Heat treating doesn't have to be expensive unless you want to rent a
high priced brain. you can do small tempering with a gas burner or
charcoal grill. Larger items are done with open forge fires or
furnaces, which runs up the price.
Tempering theory is simple: Quench from a red heat to harden, using oil
or water as a coolant.
Draw [read soften] the temper by low, uniform heat between 250 and 600
Degrees F. The surface of the polished steel will indicate the proper
draw point by the color of the oxide formed: straw yellow for razors,
gravers & other very hard [but brittle] tools. Ruby red for cutlery and
cold chisels. dark red to purple for high impact tools like axes and
pick points. Deep purple is spring temper. Black is annealed and soft
again.
That is the bare bones basics of tempering steel. Different alloys have
their own characteristics, so there's an infinite array of
possiilities.
Bugs