This method covers a procedure for calculating minimum, mean, and maximum ideal critical diameters (DI) for base chemical composition (DIB) and for the case chemical composition DIC of wrought and cast heat treatable steels. The calculated values of D are intended to serve as a guide to the range of hardenability that can be expected for a grade or family of steels. Steels without standard hardenability (H) bands can therefore be compared to those with H bands.

The calculated DI's are not intended to mean that a DI value for a particular heat of steel will never exceed the stated minimum or maximum, but that most of the time the DI will fall within the stated DI range. Calculated DI's are normally used only for comparative purposes but they may also be used for calculating hardenability bands.