Oldest method for surface-hardening steel , by heat or mechanical means to increase the hardness of the outer surface while leaving the core relatively soft.
The combination of hard surface and soft interior withstands very high stress and fatigue , and also offers low cost and superior flexibility in manufacturing. To carburize, the steel parts are placed in a carbonaceous environment (with charcoal, coke, and carbonates, or with carbon dioxide, carbon monoxide, methane, or propane) at a high temperature for several hours. The carbon diffuses into the surface of the steel, altering the crystal structure of the metal. Gears, ball and roller bearings, and piston pins are often carburized.