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Default Blueing/oil quenching of mild steel


"Kevin" wrote in message
...
Dave Osborne wrote:
When I was a lad, if memory serves correctly, we used to dip hot ferrous
parts into a bucket of old engine oil. This gave them a blue/black
finish and I'm pretty sure we called it "blueing".

Anyway, I've been Googling and I'm a bit confused, cos blueing seems to
be a rust-proofing process which does not involve oil and oil-quenching
is a hardening process, not a rust-proofing process.

Can anyone put me straight? I was idly thinking about DIY oil quenching
to achieve moderate rust-resistance.

Do I need to use old engine oil, or will new be good/better/safer?

ISTR we heated the parts up to a dull red before quenching. Does this
sound right?


Cheers,

Rumble

from memory quencing in engine oil adds carbon and surface hardens the
steel, old oil has more carbon and therefore hardens more, I did this as
an apprentice years ago and i seem to remember they still rusted


No, it's the quenching (i.e. rapid cooling) which hardens the steel, as long
as
it contains enough carbon. For surface hardening you need to heat the steel
to
red heat in a carbon rich source for long enough for carbon to diffuse into
the
surface, and then you quench it. Look up case hardening.

Oil quenching is faster and better controlled than water quenching because
you don't get the steam blanket, so it is more commonly used in industry,
also
it is better for some alloy steels.

You'll get some rust resistance from hot quenching, and more from proper
chemical blue or chemical black treatments as used on guns, for example.
Lots of proprietary suppliers or you can mix your own. Look up chemical
black
or blue