Heat treat kiln thermal expansion

T

Technohedge

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Sep 9, 2018
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Hi guys, Im partway through my build using soft firebrick (k23 or 26, cant remember which!)

Ive tried to stick the bricks together with refractory mortar and then build a angle iron frame around that clamps together with threaded rod. The base is actually held together in a welded frame section

Problem is when i fire it the bricks and frame seem to thermally expand and all the joints in the bricks crack, and im left with gaps.

Has anyone else dealt with this issue?
 
With the caveat that I have not yet built an oven - but two things: first - my local source for bricks was very clear that too rapid a heating, especially for the first several firings, can cause the bricks to crack. (They actually recommended 100F per hour). Second - when things heat up, they WILL expand. If you don’t give them room to move, you will get cracking. Mortar holds things together, and STOPS them from being able to move, creating stresses that result in cracks (concrete has cut lines in it, and bridges have moving expansion joints for a REASON).

whenever I get around to building a kiln, I will not be using mortar
 
Yea i get the whole issue but Im struggling to see how i can hold the thing together to reduce gaps when firing. How do the other 'off the shelf' kilns work as they all seem to have the bricks contained in a metal structure (which ia also how i have seen others build theirs, loose brick contained in a metal frame.

How do they cope with the differential expansion?
 
several ways. first, failure actually typically does not occur in compression, but rather in this case by shear. so press the bricks together enough to reduce the gaps, but beyond a point where you cause them to not be able to move laterally relatively to each other you will likely start to have problems .... its just likely kind of a balancing act....

At least for the designs I have been looking at .... compression really is not needed .... the bricks are really just bumping up against each other, or resting on top of each other. Honestly ... I have not examined a lot of commercial ovens, so cant speak to how they are all constructed .... but many people do talk about cracking in oven bricks....
 
I've got nothing to add as I used USED bricks form a burnt out ceramic kiln. BUT I did not compress them either during the build.
 
Mine did the same first firing then I gave it a bit of a skim after and it seemed to hold up.

Still got up to temperature no problem, but you could feel the hot air creaping out in places. Seems to be good now. Will be cranking it up higher this weekend to have a crack at some stainless so will see how that goes
 
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