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.

[user801028]

Ok to resurrect this thread, the glass cracked after what is that 8 months since making this thread when I bought it? That is disappointing.

It has a rating of 700c and yet the crack was certainly from the heat rather than an impact, the latter of which what caused it in the first instance for the one it originally replaced.

I was boiling some food and there was a definite pop/crack sound. I raise the stove off and the crack is in a circular region right where the diesel flame hits it.

For the number crunchers what average top temperatures should a diesel flame produce or is it too much how long is a piece of string? I couldn't imagine the direct diesel flame from a small stove would amount to higher temperatures than a log burner, which the glass is also geared for I think.

Anyway, regardless, it worked fine for those months in the interim so not sure why it would happen all of a sudden.

I can only guess that the daily placing on an off of the heavy pot perhaps weakened it causing micro fractures and then on that day the heat broke the camel's back so to speak. Still it was sold as stove glass so would expect it to be able to handle that too.

I am wondering where to go from here as it seems I paid around £60 for that glass and having to pay that every 8 months, not to mention the tricky job of replacing the glass, is not too enticing.

Winter is coming too, where I want to use it as a heater most. Looks like I will be getting a lot of early nights in the double sleeping bags if I don't fix it. I have been doing that already though due to working a lot of manual labor lately and just because it is going dark early so might not be much of a deprivation.

So the idea of metal would be out for something which would be comparable in price for comparable thermal expansion qualities?

Edited October 29 by user801028

[studiot]

27 minutes ago, user801028 said:

I was boiling some food and there was a definite pop/crack sound.

I wonder about the bases of your pots and pans.

 

The instructions with my AGA ceramic hob are quite clear.

You should only use pots and pans with a (ground) flat base.

Pans with ridges or dimpled or other patterned bases are forbidden.

 

I think this is because of what happens in the space between the hob and where the pan base does not touch.
A slight overspill or dirt etc can cause superheated gas between the pan and the hob which can have a powerful erroding effect.

As regards the hob itself, I was just looking at my grandmother's old welsh bakestone, made of cast iron.

This has happily gone through more than a century of gas flame burning directly under it.

The temperature of natural gas is about 2700^oC and diesel is sbout 2000^oC.

So perhaps you should consider cast iron?

A simple method would be to construct a support grid for an iron or steel plate and leave the plate free to expand and contract ie just resting on the grid.

[Externet]

Stoves use cast iron around the hot area.  Perhaps you can find in plate form; or grille.

 

[user801028]

13 hours ago, studiot said:

I wonder about the bases of your pots and pans.

 

The instructions with my AGA ceramic hob are quite clear.

You should only use pots and pans with a (ground) flat base.

Pans with ridges or dimpled or other patterned bases are forbidden.

 

I think this is because of what happens in the space between the hob and where the pan base does not touch.
A slight overspill or dirt etc can cause superheated gas between the pan and the hob which can have a powerful erroding effect.

As regards the hob itself, I was just looking at my grandmother's old welsh bakestone, made of cast iron.

This has happily gone through more than a century of gas flame burning directly under it.

The temperature of natural gas is about 2700^oC and diesel is sbout 2000^oC.

So perhaps you should consider cast iron?

A simple method would be to construct a support grid for an iron or steel plate and leave the plate free to expand and contract ie just resting on the grid.

Ah yes, useful comments thanks.

 

Cast iron sounds a good idea as also @Externet suggests.

 

Yes, as you say, with regards to the surface contact of pot to stovetop I think that might be a likely cause because the instruction manual clearly states 'no idle cooking' which I took to mean no cooking with no pot on the stove due to risk of breaking the glass. When the hood is down the fan then blows the hot air out and into the room, forming the heater part of the cooker/heater combo. When lifted the fan is not engaged so there must be something to absorb the heat.

I have been doing a lot of outdoor work lately so lots of gunk and gravel being brought into the van floor, where I set the pot down. Also I must admit I had been lax on cleaning the stove surface and quite a bit of gunge including a bit of lentil overspill having caked it when I looked this morning. So I wonder if it was a case of the pot not being flush causing the crack. Seems the most likely culprit.

For cast iron it would have to be a sealed plate as otherwise there will be diesel fumes being released in an enclosed space - which would be a nono :). Also, as above, the pot must stay flush to absorb the heat so would be no good to put a cast iron onto a glass surface.

Cast iron, from my experience of stoves is usually very thick, to the order of several cms or so. This glass was only 5mm and there is little more clearance for thicker, maybe a couple more mm, for the hood to be able to go down without obstruction. Would a thinner piece of cast iron still have the same thermal expansion resistance properties. That has not been discussed either - what is the thermal expansion or lack thereof of cast iron. As per the OP, due to the sealed nature, it has to have very little so as not to break the seal, releasing diesel fumes, during operation.

Besides the expansion properties will it have similar thermal transfer to the glass? As it is used so much for other cooking tops I guess it must be pretty good. It doesn't have to be identical and the cooker is rather efficient so I could lose some efficiency in place of reliability and sturdiness of the top.

[user801028]

Looking online it seems very hard to find a piece of cast iron metals making this option seem unattainable. All that is sold is raw iron bars.

 

Any other options that would actually within reach or ways to get a 30cm x 30cm cast iron sheet repurposed from something else? I think the latter is unlikely since it would be a real fluke to be the right size and depth and no way I want to try and fashion it myself.

 

Would stainless steel be no good? As discussed earlier that doesn't seem to warp under heat when looking at pots and pans and is superabundant.

