Thermal conductivity of two welded plates?

I am in the process of building a wood stove (details on FE Blog) mainly from scrap metal, but I have got it into my head that a stainless steel hotplate would be nice. I wanted one 10mm thick, round with a diameter of 390mm. As it turns out 6mm seems to be a bit of a threshold for stainless, and anything over has to be laser cut or similar and is loads more expensive. Like £80 is the cheapest I can find it. 

Anyway, one solution would be to buy two 5mm thick plates and weld round the circumference - that seems like it would work out quite a lot cheaper. The question is how would the thermal conductivity be affected by the tiny gaps which would inevitably remain in the centre (the two plates will never be completely flat)?
Is there some kind of thermal paste that could mitigate this, which is very heat resistant? If so how would it deal with being superheated by the welding, and would it contaminate the weld?
And lastly - this got me to thinking about copper bottomed pans - how do they bond the copper to the stainless steel (google is failing me- or I am failing google with poor search terms)??

So 3 sort of related questions:
1.Welded plate- big conductivity problem or naught to worry bout?
2.Super high temp. thermal past  - would it help?
3.How are pans copper bottomed anyway??

Thanks, Bongo

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Re-design5 years ago
Copper in pans is cast so there is a good bond. It's also thick, maybe 1/4 inch. Welding the edge of the stainless won't be very good since the better part of the metal would not be welded. After you heat it it will probably warp so that it doesn't touch.
bongodrummer (author)  Re-design5 years ago
Yep, that's exactly my concern. I wondered about drilling a number of holes in the lower plate (which will be in contact with the fire/heat), and welding them as well, while the plates are tightly pressed together with loads of clamps. Combined with the edge, might do the trick?
If you're going to ruin the look of the stainless, why not just make it out of plain steel plate?
bongodrummer (author)  Re-design5 years ago
Drilling it from the underside, where it will not be visible wouldn't ruin the look of it. Even on the underside, I could use stainless filler wire (I actually have some and everything) and grind it flat.
I am just talking about drilling through the bottom plate, in a number of places and 'back-filling them with weld.
I've never welded stainless but you'll have to be very careful not to ruin the good side with the heat. But that might work.
Why stainless at all ? Its a worse conductor than MS, its not as emissive in the IR ?

bongodrummer (author)  steveastrouk5 years ago
Mainly because it wont rust, and I could fry an egg on it ;)
Ever used cast iron cook ware ? "Season" it - theres an excellent 'ible on the subject and it becomes non-stick and pretty well rust proof.

This applies to mildsteel, like EN1 as well.

bongodrummer (author)  steveastrouk5 years ago
Interesting. Will look into it. but not sure, because I also want the option of using it as a heat source for other pots 'n pas, and especially a whistling kettle :D
Wont the build up of 'seasoning' prevent good heat transfer... I don' know enough about it.. Would I fill the room with stomach rumble inducing aromas every time I use the stove?
Seasoning puts a carbon layer on the surfaces - carbon is pretty conductive - and inert.
bwrussell5 years ago
What about cast iron?
bongodrummer (author)  bwrussell5 years ago
Haven't looked into it but guessing that it would be harder to shape into a nice circle and then require machining. Also it isn't nice and shiny like ss.
Beenay255 years ago
Just a guess, but what about plug welding the lower plate at various points. That would stop any movement...
Whats the difference if the plate is 10mm or 6mm?
bongodrummer (author)  mpilchfamily5 years ago
6mm is less, and seems more liable to warp or bend with knocks & burn through over time.. It is going to be VERY hot in there - the rest of the stove will be lined with refractory cement. Probably you are right though and 6mm would do the job - it just wouldn't be quite as satisfying ;)
Kiteman5 years ago
JB Kwik Weld is a pretty good thermal conductor, apparently.
bongodrummer (author)  Kiteman5 years ago
Hay Kiteman,
thanks for the lead. Alas a little digging and I find it "withstands temperatures up to 300° F". That.s a measly 150° C which will not cut the mustard in this instance.
What about the "normal" JB Weld?
bongodrummer (author)  Kiteman5 years ago
316 °C - still too low.