For home metallurgy, there are two sources of information for the heat treatment and tempering of steel. The first source is academic publications that include theoretical information, while the second includes the home-spun wisdom of blacksmiths who learn through trial and error. [Ben Krasnow] put up a great video that tries to bridge that gap with some great background information with empirical observations to back up his claims.
For investigating the hardness of steel, a few definitions are in order. The first is stiffness, or the ability of a material to ‘spring back’ after being flexed. The second is strength, specifically yield strength, which is the amount of strain a material can withstand before being permanently deformed.
[Ben] did all these experiments with a 1/8″ W1 steel drill rod. As it came from McMaster, this rod could handle a bit of force before becoming permanently bent, and in terms of stiffness was much better than a piece of coat hanger wire [Ben] had lying around. After taking a piece of this drill rod, heating it up to a cherry red and quenching it in water, [Ben] successfully heat treated this steel to a full hardness. After putting it on his testing jig, this full hardness steel didn’t deform at all, it simply broke.
Full hardness steel is basically useless as a structural material, so [Ben] tried his hand at tempering pieces of his drill rod. By putting a few pieces in a kiln at the requisite temperature, [Ben] was able to temper his drill rods to be stronger than the stock material, but not as terribly brittle as a full hard rod.