"Leo Lichtman" wrote in message
...
I followed your explanation with great interest.

Thanks.

You seem to take me back
to some the beliefs I held originally, and abandoned briefly as this
thread unfolded. I take it that you do not agree that a certain color
on the surface is correlated with a certain temperature in the metal.

It can't be, since I've torch tempered and oven tempered metal myself. The
oven pieces come out considerably darker, in the purple range as I
mentioned.

Since "drawing the temper" of steel, as done by blacksmiths, using the
surface colors, is a way of raising the steel to the desired
temperature, why does it work?

Because it works in the short term. As you expand time exponentially, you
get more "out" of it, but after a while, to get even a small amount out,
takes a very long time. The endpoint is arbitrary; by eye, over a few
minutes, you'll probably spot between 300 and maybe 500°F (SWAG). In the
oven, you get a bigger change out.

Why does it work as far as the metal? Two reasons. For one thing, it's
just simple carbon steel, you can't really go wrong with it (short of
overheating before quench, which makes it crunchy no matter what!). Number
two, the reactions in the metal, where bainite and whatnot break down to
more stable phases during tempering, is the same kind of time-temp governed
reaction as the oxidation is. It might not proceed at the same rate (it
would be interesting to compare this!), but who knows.

Wouldn't a longer time at a lower
temperature produce the same interference colors as a shorter time
at a higher temperature? Could
it be that the time/temperature history produces the same effect on
color that it does on hardness?

Exactly! So it may be that my purple blades at 350°F for an hour are
overtempered, while the yellow-for-a-few-minutes torch tempered jobs are
undertempered. This is where some imperical evidence comes in handy. The
last blade I tempered was the brass handled knife he
http://webpages.charter.net/dawill/I...rassKnife2.jpg
Now I've sharpened this good enough to shave with (not real comfortable, but
it cuts the hair smoothly anyway..), and in the process I don't notice much
of a burr turned up so it must be pretty hard. That's fine with me since
it's so thick it'll "never" break.

I tempered that to 350°F for an hour and it came out purple (splotchy mind
you, fingerprints for instance are prime spots to prevent oxidation). It
seems to be simple 1080-1090 (used to be a chisel), nice yellow-white bursts
on grinding, same as a file.

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms