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Below is a 150x microscope view of a 301 steel plate. It is covered by this type of small blue spots.

It appeared after the steel had been treated at 400°C. The subcontractor thinks that the steel was not cleaned correctly.

Does anyone have experience with similar results ?

AISI 301 steel x150 after 400 degrees Celsius heat treatment
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    An electron microscope with EDAX ( energy dispersive analytical X-ray) would be very handy to determine the cause. If you would need to pay someone to do it , it is likely to be very expensive. I suggest blasting or polishing the spots off if you don't want them. It looks like "heat tint" colors caused by different thickness of oxide layers but I can't imagine how they could develop on such a small scale,
    – blacksmith37
    Nov 12, 2018 at 20:22
  • The electron microscope reveal nothing more than the base metal with a lot of oxyde sadly...
    – user6131
    Feb 25, 2019 at 20:21
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    The range of colors shown : gold to blue to purple , on steel and stainless are caused by different thicknesses of oxides. They are very thin. Often they are caused in a situation with a temperature gradient like an engine exhaust pipe.
    – blacksmith37
    Feb 27, 2019 at 1:07
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    The spots look like droplets of something that got on the metal rather than local features of the metal, itself. The color could have been from the residue reacting to heat, the residue reacting with the metal when heated, or the residue turned the metal bluish through oxidation and the effects of heat. It looks like the liquid caused some oxidation around the edge of the droplets.
    – fixer1234 ♦
    May 23, 2021 at 0:01

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