DIY heat treat oven failure modes and fixes.

J

Jcdelamater

Joined
Oct 5, 2022
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Hi all. I've been doing a lot of trial and error on my HT oven and I'd be interested in hearing other's problems and how they fixed them so next time someone builds an oven hopefully they won't make avoidable mistakes.

Oven:
- Size: 7.5" w, 15" d, 4.5" h
- Heating: 1.2mm nichrome wire, ~15.5 ohm @ 240V -> ~3.7 kW

Problems:
- The classic sagging coil. Fixed by spacing coils further apart and having them slightly under tension when installed.
- Melted wire. Not really sure what happened with this yet, but there seem to have been a few points (1/2 inch long or so) on the coil that overheated and broke then arced melting some of the wire. I don't have a fix for this yet.

Anyone else have problems and how they fixed them?
 
I never used nichrome wire only kanthal wire but make some pins/staples from the same wire and pin the wire to the bricks will fix the wire sagging. Pinned good enough to the bricks and it cant sag.
I built my oven back when I first started here and built it following somebody else's posted homemade drawings and the kanthal wire was wrapped around stainless bolts. Never got long life from the elements like that , a while back I changed to using ceramic blocks to make the wire connections after seeing the oven Red Beard Ops made and major improvement
 
My guess on the melted wire would be some kind of short, but I'm not sure.

As far as problems I've had with mine, and how I've fixed them.

The biggest issues were from having the ceramic terminal blocks, and thermocouple attached to the outside of the firebrick, withing the aluminum, and steel casing. Even trying to protect them with as much kaowool as possible it was melting wires, causing excessive oxidation from the heat to the metal components of everything back there, then causing connection problems, and shorts in the thermocouple.

My solutions were moving the thermocouple to the top, and keeping the base away from any heat, and for the ceramic terminal blocks that the coils connect to. I places them on the outside of the oven, at the back, with much longer twisted leads on the coils to reach all the way outside to them. Now they don't even get warm to the touch.

Also I had an issue with long heat ups, because I'm running on 110v. So I had to make adjustments to the inside to get better results. Obviously it's still not as fast as a 220 but better. Mostly I just stuff insulation everywhere possible, and also used ht100 to coat the inside. Making sure to take out the coils and coat the grooves where they sit.
 
Thanks all,

I'll give kanthal a look and maybe replace my currently burnt out coil with that. My first coils had WAY too much slack and would find a way to sag out of just about any gap between staples.

What is ht100? I have not heard of this before.

Has anyone ever tried putting some kind of furnace cement over the coils with any success. I've seen a thing or two about embedded coils but never much on what exactly that is.
 
Jcdelamater said:
Thanks all,

I'll give kanthal a look and maybe replace my currently burnt out coil with that. My first coils had WAY too much slack and would find a way to sag out of just about any gap between staples.

What is ht100? I have not heard of this before.

Has anyone ever tried putting some kind of furnace cement over the coils with any success. I've seen a thing or two about embedded coils but never much on what exactly that is.
Click to expand...
ITC-HT100 is a coating that increases efficiency.

I would think a refractory coating on the coil is not a good idea. You have to derate them even if they are in a channel. If you coat them with refractory I would imagine they can't get rid of the heat quick enough and get too hot and burn out.
 
Jcdelamater said:
Thanks all,

I'll give kanthal a look and maybe replace my currently burnt out coil with that. My first coils had WAY too much slack and would find a way to sag out of just about any gap between staples.

What is ht100? I have not heard of this before.

Has anyone ever tried putting some kind of furnace cement over the coils with any success. I've seen a thing or two about embedded coils but never much on what exactly that is.
Click to expand...
Exactly what Hubert S. Said. It's a refractory coating that helps with efficiency. It also helps protect the firebrick, and is a good choice to put in the channels behind the coils, as that's an easily deteriorated spot. I coated everything in my kiln.

I actually read in an old thread recently about someone coating the actual coils in something. It might have actually been itc100 I'm not sure though. Look up a thread about making antiscale using boric acid. It's mentioned in the first post.(it's an old post btw from 2009ish I think)
 
I've built a couple of ovens and have used Kanthal wire for both.

In my first oven, I made some "V" shaped staples to hold the element about every 5 or 6 inches or so. It worked pretty well for the life of the oven. You still had some movement/sagging here and there, but it really wasn't an issue. Adding a few more staples would have probably solved most of that problem.

My most recent oven was built based on an oven that JTKnives built. I mounted the element to the top of my chamber using several rows of fused quartz rods through the inner diameter of my coiled element. I believe JT used a ceramic based rod for his, but the fused quartz rods priced out much cheaper. If you can find some mullite rods, that will also work. I found a place online called Joppa Glass that sells 1/4" x 30" rods, but I've not purchased any from them yet, as the fused quartz rods I used have been holding up just fine.
 
I had the same issue with kenthal. I got some ceramic rods from Amazon and during my next recoil I inserted the rods in the coils and it works great.
 
I was looking back at my notes and turns out I do have Kenthal, no nichrome. Don't know why I misremembered that.

So one point I've learned on avoiding sagging wires is that if there is too much slack when you install the coil (I stretched mine too much), I don't care how many staples are in there, it will find a way to sag. So just install coils with a little tension.

This is great, I was not aware of either quartz rods or HT100. I'm considering an oven overhaul now and I may do some work with those.
 
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