On 3 Aug 2003 13:46:32 -0700, (Jon Weaver) wrote:
I want to harden a piece of steel, and from my school metal work days,
I think that I can simply heat it to red hot and then plung it into
'oil'

Is this the right? If so, what sort of oil should I use.. Are we
talking normal engine oil, or there there a special kind??

As you can tell, I havn't really got a clue what I am talking about,
so any info would be appreciated

There's a bit more involved. First you have to know the type
of steel. Some steels can be heat treated, others can't. Basically
this is a matter of the carbon content of the steel. Below about
20 points, you have mild steel, which won't harden. Above about
90 points carbon, hardening becomes tricky (big risk of cracking
if you don't do it exactly right). Other alloying elements can also
change the heat treating process.

Each kind of steel requires heating to a certain temperature, then
quenching in a certain medium (sometimes water, sometimes oil,
sometimes just cooling in air, depending on the particular requirements
of the particular steel). Then it needs to be tempered because
hardening makes the steel brittle, and the "temper" of the metal has
to be drawn back a bit to restore some toughness. (Tempering is
usually done between 400 and 600 degrees, depending on the
particular steel, and the final properties you want.)

So with an unknown steel alloy, you're shooting in the dark.
If you know the alloy, then you have a better chance of being
able to harden it successfully. For example, suppose your
steel was O1. Then you'd heat it to cherry red (just above
the temperature where a magnet won't stick to it), then plunge
it in oil (vegetable or mineral oil preferred, though petroleum oil
can be used). Then reheat to about 475 degrees to temper
it.

OTOH, if the steel is W1, you'd have to heat it a bit hotter,
quench it in *water*, and draw the temper at about 575
degrees. If it were A1, then you'd have to let it air cool.
Quenching in either water or oil would cause it to crack.

Gary