AISI 301 steel x150 after 400 degrees Celsius heat treatment


This is a x150 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.

Do you have some experience with similar results ?


  • 1
    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 '18 at 20:22
  • The electron microscope reveal nothing more than the base metal with a lot of oxyde sadely... – user6131 Feb 25 '19 at 20:21
  • 2
    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 '19 at 1:07

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