I would like to talk a bit about tempering steel. This post is going to be very broad and as simple as I can make it without going into great detail. There are almost as many kinds of steel and iron flavors as fans at a baseball game. I generally re-purpose steel that I find. If you know you specific flavor of steel you can research the proper tempering method for it.
This subject has books written on it. This is just a crash course for simple tool making.
The three topics I want to discuss are, annealing, quenching and tempering.
Annealing is a heat treatment that alters a material to increase its ductility and to make it more workable. It involves heating material to above its critical temperature, maintaining a suitable temperature, and then cooling. Annealing can induce ductility, soften material, relieve internal stresses, refine the structure by making it homogeneous, and improve cold working properties.
Quenching is the rapid cooling of a workpiece to obtain certain material properties. It prevents low-temperature processes, such as phase transformations, from occurring by only providing a narrow window of time in which the reaction is both thermodynamically favorable and kinetically accessible. For instance, it can reduce crystallinity and thereby increase toughness of both alloys and plastics (produced through polymerization).
Quenching Rate: (FASTEST) Salt water > water > oil > air. (SLOWEST)
Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys. It is also a technique used to increase the toughness of glass. For metals, tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to a much lower temperature than was used for hardening. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, while springs are tempered to much higher temperatures.
Please friends ask questions, that is how I learn.