Hardening oven saga

Ferrous Turner

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Didn't know anything about these types of ovens except what I gleaned from watching a few YouTube videos. How hard could it be, it looked so easy in the videos! Actually it wasn't too bad, but like every other venture into the unknown there were a few screw ups.

I thought the ceramic fiber bricks were ridiculously expensive, so I picked up some regular fire bricks instead. Cutting these turned out to be quite time consuming (they are very hard). The regular masonry disc didn't do any better than standard metal cutoff discs, so I used those.

I also bought some high temperature mortar which came in a tube (much like silicon seal). I used it to glue the bricks together and it worked OK. It’s water based and cures by drying out. This burned me later.

This is how it started out:



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I traced out a groove in the top in which I was going to put the 1500 watt heating element. After hours of grinding out the groove, I found out that one must stretch the heating element coil out in order for it to work properly. So much for the groove.

I stretched out the coil and ran it through wire hangers that I “glued” into little slits I cut in the brick (F#@k the groove). I used the tube mortar for glue and it seemed to work OK.

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I then tested the coil and got very little heat from it. I measured the resistance and got about 26 ohms across the coil. This gives about 500 watts @120 Volts. Gee, I guess this element was meant for 220 Volts.

I didn’t want to run on 220V because my control box was all wired for 120. So, I cut the element in half and ran the two pieces in parallel. It lit right up to a nice glowing orange in that configuration. Nevertheless, I had to rerun my coils again. The top of my oven is starting to look ugly, but who cares as long as it works.

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I mounted the thermocouple in the bottom rear of the oven. I enclosed it in a steel pipe and welded a piece on the end for it to screw it into.



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It sticks out a ways because I’ll be adding regular mortar to the outside for more insulation.
(continued in next post)
 
I got the element ($7), thermocouple, SSR and controller on Ebay. I'll get to that other stuff in the next post.
 
Next thing was to build a frame. I got a few lengths of angle iron and got to work with the welder.

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I wanted the top element work all done before I buttoned it up. At this point I just cemented the top on and slid it in the frame. I left a few inches on the vertical pieces to act as legs.
Once I got that done, I cut part of a brick off to fit with another one for the door. I built a frame similar to the other parts which fit around the outside of the door. I mortared the bricks to the frame and welded a couple of hinges between the frames. I also welded on a bracket to hold the control box.

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I already had the control box built by that point, but I'll go over the details of that later.
I used another hinge for the latch. I welded a length of 5/8" threaded rod to the inside and used a butterfly nut for the hold down.

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I had to seal the door, so I used some more of the tube mortar. I spread it around the opening, put some wax paper over it and closed the door down on it. I let it set for a day, but then you have to remember that this stuff dries out to harden and it wasn't doing a lot of drying all covered over with wax paper. So, I opened the door and carefully peeled the wax paper off. Of course, the wax paper had wrinkled and the surface was a little goofy, but I had the right shape anyway.

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As you can see in the picture, I already had a couple of the sides mortared up. To do this I framed the sides with a combination of 1" angle iron and 1" flat stock. I just tack welded the pieces in place to make a mold. I mixed the mortar real soupy so I could easily pour it in the mold and smooth it out. I've done some casting this way and it works good. Doesn't crack either. I also mix my own mortar using just sand and portland cement. Nothing complicated or precise about it, I use about 2 thirds sand and a third mortar (and there's a lot of fudge factor there). The hardest part is estimating just how much you need per batch. You also need at least a day between sides to let it set, so 6 days minimum to do all 6 sides.
After the outer mortar is all done the thing weighs about 100 lbs (feels like it anyway), but it's pretty well insulated.

(continued next post)
 
Do you have a safety switch on the door to cut power feed to the SSR so there are no live parts accessible with the door open?
 
Just a thought, the steel in your assembly, when heated will expand. If an 8 inch piece of steel reaches 1500º, it will expand .08 inches. Twelve inches will expand 1/8". Who knows what rate the fire brick will expand at, but I suspect there will be a resulting disassembly of the mortar, at the very least. The furnaces I've examined used a fibrous mat as insulation between pieces.
 
A safety switch would probably be a good idea, especially if the element were running on 220V. However, I have an on/off switch right next to the door latch, which if I fail to turn off before sticking anything inside, makes me deserve a reminder. I also have bright red lights to remind me that the unit is on. The kids are all grown and gone, so I don't have to worry about curious fingers either.
 
Just a thought, the steel in your assembly, when heated will expand. If an 8 inch piece of steel reaches 1500º, it will expand .08 inches. Twelve inches will expand 1/8". Who knows what rate the fire brick will expand at, but I suspect there will be a resulting disassembly of the mortar, at the very least. The furnaces I've examined used a fibrous mat as insulation between pieces.

The steel seems to be on the Outside?
It should stay within a temperature range which doesn't cause 3rd degree burns on brief contact ;-)
 
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