Heat treatment oven build (photo heavy)

[Jaakko Fagerlund]

All the photos can be clicked to view bigger. CC BY-SA version 3.0 unported license applies to them all.

My excuse was because I think I can. After that was the need for case hardening and hardening alloy steels. And cooking sausage on top of it.

So first one needs an oven that will go to high temperatures and be able to hold it long enough. One could buy the oven, but where is the fun in that I may ask. After all, this is my hobby so of course I make it myself! Sorry for not having lots of photos of the build process, but I hope you get the idea.

I welded a frame and a door out of 30x30x3 angle iron, added hinges and painted it black with high heat resistant spray paint. All the sides were covered with 0.7 mm thick galvanised steel sheet metal, attached with pop rivets.



Once the oven frame was ready, I bolted it to a stand that I made from square tubing. This serves to raise the oven to a nice working height and provides me with space under the oven for all the supplies etc. to be stored, as space is always a concern.

In this picture you can see that I have mortared the bricks together. The bricks already had the grooves cut in them to hold the heating elements and all the brick surfaces were vacuumed with a brush to remove the fine particles so that the mortar would cling better. The mortar I used is a high temperature mortar designed to withstand 1200 degrees Celsius and the bricks are type 23 insulation bricks.



Couple of words about using the mortar. You want it to be quite a slurry, so you can just dip the end of the brick in the vat to get a smooth and thin layer of mortar on it. It is also advised to spray a little water on the mortar first, as the very porous mortar sucks up water like a sponge from the mortar, so it otherwise tries to dry up instantly. And by thin layer I mean something thatt fills any small gaps between two bricks, not the kind of thick layer used in regular construction.

While the mortar was drying (5 days), I had time to complete the electrical side of the oven. I ordered a box and parts and assembled them. The most work was doing the necessary holes to the box for the PID controller, the wires and the switch, as the box is 1 mm thick steel plate. Cut off disc on an angle grinder, hand held drill, a little filing and a DIY knockout punch gave me the holes needed.

The main power switch and PID ramp controller I installed on the door. On the upper portion of the right hand side is the solid state relay (SSR) that the PID uses to control the power to the heating elements. Below that is a DIN rail with all the components, from right to left: power in connectors, 10 A automatic fuse with fault current detection (30 mA limit), 24 VAC transformer, a contactor with 24 VAC coil, door safety switch connectors and power output connectors.



Because the SSR only breaks one cable, it is not safe as the live voltage can be at the elements even though the SSR is off. And as SSR's have a very low leakage current anyway, I installed a safety cutoff. When the door is opened, a microswitch opens up and cuts of the 24 VAC power from the contactor, which then opens up and cuts both the live and neutral lines from the heating elements.

I read Dudley's paper on heating elements (found from Google) and picked up a nice idea for the electrical connections behind the oven. As the heating elements end here, they will heat up this connection box too somewhat, but the main problem is the heat radiating from the back wall of the oven. In the paper it was suggested to make a chimney pipe where all the connections are made, so that when the air inside heats up, it rises up and pulls cool air in from the bottom and this cools down the electrical connections area.

Sounds like a good idea, so I folded a 100 x 100 mm sized box that is as tall as the oven and capped the ends with pop riveted steel mesh to prevent sticking fingers or anything in to the connections, but still providing free air flow. Also shown in the picture is a piece of fibre glass insulator board (the white square).



The idea is to put that insulator board on a little standoffs and attach a ceramic electrical connector to it with screws. The power lines are brough to this with heat resistant silicone cables and the heating elements ends are brough through the holes in the insulation board and connected to the ceramic connector. The standoffs provide air flow around the connections and as the ends of the heating element are folded and twisted, they have less resistance and thus heat up less. Basically the ends don't glow hot like the elements inside the oven.

All the side panels on the oven I riveted using pop rivets, but the electrical connection box and the insulation board etc. were attached with M4 screws that I could attach by using those rivet nut inserts. Great little things thoug they cost way more than pop rivets.

(Continues in the next post due to photo limit)

[Jaakko Fagerlund]

Once I had insulated the oven with fire resistant wool all around, it was time to fire it up. I fired it to 100 degrees Celsius for a few hours, let it cool down with the door open and repated this for a couple of days, bringing up the heat in 100 degree increments to drive off any remaining moisture in the bricks and mortar.

