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Author Topic: Heating metal  (Read 3429 times)

Offline Les the Scientist

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Heating metal
« on: 05/11/2004 06:38:05 »
Why, when you heat up something made of stainless steel like a spoon, does it turn multicoloured afterwards ?
« Last Edit: 30/11/2004 17:06:54 by NakedScientist »


 

Offline Ylide

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Re: Heating metal
« Reply #1 on: 05/11/2004 07:36:21 »
I'm not 100%, but I believe it's because of oxides forming on the surface of the metal, which refract the light that normally gets reflected from the surface.



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Offline chris

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Re: Heating metal
« Reply #2 on: 05/11/2004 09:09:30 »
This is a great question. I am pretty sure that Ylide is correct - you do indeed get some blue iron (Fe3O4) which, as the name suggests, is a very pretty bluish colour. I therefore agree that the colourations are oxides of iron.

But, by sheer serendipity, Dr. John Emsley is the guest on this week's Naked Scientist radio show. John is a professional chemist who has substituted the writing desk for the lab bench and now pens some stunning books on his subject.

His Shocking History of Phosphorus - http://nakedscientists.com/HTML/books/chemistry.htm - is a masterpiece.

This weekend he's coming in to talk about pyrotechnics and the chemistry of fireworks - but I'll ask him about why a piece of stainless steel changes colour on heating too !

For the answer you can catch the show via http://www.thenakedscientists.com/listen - it will be archived there for a week in Real format, then moved to our archive where it will be available as a streamed MP3, or as a download MP3 (for iPod or MP3 player).

Chris

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Offline Ylide

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Re: Heating metal
« Reply #3 on: 07/11/2004 13:04:06 »
Cool, I'd be curious to know if I nailed that one.  I'm not that up to date on my metallurgy.  =/  





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Offline tweener

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Re: Heating metal
« Reply #4 on: 08/11/2004 00:34:55 »
I don't know any chemistry at all, but I don't think it's oxides on the surface.  I say this because if you cut the metal, the discoloration goes well below the surface.  This happens in other types of steel besides stainless and causes real problems when sharpening knife blades or saw blades.  Hardened steel that is heated enough to discolor no longer holds an edge.  I'll be curious to know the answer.

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Offline Ylide

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Re: Heating metal
« Reply #5 on: 09/11/2004 18:43:31 »
Given that information, I would suggest that the heat causes positional changes in the crystalline lattice of the atoms making up the steel.  That would make sense considering it changes the hardness, as John mentioned.  The change in lattice structure would account for the change in light absorption.  





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Offline neilep

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Re: Heating metal
« Reply #6 on: 09/11/2004 19:59:58 »
Thanks for a very interesting thread guys...Though I'm a voyeur on this one I find it fascinating...cheers.

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Re: Heating metal
« Reply #6 on: 09/11/2004 19:59:58 »

 

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