Gregor, while I am no expert, generally there are three terms that
are used in heat treating - annealing, hardening , and tempering.
each metal has its own specifics but for example for o-1 ( aka oil
hardening ) its is annealed by heating it to a critical temperature (
for 0-1 when it is no longer magnetic ) then allowing to cool
slowly, often while packed in ashes. Once annealed it is at its
softest state and that is when it is shaped to rough form. The next
step is to again heat it to the critical temp then plunge it in the
appropriate coolant ( for o-1 warm oil) once it is quenched it is as
hard as it will get, but it is very brittle - to make it less brittle
it is tempered by heating it to a specific temp ( depending on how
hard you want it - the 350 -400 range is common for hand tools for
working wood). If you overheat it at this stage and make it too
soft, you can start again ( though I usually skip annealing and just
heat treat it, then after quenching, temper it again)
From another site I found this- The proceedure for 4140 is preheat to
1250 deg F then bring it up to 1550 deg F. Let it soak at temp for
probably 15-20 min and oil quench. Max hardness on 4140 is around 55
Rc I don't think you would want it that hard my sugestion would be to
draw it back to about 46-48 Rc.
Unless your part is very big you should have no problem getting it up
to 1550 with a MAPP torch, though I don't know how you would confirm
you reached that temp. Here is a site with a short description of
the temps for that metal
Family=Alloy+Steels&MetalName=4140
