From: arno@antares.utu.fi (Arno Hahma)
Subject: Re: Knives made of Damascus Steel
Date: 1998/02/23
Message-ID: <6csp15$gj5@antares.utu.fi>
Newsgroups: sci.chem,sci.chem.analytical,sci.chem.coatings,sci.chem.electrochem,
	sci.chem.electrochem.battery,sci.chem.labware,sci.chem.organomet

In article <6crvk4$nq$4@news.iinet.net.au>, Trevor Calder <No> wrote:

>I'd want an idea what you mean by 'ultrahigh' - I certainly wouldn't want
>a blade made of ultrahigh carbon steel - it'd break far too easily.
>About 0.9% carbon for a knife, 0.7% for a sword is OK.

Ultrahigh carbon steel is steel with 1 to 2.5 % of carbon in it. Yes,
it is very brittle, if it is not treated properly. However, if you do
treat it properly, the strength can be about 20..25 % more than the
best, high alloy steels.

Heat treatment should be done in three steps. First normalize the steel
at 1100..1150 oC for 12..16 hours, cool slowly (10 oC /hour). This will
cause a coarse grain, continuous cementite network to form and convert
as much as possible from the carbon into iron carbide and distribute
the carbon evenly throughout the metal. As such, the steel is very
brittle and not usable. Next, reheat to 650..800 oC and roll or forge
up to 5..10 fold elongation (or more, if desired). This will reduce the
cementite network into a fibre-like structure in the roll direction,
but does not break the chemical carbide bonds (at least not much).
Overheating effectively destroys the steel and you will have to start
over.

The final step is hardening. The steel has to be heated only up to
730..750 oC and then quenched into brine. This low a hardening
temperature prevents cementite grains from growing while still
releasing some carbon into the lattice. The lesult is a very finely
grained, oriented structure steel, i.e. tough and hard. The steel won't
have a damasc pattern, but it is stronger than the "real" damascus
steels. If damasc pattern is desired, heat up to 1000 oC for a short
time, then air cool to 800 oC and quench. That will cause cementite
grains to grow here and there and a damasc pattern will appear.
However, the steel is not as strong as without the pattern.

>The cutting abiltiy is caused by the serrations between the different
>steels. Being of differing hardnesses, the steels are removed differently
>during sharpening. This leaves an edge with sub-visible serrations.

You can also achieve that with the above heat treatment. The damasc
pattern means you have an uneven distribution of carbide grains in the
blade. Grinding such a blade will also make a serrated edge, as the
carbide is harder than the surrounding metal.

The japanese blades and the Damascus blades are about equivalent in
their properties. The Japanese just did it the hard way, while the
Damascus smiths knew, how to get to the same result much easier. Both
methods produce a fine grained, oriented structure, which is tough but
still relatively hard.

>European smiths had been forge welding blades of differing steels for
>a long time. Long before the Crusades.

Yes, but they did not know how to treat high carbon steel. Europian
smiths used lower carbon steel, that could be forged at higher
temperatures. If you try to forge ultrahigh carbon steel at the usual
1000 oC temperature, it will crumble like cast iron.

>Trevor Calder

ArNO
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