Harden ability of steel is a measure of

1. the maximum hardness that can be obtained when it is austenitized and then quenched
2. the depth to which required hardening is obtained when it is austenitized and then quenched
3. the ability to harden when it is cold worked
4. the ability to retain its hardness when it is heated to elevated temperatures

Harden ability is a measure of the capacity of a steel to be hardened in depth when quenched from its austenitizing temperature.