The temperature at which steel properties change depends on the specific chemical content. The critical point is the temperature above which the material will harden when quickly cooled. For most steel, the critical point is between 1200 and 1800 F. Fast Cooling When the steel is heated to these high temperatures, it must be cooled so the steel is left with the desired physical properties. Quick cooling, or quenching, is done in a number of environments, including air, water, oil, sand and chemical baths. This results in harder steels.
Water is a quick and inexpensive medium for quenching.
With low carbon steels, water quickly cools the material, increasing its hardness and strength. Quenching with oil and air results in slower cooling, which means less strength and hardness. As with most things, there is a trade-off. Quicker cooling leads to stronger, harder materials, but it also means more distortion and cracking of the material. Slower cooling provides less strength and hardness, but also less cracking and distortion.