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.

W1 etc many times can be heated quickly and quenched.

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. ??

Alvin in AZ