The hardenability of steel decreases with

ANS:D - decrease in grain size.

Decreasing the grain size of steel can actually increase its hardenability. Grain size refers to the size of the individual crystalline grains in the microstructure of the steel. In metallurgy, finer grain sizes are generally associated with improved mechanical properties, including higher strength, toughness, and hardness. When steel is quenched (rapidly cooled), the transformation of austenite (the high-temperature phase) into martensite (the hardened phase) occurs more rapidly at the grain boundaries. Therefore, smaller grain sizes mean more grain boundaries, providing more sites for the nucleation of martensite during quenching. This results in a greater depth of hardening and increased hardness throughout the material. In summary, decreasing the grain size of steel can enhance its hardenability, allowing it to achieve greater hardness and depth of hardening during heat treatment processes like quenching.