 

From the previous replies stainless steel should have similar thermal expansion properties to pyrex? Hmm that is weird...so mild still should have even performed better due to lower rating yet it is the one which spurred this thread in the beginning. Lower is better right? I am presuming so because the pyrex and such glasses, made for heat resistance, are also low listed here.

I don't get how the mild steel which caused me problems from the start is supposedly the same range for thermal expansion as pyrex?

Edited November 3 by user801028

[exchemist]

1 hour ago, user801028 said:

Looking online it seems very hard to find a piece of cast iron metals making this option seem unattainable. All that is sold is raw iron bars.

 

Any other options that would actually within reach or ways to get a 30cm x 30cm cast iron sheet repurposed from something else? I think the latter is unlikely since it would be a real fluke to be the right size and depth and no way I want to try and fashion it myself.

 

Would stainless steel be no good? As discussed earlier that doesn't seem to warp under heat when looking at pots and pans and is superabundant.

 

From the previous replies stainless steel should have similar thermal expansion properties to pyrex? Hmm that is weird...so mild still should have even performed better due to lower rating yet it is the one which spurred this thread in the beginning. Lower is better right? I am presuming so because the pyrex and such glasses, made for heat resistance, are also low listed here.

I don't get how the mild steel which caused me problems from the start is supposedly the same range for thermal expansion as pyrex?

The thickness of a metal sheet will very likely make a big difference. A thick sheet will warp less easily, I suspect. It will have greater mechanical strength and the thicker material may be able to conduct heat away better and so reduce peak local temperatures. But not sure we can be of much more help. You've got onto the coefficient of thermal expansion, which is must be the key point of science involved in this application. Sounds to me as if you were very nearly there with the glass, if it lasted 8 months. But obviously not quite. Pity.   

[studiot]

2 hours ago, user801028 said:

Looking online it seems very hard to find a piece of cast iron metals making this option seem unattainable. All that is sold is raw iron bars.

 

Any other options that would actually within reach or ways to get a 30cm x 30cm cast iron sheet repurposed from something else? I think the latter is unlikely since it would be a real fluke to be the right size and depth and no way I want to try and fashion it myself.

 

Would stainless steel be no good? As discussed earlier that doesn't seem to warp under heat when looking at pots and pans and is superabundant.

 

From the previous replies stainless steel should have similar thermal expansion properties to pyrex? Hmm that is weird...so mild still should have even performed better due to lower rating yet it is the one which spurred this thread in the beginning. Lower is better right? I am presuming so because the pyrex and such glasses, made for heat resistance, are also low listed here.

I don't get how the mild steel which caused me problems from the start is supposedly the same range for thermal expansion as pyrex?

Yeah you really need a foundry for cast metals, which is why cooker manufacturers, like AGA, often use inserts in the hob.

I can see that this is difficult with your diesel flame and fumes.

 

But it is not only the expansion that is important.

The heat transfer coefficient is also important.

Basically ferrous alloys  - iron, steel etc have good coefficients.

Fancy steels, especially stainless steels, have significantly poorer coefficients.

This is why all good quality pan bases are not solid stainless steel but some sort of sandwhich (often copper based).

Otherwise there is not only the heat loss from having to drive the system harder but also the greater danger of burning the contents.

Many very thin bottomed pans as susceptible to this burning, if they are made of stainless steel.

[John Cuthber]

On 10/29/2024 at 8:57 PM, Externet said:

Stoves use cast iron around the hot area.  Perhaps you can find in plate form; or grille.

 

The clever bit about those is not the choice of material so much as the big gaps. That means they are less troubled by expansion.
They don't buckle and warp because, for the most part, they can just expand a bit.
They use cast iron because it's strong enough, has a high enough melting point, and it's cheap.

A sheet of cast iron would only be slightly better than a sheet of glass.

[user801028]

3 hours ago, John Cuthber said:

The clever bit about those is not the choice of material so much as the big gaps. That means they are less troubled by expansion.
They don't buckle and warp because, for the most part, they can just expand a bit.
They use cast iron because it's strong enough, has a high enough melting point, and it's cheap.

A sheet of cast iron would only be slightly better than a sheet of glass.

Only slightly? How about the huge advantage of not cracking like glass. :)

Yes sadly a must is that it has to be sealed, thus ruling out many options, in order to keep the diesel fumes contained in the appliance's chamber and exhausted outside.

Seems taking my chances with glass again is my only viable option.

I suppose £60 for 8 months of daily usage is not the worst as a lot of things require servicing every 6 to 12 months and would cost more. Certainly not ideal and a faff to have to change it out each time. I would try and prevent by keeping a cleaner stovetop as I suspect the uneven base due to old food and general muck may have caused the overheating and cracking where the pot was not flush with the surface.

[John Cuthber]

On 11/3/2024 at 3:40 PM, user801028 said:

Only slightly? How about the huge advantage of not cracking like glass.

Cast iron will crack due to uneven thermal expansion if you are not careful.

Edited November 9 by John Cuthber

[zapatos]

On 11/3/2024 at 2:40 AM, user801028 said:

Looking online it seems very hard to find a piece of cast iron metals making this option seem unattainable. All that is sold is raw iron bars.

 

1. Go to Amazon and look at replacement parts for stoves, BBQ pits, etc.

[StringJunky]

16 hours ago, zapatos said:

1. Go to Amazon and look at replacement parts for stoves, BBQ pits, etc.

I just looked and 5mm or 10mm mild steel is used for  BBQ hotplates. 

OP Can you mod the top to take a 5 or 10mm steel plate and have the necessary seal? I'd be inclined to use 10mm.