How is steel typically hardened?

Steel is typically hardened through a process known as quenching, which involves heating the steel until it reaches its austenitic phase, where the iron atoms rearrange into a more face-centered cubic structure, allowing for significant changes in the steel's properties. Once the steel is in this heated state, it is rapidly cooled or "quenched" in a liquid, such as water or oil. This rapid cooling transforms the austenite into martensite, a much harder and more brittle structure.

This process of heating to a specific temperature and then quickly cooling is essential for achieving the desired hardness in steel. The transformation of the microstructure significantly increases the hardness and strength of the material, enhancing its performance in various applications.

The other options do not effectively describe the hardening process of steel. Gradual heating and cooling would not produce significant hardness improvements; it may even negate the benefits derived from the transformation of microstructures. Mixing steel with other metals could alter its properties but does not specifically harden it in the way quenching does. Lastly, slowly heating steel to remove impurities may improve its ductility but is not a method for hardening.