[Bruno]

Last week I got this heat treatment explanation from Wayne D.
It is for O1 (or in belgium / germany: 1.2510) steel. I am posting it here because it might help others as well, and this way I know where I can find it when I need it.

O1 is an oil hardening steel, so wants quenching in oil. If you do it in ice water you run the risk of cracking the steel.

From Wayne:

I use motor oil in a large turkey roasting tray
it's simple to heat treat O1,
you need a pan of motor oil a magnet aluminium kitchen foil (bacofoil) and a kitchen cooker preheated to 250 oC or gas mark 8

heat the steel in the fire until it goes a cherry red colour then touch it against the magnet if it's attracted it's not hot enough yet,
keep heating until the steel is not attracted (antimagnetic) then quench in the oil keeping all of the O1 under the surface move it around a little to stop the oil getting too hot.
take out of oil, clean with a cloth and repeat this heating 2 more times.
* at this point the O1 is very brittle so don't drop it *

when you've done this wrap the O1 in the aluminium foil - it stops your kitchen smelling of burned motor oil and helps even out the heating process

then put it in your preheated oven for 1 hour.
take it out of the oven and allow it to sit on the worksurface until it's cool enough to pick up with your hands comfortably - but not cold
then place back in the oven for 1 hour, remove allow to cool down to hand temperature,

place back in oven for 1 hour and then when you take it out allow to cool totaly.
when you remove the foil the metal should be in the range of light brown/ straw / golden/ yellow colour

this means you have heat treated it to between Rockwell 60 and 62.