Hardenability is a measure of a steel's ability to form martensite (harden) to a certain depth when quenched. AISI 4140 has very good hardenability due to its Cr and Mo content. This is quantitatively represented by a Jominy End-Quench test curve, which shows how hardness decreases with distance from the quenched end.
Importance: Good hardenability means that thick sections (like the center of a large-diameter round bar) will still transform to a significant depth of martensite during oil quenching, leading to high and uniform strength throughout the part after tempering. For a 4140 round bar, effective through-hardening can be achieved up to approximately 75-100mm (3-4 inches) in diameter with an oil quench. This makes it suitable for large, high-strength components where core properties are critical.