ABSTRACT
Perhaps t he o ldest, most co mmon, a nd l east ex pensive qu enching m edium i s a ir. Heat transfer coef cients for air with respect to surface temperature for steel, aluminum, nickel, and copper are shown in Figure 13.1 [1]. Like other quenching media, air cooling is dependent on the ow rate past the cooling metal surface, as shown in Figure 13.2 [2]. Although increasing the ow velocity does accelerate the cooling rate, it is still insuf cient to harden many steels. For example, Figure 13.3 shows that the ability of air to harden carbon steel is dramatically dependent on increasing the carbon content of the steel [3].
