How does manganese improve the hardenability of steel?

Manganese improves the hardenability of steel primarily by influencing the kinetics of phase transformations during heat treatment.

Alloying elements like manganese slow the diffusion of atoms through the iron lattices, which effectively delays the allotropic transformations that occur as steel cools. By slowing these transformation rates, manganese allows the formation of the extremely hard microstructure known as martensite to occur at lower cooling rates than would be required for plain carbon steel.1

This reduction in the necessary cooling rate provides two significant benefits:
* It results in less internal stress within the metal during the quenching process.1
* It produces a deeper hardened zone in the workpiece, which is the defining characteristic of hardenability.12

Beyond its role in hardenability, manganese also serves as a critical desulfurizer and deoxidizer, helping to remove impurities that could otherwise compromise the integrity of the steel.34