Gear carburizing is essentially a chemical heat treatment that alters the chemical composition of the surface layer. It diffuses carbon into the surface of gear teeth to turn the outer layer into a high-carbon material, while keeping the inner body of the gear soft and tough to withstand impact loads and avoid overall fracture. Carburizing itself does not harden the gear immediately; it only modifies the surface composition and prepares the gear for subsequent quenching.
Quenching is essentially a physical heat treatment that changes the metallographic structure of steel without altering its chemical composition. The treated gear is rapidly cooled to harden the surface, forming a durable wear-resistant tooth surface that resists wear and pitting, while restructuring the internal steel matrix to improve overall strength. However, quenching makes the steel brittle and introduces residual stresses, so tempering is generally required to reduce brittleness, prevent cracking and deformation, and stabilize dimensional accuracy.