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Heat-Treating and Steel Characteristics
You may have heard people refer to high-carbon steel or special steel alloys like those used for super-hard applications -- alloys containing extra-hard elements like cobalt, for example. While it's true that the composition of a steel alloy has a lot to do with its characteristics, including its hardness, the alloy will make little difference if it's not properly heat-treated. Any heat-treating can be accomplished with an oxyacetylene torch.
Hard, Harder and Hardest
Steel is an iron-based alloy. Iron is hard even before it is turned into steel, so "hardening" is a relative term. Every steel alloy will have a molecular structure that falls in a relative spectrum of hard, harder and hardest. Another way to think of the three stages is annealed, tempered and hardened. You can harden or soften steel in relative increments.
Tempering
If you start with "soft" steel -- steel that has either never been heat-treated or been heat-treated, then annealed, softening it for workability -- you may want to temper the steel with an oxyacetylene torch. Tempering is not the most extreme hardening process, but it will make high-carbon steel hold a good edge, for example, on a chisel. To temper steel, you heat it with your oxyacetylene torch, then allow it to cool slowly. There is a range of tempering. The harder the temper, the more brittle the steel. Tempering may be achieved by heating steel to anywhere from 350 to 1,250 degrees Fahrenheit. You may also refer to a matrix that shows you how to correlate the color the steel turns to the hardness or temper after it cools. However, you should be aware that heat and color guides are only approximations unless they specifically refer to the kind of steel alloy you're tempering.
Hardening
To make steel even harder than the process of tempering, you'll get it past blue, and maybe even past red until you reach a straw color or near white. The steel may then be quickly cooled by plunging it into a bath of water or oil. This very fast cooling process aligns the molecules of the steel, making the extremely hard, but very brittle and potentially easy to break. While you may be able to add temper to steel with a simple hand torch, oxyacetylene torches are a good choice for hardening, as they can generate the necessary heat.