Hardenability of steel is a measure of

The correct option is: the depth to which required hardening is obtained when it is austenitized and then quenched

Explanation:

Hardenability is a fundamental metallurgical property of steel that describes its capability to be hardened in depth under a given set of conditions. It is important to distinguish "hardenability" from "hardness" itself:

1. Hardness refers to the resistance of a material to localized plastic deformation (such as an indentation or scratch). The maximum hardness of a steel is primarily determined by its carbon content.
2. Hardenability, on the other hand, is a measure of the depth or distribution of hardness that can be achieved throughout the cross-section of a steel component. A steel with high hardenability can form martensite (the hard phase of steel) deeper into its interior when cooled, rather than just on the outer surface.

To achieve hardening, steel is first heated into the stable austenite temperature range (austenitized) and then rapidly cooled (quenched). During quenching, the surface cools faster than the interior. If the steel has high hardenability, even the slower-cooling interior regions will transform into martensite, resulting in a deeper hardened layer. Therefore, hardenability is indeed a measure of the depth to which the required hardening is obtained upon austenitizing and quenching.