Dark magenta colored Stainless Steel Heat treatment

[MechatroPro]

I have a couple of parts made of Stainless steel 403, the surface is dark magenta/Red color, I'm wondering what type of heat treatment/hardening process results in such a surface. Any hint is welcome.

Here's a picture of the parts:

http://s30.postimg.org/jaqu0fgu9/hardening.jpg

 

[Ron]

Any possibility there was a film of oil or other contaminant on the surface prior to heat treatment?

 

[CoryPad]

Looks like temper scale, which seems plausible since 403 is a martensitic alloy that would be hardened and tempered.

 

[metengr]

Agree with CoryPad. Tempering in air will cause surface oxidation and depending on oxide thickness will cause the subsequent temper color.

 

[EdStainless]

And of course this surface will have much lower corrosion resistance.

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P.E. Metallurgy, Plymouth Tube