Hardenability Curve

A hardenability curve plots hardness versus distance from the quenched end of a standardized steel specimen, typically the Jominy end-quench test. It shows how deeply hardness is retained as cooling rate decreases. Hardenability differs from hardness: hardness resists indentation, while hardenability is the steel's ability to form martensite to depth. It compares heat-treat response; for example, 4140 retains hardness farther from the quenched end than 1018.

On the shop floor, the hardenability curve predicts whether a section thickness will harden through the core or remain case-biased after quenching, directly driving machining sequence, fixturing, and finishing strategy. Heat-treat planners use the curve to choose steel grade and quench severity that achieve required hardness depth with minimal distortion. In CNC work, the curve matters when roughing in annealed or normalized state is followed by finish machining after hardening; it indicates how much cross-section becomes hard, raising tool wear and determining whether final tolerances can be held. The curve also lets programmers compare materials such as 4140 and 1018, so section size and cooling conditions can be matched confidently. For carburized or induction-hardened parts such as shafts, gears, dies, and fixtures, the curve confirms that the core remains tough while the surface reaches wear-resistant hardness, preventing brittle-through failure.