Carburizing is a thermo-chemical "case-hardening" process. Low-carbon steel parts are heated in a carbon-rich environment (gas, liquid, or solid), allowing carbon atoms to diffuse into the surface of the metal.
The Result: A "dual-nature" component with a high-carbon, glass-hard outer shell for wear resistance, and a low-carbon, ductile inner core to absorb shocks and prevent snapping.