How is steel hardened?

• A. It is heated to its critical temperature and quenched in water, brine, or oil.
• B. It is heated to a temperature below the critical point and cooled slowly.
• C. It is left to air-cool slowly.
• D. It is treated by applying a protective coating.

Answer: (A)

Hardening steel relies on transforming its crystal structure to martensite by heating into the austenite region and then quenching rapidly. Heating above the critical temperature changes the structure to austenite; quenching in water, brine, or oil cools it so quickly that diffusion is suppressed and austenite becomes martensite, a very hard yet brittle phase. If you heat only to a temperature below the critical point and cool slowly, or if you let the steel air-cool, you form softer structures like pearlite or ferrite rather than martensite. A protective coating changes surface properties but does not produce the internal hardness achieved by quenching after austenitizing.