Re: [Phys-L] Blue color heated steel

[John Denker <jsd@av8n.com>]

On 10/6/21 4:11 PM, Paul Nord wrote:

> I googled my assumption and found out that welding
> produces a whole range of colors:
> https://www.millerwelds.com/resources/article-library/welding-colors-what-they-mean-and-why-they-matter

Steel is fantastically complicated.

The article cited above was written by a welder who is in large
measure an artist. I assume he is primarily interested in the
iridescent colors produced by surface layers.

Everything he says is true, but there's a lot more to it.

The iridescent surface colors come from layers that are mostly
transparent and half a wavelength thick.

There is also a non-iridescent color called "steel blue" that
is another type of surface layer, a blue-black layer of Fe₃O₄
produced via an electrochemical passivation process.

And there is still more to the story. Surface layers are not
the whole story.

For example: If a piece of tool steel gets burned blue, that is
not a surface layer. The blue goes deep into the metal. To fix
the tool, you need to grind away all the blue and then refigure
the working edge from scratch. Not fun. The tool will be useless
until you do that.

To repeat: When tool steel has been burned blue, it is not a
surface layer that you can wipe away with a bit of emery paper.

There are different kinds of steel, each of which has its own
very complicated microcrystalline structure, produced by clever
chemistry and clever heat treatment schedules. The size and shape
of the microcrystals determines the macroscopic properties such
as hardness and malleability. I'm not an expert, but it seems
plausible that tool steel starts out with very small microcrystals,
and when it is overheated, the microcrystals grow large enough
to scatter blue light.

Blue isn't the whole story; sometimes you observe a yellowish
straw color in burned tool steel.

Burned here is jargon for overheated. The tool isn't oxidized to
any significant extent.

Encyclopedia Britannica has a wonderful article on steel and
steelmaking in the Macropedia section. AFAICT it's not readily
available online, but you should be able to find it in a library.