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Ed Huntress

On Mon, 19 Dec 2011 02:37:24 +0000 (UTC), wrote:

Ed Huntress wrote:
In general, it's the result of heating carbon steel in air -- the
hotter you get the steel, the worse the condition is. Heating it
above transition temperature (Curie point) without protection from
oxygen in the air will start decarb immediately. The carbon
combines with the oxygen. The higher the temp and the longer the
part soaks at high temperature, the worse it gets.

Right on the money, EH.

Plain high-carbon steels (W1; music wire; AISI/SAE 1070 and above)
are the worst offenders. High-alloy steels, in general, present
less of a problem.
Ed Huntress

Yeah but. LOL

Except high alloy steels have the worst reputation for reasons you
gave in the first part tho. The high alloy steels need longer soak
times at higher temperatures and so cook off more of their carbon.

Hmmm...Ok. That could get unnecessarily complicated for what's being
discussed here, but, in a broad generality, the diffusion rate of
carbon in plain-carbon steel is higher than for many, if not most
alloys, and thus it reaches the surface and oxidizes more quickly at
high temperatures. It's also true that alloy steels containing
chromium, molybdenum, and maybe some other elements require less
carbon to achieve a given hardness, so, in terms of hardenability,
it's a more critical issue with plain-carbon steel.

Beyond that, it's probably not important to get into the details.

W1 etc many times can be heated quickly and quenched.

True. You sacrifice some strength if you also temper it quickly, but
it's done all the time in blacksmith work and in other heat treating
done in a forge or with a torch, as a practical matter. And the
initial heating/quenching rate itself doesn't have much effect as long
as the piece has reached the transition temperature all the way
through.

But yeah, even heated and quenched quickly there's still a reduced
carbon layer of some sort when using my propane burners. In my case
it's expected and I just grind it off but also I'd grind it thinner
anyway since a thin knife blade cuts most stuff better.

With my gun and knife springs made from 1095 or O1 the thin reduced
carbon layer doesn't seem to effect anything. ??

It affects something, but not something that may be noticed or that
may matter in your applications.

A skin of decarb reduces the yield point and hardness of that surface
layer, right where it matters most. However, it doesn't affect the
Young's modulus, so it has no effect on spring rate -- within the
elastic limit.

So a spring with a very thin decarbed surface will perform
identically, as a spring, to one that's hard all the way through. It
just can't be bent quite as much without taking on some degree of
permanent set. In a normal application, your spring probably is
operating well within the yield-strength envelope of that steel, so
you'll never notice it.

--
Ed Huntress

Alvin in AZ

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