toughness of the steel.
If anthonyget were to use a steel with a higher alloy content( =Stainless or HSS), he would have to choose a higher value for the minimum quenching temperature, since his objective is to dissolve not only iron carbide, but also chromium, vanadium and tungsten carbide, if those elements had been added to the steel. But his upper temperature limit would remain the same at 1100-1150 ^oC, since he must keep the aluminum nitride precipitated, or have massive grain growth with its concomitant drop in toughness. He will most likely notice this toughness drop by fracturing his knife when quenching. If not then, then soon thereafter, as this toughness loss is not averted by tempering.
On alloy steels, tempering cannot be avoided, since the objective here is to re-precipitate the chromium, vanadium or tungsten carbides, as they are the source of the improved properties of alloy steel. And that means not only the tempering temperature, but also the rate of heating must be controlled so that precipitates may be obtained with the right size, the right quantity, embedded in a matrix that does not change over time. And the latter means that tempering needs to be repeated several times until the matrix has a stable composition.
You don't believe in stabilising? This is what is behind JKeetonKnives "Straightening Plates For The Knifemaker".

And lastly, these temperature tolerances are applicable over the whole length of the knife, excluding, maybe, the tang. And we will have to assume that the blade is 200mm long.

[Frank S]

What CanBeDone stated in his last post reminded me of something a blacksmith once told me back when I was a teenager. He was noted for making some of the toughest most wear resistant agriculture and oilfield tools anyone had ever seen or used. Of course he used a coal forge and quenching tubs but he did other things as well a lot depended on what he was needing to accomplish with a particular type of steel to add or remove certain properties he would heat his metals to near liquid state pour on fluxes or powdered metal oxides or salts and do what called beat the sponge out of it much the same way ancient ore workers would make pig iron out of ore by heating and beating the ore until it became what was called sponge then continue repeating the processes until they had made steel out of iron.
His tempering methods were all over the scale both in how he would go about it to the media used to control the cool down. Sometimes what ever it was he needed to temper and retain maximum hardness he would weld up a thick steel box or use a thick walled pipe then weld caps over the ends of the pipe with the object inside then spend sometimes an hour to slowly het the container to the color he wanted then pack that in lime or even heated salt for it to cool slowly over night . cut open the vessel to retrieve the item he was making.

[DIYSwede]

Thanks, CanBeDone for your elaborate and clarifying explanations - much appreciated!
I could hopefully follow and understand your reasonings & experiences on furnace construction
and advanced heat treatment of proper, top-notch alloys for a lab or production facility.
I also realize that your suggested embedded coils would have benefits regarding
working lifespan, thermal shock when opening, harmful vapors and temp gradients within.

Without intention of being polemic:
I perceive most DIYers (incl yours truly) neither have the means nor the resources for achieving these optimal results,
we just have to cope with our limited financial, material and cognitive resources to get "good enuff" results:
Like "Mystery metals" from scrap yards, cobbled-together furnaces with unclear repeatability for their results.

I personally embrace each step to minimize all uncertainties and tolerances within the limitations above,
in which I'm obsessed with putting in an unresonable amount of work and search for info & methods.

But I'm still stuck with what I've already got: 2 dozens of Grade 26 insulating firebricks, a few cartons of Kaowool cutoffs,
a cheapo Chinese PID controller, a roll of 1,2 mm dia Kanthal A-1 wire and some steel sheets and angle leftovers.
I just have to wing these pieces together as fast, cheaply and operational I can.

AFAIK; I haven't seen ANY available (for privates) cements suitable for Kanthal A-1 wire,
so my hypothesis for now is to assemble the bricks entirely without mortar/ cement
with the coils suspended in angled slots in the sides, devised so they won't drop.
The lifespan/ results of my furnace will only be exactly as long (or short) as I deserve thru my own choices and use.

The web and YT is filled with DIY furnaces, some better than others (from my layman perspective),
but I've yet to come by a DIYer or even a supplier offering a DIY solution having the properties you suggest.
Until then I'll just continue to "wing my own with what I've got" -
keeping an open mind (and ditto construction) to modifications when needed,
while always remembering (or being informed of that) I'm neither omniscient nor omnipotent.

Cheers

Johan