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Year 11 chemistry student here. I'm curious about how the case hardening effect works on metals. I know that when the electrons gain an energy 'level' or lose an energy 'level' they emit light at differing frequencies. So why does the outside of metal, after being heated, stay those pretty rainbow colors after they cool down?

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    $\begingroup$ I have never heard about "case hardening", but rainbow colours are interference patterns of reflected light on thin oxide layers.Nothing to do with the actual metal, except that it is below the oxide and reflecting. $\endgroup$
    – Karl
    Sep 18 '20 at 6:33
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    $\begingroup$ Steel tempering, rainbow colors, and electron transitions are very different things, almost unrelated to each other. That is, tempered steel does show some colors, but it doesn't tell us much of its internal structure and the mechanism of hardening. Also, thinking of colors in terms of electronic transitions doesn't help us much in this case. $\endgroup$ Sep 18 '20 at 8:37
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Case hardening (carburizing or induction or flame) does not color steel. You are seeing “temper” colors which are unrelated to case hardening. It is caused by different thickness of very thin layers of iron oxide. These colors were developed during tempering heat-treatment after case hardening. Starting with yellow, then blue and purple in a continuous transition. Faint yellow starts about 300 °F, longer times give more color development of color.

The same colors also develop on stainless steels and titanium at different temperatures and times. A stainless motorcycle exhaust pipe will show the range of colors as temperature changes along its length. If a titanium weld shows yellow color it has been exposed to air at elevated temperature (and is likely defective because of absorbed oxygen and nitrogen).

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