What causes rainbow color in heated steel?

[bigfooted]

Hi,

This question came up after some stainless steel samples were exposed to high temperatures (metal flame holders in a combustion chamber). Some of them had areas with rainbow colors. What exactly happens to the steel that causes these local rainbows? If the colors in point A and B on the sample are different, does that mean that in point A the steel has been exposed to different temperatures than in point B?

 

[AlephZero]

The thickness of the oxide layer depends on the temperature, and the thin layer creates interference effects for different wavelengths of light, and thus show colors - similar to a thin oil film on water showing "Newton's rings".

http://en.wikipedia.org/wiki/Tempering_(metallurgy)#Tempering_colors