The PID controller has an auto tuning function, so I set the oven to heat up to 300 degrees Celsius with the tuning function. It heats up until the temperature goes above 300, then lets it cool down below 300 and repeats this a couple of times and sets its own P, I and D parameters. I noticed no need to change these, it rises to this temperature steadily fast and doesn't overshoot. This autotuning is best done at a lower temperature like this, as higher temperatures are easier for it to control due to excessive heat losses.

My main goal was to have an oven that is capable of hitting 1000 degrees Celsius, so I finally set the controller to manual control, input 100 % power and let it roast away. The controller keeps a record of time in minutes of how long it has been running, so I used to check the time in 100 degree Celsius intervals and jotted in down on paper. The following graph shows this slope. I also had a hand held thermocouple meter that I had put in between the frame and door. Its readings are on the same graph, the lower line. It can be seen that the outside temperature starts changing after the temperature inside the chamber has hit 500-600 degrees Celsius. So the 100 mm thick insulation really works well.



The graph also shows the usual slope of time vs. temperature. Temperature is in proportion to the square root of time. From what I've learned is that an oven hits half of its maximum temperature in one hour, then half of the remaining in the next hour, then again half of the remaining in the next hour and so on. So by looking at my graph, I would estimate my oven to be capable of reaching 1200 degrees Celsius. But I won't fire it up that high, as the life of the heatings elements would be exponentially shortened.

After 4 hours and 32 minutes at full blast, the PID showed a temperature of 1000 degrees Celsius. Objective completed, project succesful and wife NOT happy that the whole house stinks like hell.



Here is a photo of the room when the oven was at 1000 degrees. You may notice a little foggy atmosphere and this was due to all kinds of oils and residues left on all the metal parts and all the moisture running off from the outer portions of the oven. Room temperature was about 38 degrees Celsius, normal being around 22.



I didn't want to open the oven door at this temperature, so I just switched off the heating and left the controller on to see the temperature. It plummeted very rapidly to about 800 and began its slow descent after that. This was at the evening, and at the next morning it still showed 250 degrees Celsius. I must say that the insulation work quite good and that the oven has quite a mass.

After my work was over that day, I opened the oven to investigate. The door seal was really a toast. It was supposed to handle 900 degrees Celsius, but it was already in my firing tests that it started smoking and turned a little black. The following photo shows that. But by now it was almost completely black, a little fuzed, crumply and I just pulled it off, packed it in a plastic bag and returned it to the store that I bought it from, as it clearly was not able to withstand even 600 (the point at which smoking started), so it was false advertisement.



(Continues in the next post due to photo limit)

[Jaakko Fagerlund]

Got my money back, didn't want a new one to test out but contacted my ceramics supply house and bought a roll of 13 mm thick ceramic fibre wool, designed to withstand 1400 Celsius. Cost was next to nothing, I think the shipping was more than the product. I cut off a small patch from it and attached it to the door with pins.



Here is an inside view of the oven after repeated firing. The heating elements show no creep, so the pinning that I did with about 50-100 mm increments was good. The lining (hand slapped/smeared mortar) has held up fine, just shows interesting color changes. It was white'ish when dry powder, gray when mixed, light gray when dried and turned to yellow when fired.

You can see the themocouple end in the center near the ceiling. It protrudes about 100 mm in to the oven. Below it can e seen the ceramic insulators that bring the ends of the heating element to the back of the oven.



Here is a couple of photos of the oven installed in its own place and with the controller properly mounted on the wall. You may notice that the power cable to the controller box is just ordinary rubber insulated, but the left one that goes to the back of the oven is brown colored. It is heat resistant silicone cable, designed to be used in electrical sauna installations. The same cable type was also used for the door switch.





Details of the oven
-230 VAC input
-1.9 kW power (operates behind standard 10 A fuse)
-heating element Kanthal A1 wire (1.2 mm, 20 m, coiled)
-K type thermocouple
-PID ramp controller, 50 points maximum
-chamber size height 150 mm, width 225 mm, depth 455 mm
-0.45 W / cm²
-insulation 75 mm brick and 20 mm fire resistant wool
-door gap insulation 13 mm ceramic fiber wool
-cost approximately 600-700 EUR

Pros
-works just as expected
-easy to program and control
-temperature difference only 10 degrees between ends

Cons
-could be bigger or different shape
-control box needs some light or buzzer to inform end of program
-project complete - now what?

