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Normalizing and carburizing

Normalizing heats the steel above the austenite range and cools it in air, giving a fine-grained and uniform structure. Carburizing (case hardening) increases the carbon content at the surface so it becomes hard while the core stays tough — widely used for gears that need both wear resistance and impact resistance.

normalizing: fine-grained, uniform structure\text{normalizing: fine-grained, uniform structure}
carburizing: hard surface, tough core\text{carburizing: hard surface, tough core}

Example

A gear is carburized so the teeth withstand surface wear while the core withstands impact without shattering.

Alloying elements like chromium and molybdenum increase hardenability — how deep steel can be hardened by quenching.
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Part of Materials Technology: Heat treatment and corrosion.