Roberto,The standard reason given for example at Wikipedia https://en.wikipedia.org/wiki/Tempering_(metallurgy)is the progressive accumulation of a thin translucent surface oxide layer on carbon steel when reheated after quenching in an oxidising atmosphere. This provides color by interferometric means.
A moment's consideration will suggest however, that a layer of increasing thickness provides light reinforcement at progressively lower frequencies , rather than blue representing a spring temper requiring a higher temperature andpresumably thicker oxide layer. The answer may lie in the growth of microcrystalline structure of a particular kind with heat, but I did not quickly find a sensible reference.
On Monday, October 4, 2021, 09:26:11 AM CDT, M R Carabajal via Phys-l <firstname.lastname@example.org> wrote:
I would greatly appreciate a bibliographic indication on the subject of
heating steel. I have not been able to find a clear explanation of the
reason for the blue color, that steel presents when heated to about 600
Kelvin. That is, to substantiate the deviations of the blackbody heating