All in all this project was a success, although in retrospect I would do a few things differently. My main limit for the whole project has been the 10 A fuse, as this oven sits in a basic room in a basic house, so the fuse is 10 A. If this was a garage, then it would be 3x16 A and I would be able to up the size and power considerably.

If (actually when) I build another oven, I want one that is higher but not so deep as this one. This one was made deep because I need one. And now wife wants an oven too to melt her glass stuff, as I forbid using my oven for that. Bought her a microwave melting box to get started while she waits for me to build her an oven.

With the next oven I don't do coiled elements and grooves in bricks, but instead just snake the wire around the inside walls and pin it in place. A lot easier to make and install, provides more uniform heat and doesn't need grooves.

Would be nice to build a large one to use at work, something with like 400 x 400 x 400 mm chamber and powered with 3 x 16 A fuses. But it would require that I make it CE certified and that is one hassle to begin with, though it is just paper work.

You may be interested in reading my other thread on case hardening steel (a.k.a. carburization) with this oven, so that you too have an excuse to build one. Here is the link: http://bbs.homeshopmachinist.net/thr...dening-success

TL;DR: well shame on you.

[The Fixer]

Looks like a great addition to any small shop, nice work.

[DICKEYBIRD]

Great post Jaako. Just like a magazine article but free!

How much was the controller?

[Jaakko Fagerlund]

How much was the controller?

The PID cost 40 USD I think, bought it through eBay from Thermalmart I think (it has been long since I bought it). Basically costs next to nothing, considering all the advantages it gives

[lazlo]

Nice job Jaakko!

That would be a great article for HSM Magazine

[Jaakko Fagerlund]

Nice job Jaakko!

That would be a great article for HSM Magazine

If George wants it, then yes, it would be nice. Though there should be a little more photos, like from the electrical connections box. Just requires moving the oven a little from the wall and screwing off the box to take a shot.

[Toolguy]

When you make the next one, give your wife this one!

[Jaakko Fagerlund]

When you make the next one, give your wife this one!

No way I'm giving away my oven! It is mine..mine I say! My preciousss...

Nah, the real reson is that the glass works really would require an oven with top heat and more width, so that is the reason why I would have to make her a different oven.

[rohart]

That looks great ! I bought used a small oven that I enjoy using from time to time. It has a 50mm long 20mm diameter hole in the insulation in the top, that is normally closed by a flap of 1.5mm aluminium. The hole is just offset from the top surface, so to use the hole for access to the main chamber you would have to turn a bend as you reached the chamber. I wondered for some time what this was for. Was it for an extra thermocouple ? Was it so that you could fine adjust the rate of cooling ?

I ended up using it to pipe argon in to the oven when hardening O1 and silver steel, although I had no real way of knowing if the argon flow I used was enough to overcome the leakage out of the front door and elsewhere. I guess I could heat vertical rods and look at the vertical distribution of scale at various argon flow rates. I'm not sure I have the patience for this.

Stainless steel foil is expensive.

But I'm glad I have the hole - it gives me a lot of flexibility. And the thin ally flap doesn't get very hot, so closing the hole is no problem and doesn't lose much heat.

I'd really like a window, but that would be expensive, and I don't know what I'd learn.

[madwilliamflint]

Very nicely done. I'm about 30-40 years of experience in each of 4 different fields away from attempting anything like that.

[Paul Alciatore]

That is a really great build. I am impressed. You really should write it up and submit it to Village Press for publication. You are about 3/4s of the way there with the above posts.

[Evan]

Looks very nice to me. On a cold winter night it also looks really nice inside when you open it hot.

[iMisspell]

After my first basement heat treating session the thought of a home made oven drove me to search the net, most designs seam similar to this but with out the walk-through, thanks for providing that. It sure does look good, nice job.

I did not read the other thread (sorry if you answered it there), does this oven cool down fast enough that you will also use it to temper your parts ?
Not knowing much about this stuff, going from 1500°F to 300 in a couple minuets seams like a big jump, is it ?
Can you control how fast it cools down, say 50° per hour ?

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