Steel inner dome build

[MarcD]

Starting an oven build with a 36 inch ID steel tank end that's 1/4 inch thick. The plan as of today is to brick and motor the outside followed by concrete board box to be filled with a loose insulator. This could change depending on the cracks that develop in the masonry layer. If it cracks less than we anticipate I'll use a ceramic blanket with a tile shell.

Still working out the exhaust design but the tentative plan is an 8 inch pipe through a brick archway.

Here are some pictures of the dome with the door cut out and the hole patched. I dug this thing out of my back yard where the previous home owners used it as a fire pit. I currently have no plans to cut off the two inner bungs or fill the seam. I don't anticipate either creating convection issues.

[Disco Stu]

Surely the steel and brick layers won't play nice together with different rates of expansion / contraction, maybe consider a sand or crushed rock heat bank layer behind the steel instead of brick?
One of my ovens built over a steel inner dome at the end of this thread....
https://community.fornobravo.com/for...-mass-question
Good luck with the build.
Stu

[fox]

I think the inside will rust really badly and then the rust could be dropping on your food?

[MarcD]

Surely the steel and brick layers won't play nice together with different rates of expansion / contraction, maybe consider a sand or crushed rock heat bank layer behind the steel instead of brick?

I'm expecting the brick to crack but not enough to harm anything, maybe not even enough to notice. Another build here used a basalt and cement cladding as the thermal mass and another used the same brick method I'm planning. Both seem to work with the brick method having some documented longevity that took a falling tree without damage. Either way I think I can use carefully placed slip joints to help. I'm not yet ruling out a basalt thermal mass cladding but it will come down to whatever material is easiest for me to access. Right now I have a pile of red brick that's free but I'm not married to the idea.

I think the inside will rust really badly and then the rust could be dropping on your food?

Lots of steel cookers out there. I've got a steel tank bbq pit that I dragged out of a scrap yard where it sat for 20 years. It sits outside at the camp in a salt water environment without issue. I don't think we will see any issues with this oven. At least not any time soon.

[fox]

Ah ok I just thought that steel started to spall at around 400c and my ovens go up to 500c, in the top of the dome sometimes 575c .

[david s]

Spalling can occur in concretes or refractories if contained moisture either expands rapidly (steam spalling) or freezes and expands or corroding reinforcing expands. Spalling can also occur if the cement breaks down from excessive heat. Portland cement begins to fail north of 300C, lime north of 500C and calcium aluminate cement vulnerable in the 500-650C range. Clay just becomes harder as it continues to vitrify more as the temperature increases over 600C, but is also vulnerable in the 500-650C range.
I’ve never heard of steel spalling, but anything galvanised can lose the zinc coating north of 400C (approx zinc melting point). These fumes are highly toxic so never have anything galvanised in an oven. Thin steel (.around 0.5 mm) will corrode quickly, but >2mm thick, although it will rust on the surface does not become a problem, the thicker the better.

[MarcD]

Test fired the dome this morning while finishing other projects to get an idea of how evenly it will heat. Very little temperature variation across the outside of the dome. Within 15 minutes the hot side of the dome was over 650F and climbing. Within 45 minutes the entire surface was anywhere between 415F and 450F. Once up to temp the natural convection appeared to be good. I'm calling this part done, no more work will be done to the steel.

I'll frame the slab tomorrow and place the order for the concrete on Monday. Hopefully next weekend I'll have the table assembled and the hearth framed. Still working out exhaust details which may end up being done on the fly with whatever I have laying around.

[fox]

It will be interesting to see how it goes but what we have inside a pizza oven is a very high temperature oxygen rich environment, it is the nature of how the oven works buy sucking in clean air to feed the fire.
Although steel wont melt until 2600f but anything above 1500 ºF and the steel is corroding away in a very spectacular way.
As such, the damage is done waaay lower than melting temperature and spalling and rusting will acure.
That is why box stoves are made from cast iron, BBQs work in a different manor with low oxygen.

[MarcD]

It will be interesting to see how it goes but what we have inside a pizza oven is a very high temperature oxygen rich environment, it is the nature of how the oven works buy sucking in clean air to feed the fire.
Although steel wont melt until 2600f but anything above 1500 ºF and the steel is corroding away in a very spectacular way.
As such, the damage is done waaay lower than melting temperature and spalling and rusting will acure.
That is why box stoves are made from cast iron, BBQs work in a different manor with low oxygen.

You make an interesting point. The truth is I don't know for sure one way or the other. It's been about 15 years since I slept through a materials science class so I won't even bother trying to look it up because I don't remember enough to make sense of what I'm reading. I don't even know what type of steel is in the dome. In fact, nobody knows what kind of steel is in the dome. What I do know is I've seen enough propane tank ends used as fire pits and BBQ's to have enough confidence in this dome. I don't see a naturally aspirated oven being any more oxygen rich than any other outdoor fire but I've been wrong before so we will have to wait and see. There are documented steel dome builds that perform as intended and don't fall apart, there is even a link to one in this thread so I feel pretty good about it.

The fun thing about this oven for me is I'm going to build it with whatever material that is easiest and cheapest for me to acquire. My dome was free, bricks were free, blocks were free, the wood for the framing I got out of the trash. Even the fire brick is coming from local classifieds for under $1/brick

I really like this build https://community.fornobravo.com/for...ome-oven-build

[fox]

Well i dont mean to be negative its just that I have a feeling the steel will drop rust on your food but I really dont know for sure and as you point out you dont have much to lose.