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I have an aluminium baking plate that I did put in a very hot oven and now it is soft, so I can bend it or almost roll it up. Why?

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How hot an oven, and for how long? –  rob Jun 11 at 1:50
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If the temperature of the plate was high (T>550F) for several hours, you may have annealed the metal. Work hardening is sometimes used to temper aluminum objects, and this would have been sufficiently hot to anneal some aluminum alloys.

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How specific is that 550ºC ? Most self-cleaning ovens go up to 900ºF = 500ºC and stay there for a few hours; I can't think why a home oven would get any higher. –  rob Jun 10 at 0:35
    
Good catch. The reference figs I linked to are actually shown in F degrees, I wrote C, I corrected the answer. Thanks –  Mark Rovetta Jun 10 at 16:44
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I don't know whether or not this is what happened in this case, but some metals have multiple crystal (solid) states. Changing heat changes the crystal structure, which changes the metal's physical properties, such as stiffness. Tin is a good example - if cooled down cold enough and long enough, it becomes very weak, crumbling under the slightest pressure. It could be that you heated up the pan hot enough to do something like this. Again, I don't know if this was what happened, but it is possible.

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