Heat treatment of copper

[nsv]

A 0,25mm copper braid consisting of 0,08mm strands is heated to 220°C for 35 minutes which causes the surface to take a golden hue.
The material feels the same, but still I would like to know what causes the color change and if it influences the ability to bend and withstand vibrations.

NSV

 

[kenvlach]

Probably interference coloring. Heating in air creates an oxide thickness of about 300 nanometers (0.3 microns)* which produces a yellow optical interference coloring. The interference is between light reflecting off the metal surface (refracted as it passes in & out) and light reflecting off the oxide topr surface.
There is also some natural coloration of the copper oxides, Cu2 being ~pink and CuO being black.

*0.2 microns per table here:

http://mitghmr.spd.louisville.edu/course/colors.html

 

[nsv]

Thanks kenvlach, I
take it that I can use
the braid as before,
without worrying about
increased brittleness.

NSV

PS What happens to the
text editor in this
thread? I can see only
the left half of the
page

 

[nsv]

PPS: That interference
color chart is cool!

NSV

 

[kenvlach]

nsv,
The oxide coating increases the overall wire diameter by about 0.1%, with the copper core diameter decreasing by about 0.1%. There will be a slight decrease in mechanical properties; surface oxide films are brittle and reduce fatigue properties.
Also, the thermal emissivity of copper wire is increased by surface oxidation.

I apologize for the oversized interference color chart; the smaller ones I found last night were illegible & couldn't find the earlier thread. The ones in the following thread are more straightforward
Blueing steel parts
thread818-144274