What affordable sheet metal will be resistant to high temperatures without warping?

[user801028]

Maybe a rudimentary question for yall but was not getting much useful information from diyers.

I have a cooktop in a van with the normal heat resistant glass but due to the bumping around when driving as well probably to poor design quality of the glass it cracked.

Rather than replace with another glass one which is liable to crack I thought why not replace with metal instead.

The size I want is relatively small at 29x29cm. Thickness 1-3mm will do.

In my naivety I did not consider the issue of warping at all and just bought standard 3mm mild steel since it was cheaper.

Placing the metal in the recess and firing up the cooker the appliance didn't even have a chance to get through its run up cycle to reach full heat and the metal was already bending. At first I wondered what on earth was happening when I saw it raising off the ground then I twigged it was warping from the heat. This also caused diesel fumes, as that is what it runs on, to bellow out so I had to shut it down quick.

Now I still have the option to use glass which is made for the job of withstanding cooking heat but if there is some metal that will resist the bending under cooking temperatures, I suppose a few hundred Celsius, then metal would still be preferable due to lack of breakage factor.

As I thought and read a little more I realized stainless steel is used for much cookware and that doesn't warp. Also bbq and fast food cooktops I noticed use some kind of sheet metal where they fry stuff right off the cooktop so there must be some metals which will withstand it. Is that likely stainless steel too? Price wise it also seems not too much more than normal mild steel. So will that suffice? It really does have to be well resistant to bending because once placed on the cook surface, after testing it works, it will have to be sealed with heat resistant sealant to keep the fumes contained in the burn chamber and vented out. So it must maintain its form under constant heat and over time.

Will stainless steel still be suitable given those requirements or is something else better, again, considering price? It should be at most as much as a ceramic/glass cooktop which comes to about 40-80 gbp for the size I want.

Edited February 17 by user801028

[swansont]

I suspect your issue is due to the thermal expansion; various kinds of steel will expand probably 10-20 times as much as the glass used in stovetops. If it's constrained in any way, it will tend to buckle. Being thin probably doesn't help.

 

[exchemist]

7 hours ago, user801028 said:

Maybe a rudimentary question for yall but was not getting much useful information from diyers.

I have a cooktop in a van with the normal heat resistant glass but due to the bumping around when driving as well probably to poor design quality of the glass it cracked.

Rather than replace with another glass one which is liable to crack I thought why not replace with metal instead.

The size I want is relatively small at 29x29cm. Thickness 1-3mm will do.

In my naivety I did not consider the issue of warping at all and just bought standard 3mm mild steel since it was cheaper.

Placing the metal in the recess and firing up the cooker the appliance didn't even have a chance to get through its run up cycle to reach full heat and the metal was already bending. At first I wondered what on earth was happening when I saw it raising off the ground then I twigged it was warping from the heat. This also caused diesel fumes, as that is what it runs on, to bellow out so I had to shut it down quick.

Now I still have the option to use glass which is made for the job of withstanding cooking heat but if there is some metal that will resist the bending under cooking temperatures, I suppose a few hundred Celsius, then metal would still be preferable due to lack of breakage factor.

As I thought and read a little more I realized stainless steel is used for much cookware and that doesn't warp. Also bbq and fast food cooktops I noticed use some kind of sheet metal where they fry stuff right off the cooktop so there must be some metals which will withstand it. Is that likely stainless steel too? Price wise it also seems not too much more than normal mild steel. So will that suffice? It really does have to be well resistant to bending because once placed on the cook surface, after testing it works, it will have to be sealed with heat resistant sealant to keep the fumes contained in the burn chamber and vented out. So it must maintain its form under constant heat and over time.

Will stainless steel still be suitable given those requirements or is something else better, again, considering price? It should be at most as much as a ceramic/glass cooktop which comes to about 40-80 gbp for the size I want.

I'm no expert in this area but I'll have a go, to start off the discussion.

I  presume the warping you refer to is due to thermal expansion of the metal, in those areas where it gets hot, whereas the rest of it stays cool. There is a table of coefficients of thermal expansion here:

https://www.engineeringtoolbox.com/thermal-expansion-metals-d_859.html

According to this, the coefficient for stainless steel almost double that of mild steel, so it would be expected to expand more with heat and warp more. This is just for metals but here is one that includes glass: https://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

According to this, plate glass is similar to stainless steel while unqualified "glass" is similar to mild steel.

Given that glass is brittle, it will crack if it experiences too wide a temperature variation across the specimen. That being so I am wondering what the design of this cooker is and whether you may have inadvertently removed something that avoids a large temperature differential from being created, or else you have not included a gap, or cut, or flexible fixing, somewhere, that permits differential thermal expansion.  

Alternatively it may be a question of thickness. Frying pans are commonly made of steel and cheap ones can bow upward in the  centre if they are too thin. However better quality ones (heavier, thicker ones) seem not to, presumably because they can contain the thermal stresses within the metal without bending appreciably.

Regarding glass if, it was not just glass but Pyrex, this has a lower coefficient of expansion (~3 x 10⁻⁶mm/mm/ Cdeg) than any of the above materials and can be made thicker and a lot more thermally resistant. 

P.S. cross-posted just now with @swansont

Edited February 17 by exchemist

[swansont]

15 minutes ago, exchemist said:

According to this, plate glass is similar to stainless steel while unqualified "glass" is similar to mild steel.

I used numbers for borosilicate glass (like the old pyrex) which is smaller.

[user801028]

On 2/17/2024 at 12:20 PM, swansont said:

...

 

On 2/17/2024 at 12:24 PM, exchemist said:

...

Thanks both. Your information made me see it is probably better to just go for the glass made for the job. I also learned a new thing in thermal expansion. When I was looking at the ads for the stove glass again I suddenly did notice thermal expansion mentioned for one and lauded at "nearly 0%".

Edited February 20 by user801028

[exchemist]

5 hours ago, user801028 said:

 

Thanks both. Your information made me see it is probably better to just go for the glass made for the job. I also learned a new thing in thermal expansion. When I was looking at the ads for the stove glass again I suddenly did notice thermal expansion mentioned for one and lauded at "nearly 0%".

0% is interesting. There may I suppose be special "stove glass" with an expansion coefficient even lower than Pyrex. Anyway yes, thermal expansion of a part in the middle of a plate, that is hotter than the periphery, will tend to make it bow up or down, or twist, to relieve the strain  - i.e. warp. One other thing: glass is a good thermal insulator. So the glass top may also protect whatever is underneath from getting too hot. 

Anyway, glad you found the comments helpful and good luck with the repair.

 

[John Cuthber]

https://en.wikipedia.org/wiki/Glass-ceramic#Glass_ceramics_in_cooktops

And 
"An interesting property of these glass-ceramics is their thermomechanical durability. Glass-ceramic from the LAS system is a mechanically strong material and can sustain repeated and quick temperature changes up to 800–1000 °C. The dominant crystalline phase of the LAS glass-ceramics, HQ s.s., has a strong negative coefficient of thermal expansion (CTE), keatite-solid solution as still a negative CTE but much higher than HQ s.s. These negative CTEs of the crystalline phase contrasts with the positive CTE of the residual glass. Adjusting the proportion of these phases offers a wide range of possible CTEs in the finished composite. Mostly for today's applications a low or even zero CTE is desired. Also a negative CTE is possible, which means, in contrast to most materials when heated up, such a glass-ceramic contracts. At a certain point, generally between 60% [m/m] and 80% [m/m] crystallinity, the two coefficients balance such that the glass-ceramic as a whole has a thermal expansion coefficient that is very close to zero." from teh same page.