the severity of the quench, and the size of the bar. Let us consider first the effect of changing the diameter of the bar of steel. Suppose that a number of bars of the same steel are given an identical quench in a brine solution and then sectioned so as to obtain hardness contours. The results are shown schematically in Fig. 19.5. An investigation of these curves shows that there is one unique diameter, the value of which is 1 in. The bar with this diameter hardens so that it has the 50 percent pearlite–50 percent martensite structure just at its center. All bars with smaller diameters are effectively hardened throughout, while any bar with a larger diameter has a soft core containing pearlite. This particular diameter is called the critical diameter. Its value depends on the steel in question and the means of quenching, and its importance lies in the fact that it gives a measure of the ability of the steel to respond to a hardening heat treatment. The particular steel being discussed has a moderate ability to harden, or, as it is more properly stated, it has a moderate hardenability. According to Fig. 19.5, its critical diameter D is 1.0 in. The addition of suitable alloying elements to steels can greatly increase their hardenability and this is shown by corresponding increases in their critical diameters. The critical diameter D of a steel is, consequently, a measure of its hardenability (ability to harden), but it also depends on the rate of cooling (the type of quench). In order to eliminate this latter variable, it is general practice to refer all hardenability measurements to a standard cooling medium. This standard is the so-called ideal quench, which uses a hypothetical cooling medium assumed to bring the surface of a piece of steel instantly to the temperature of the quenching bath, and maintain it at this temperature. The critical diameter corresponding to an ideal quench is called the ideal critical diameter and is designated DI.