Hardenability is quantitatively measured using the Jominy End-Quench (Jominy) Test (ISO 642 or ASTM A255). A standardized bar (25mm dia, 100mm long) is austenitized and then quenched at one end with a controlled water jet. This creates a range of cooling rates along the bar. Hardness is then measured at intervals from the quenched end.
Typical Jominy Curve for 42CrMo: The curve shows high hardness (~55-58 HRC) at the quenched end (J1, fast cooling, full martensite). The hardness decreases gradually with distance (slower cooling). Due to its high hardenability, 42CrMo maintains a high hardness level (e.g., >45 HRC) even at a significant distance (e.g., J20 or J30), indicating that a large volume of material will transform to martensite even with moderately slow cooling (like in the center of an oil-quenched bar). The exact curve depends on the actual composition within the specified range (a "hardenability band").