The hardenability of a steel depends upon the actual rate at which its austenite transforms to fine pearlite at the particular temperature at which this reaction sets in most promptly. The precise temperature at which this transformation rate is greatest depends upon the composition of the steel; in any case, it is this maximum rate at temperatures near 950 degrees Fahr. (500 degrees Cent.), which determines the critical quenching speed which must be exceeded if the steel is to be hardened. This maximum rate of austenite transformation to products other than martensite is in turn largely determined by the condition of the austenite at the moment of quenching with respect to two factors: (1) its composition with respect to dissolved elements, most of which retard, though some may hasten, its transformation to pearlite. Manganese, chromium, nickel, silicon, and aluminum definitely retard the reaction and so contribute directly to deep hardening about in the order named; on the other hand, tungsten, cobalt, molybdenum, vanadium, and possibly oxygen appear to induce shallow hardening, though most probably indirectly by restricting grain growth. (2) its effective grain size. The finer the grain, the more rapid is the transformation to fine pearlite, and correspondingly the lower is the hardenability. Effective grain size seems to be the most potent single factor influencing hardenability; it in turn is probably controlled largely, for any specified temperature, by the obstruction to grain growth offered by large numbers of very finely dispersed particles, of exceedingly small aggregate mass, comprised presumably of stable oxides such as alumina, vanadia, and probably silica or silicates.