The earliest method of hardening steel surface is surface hardening or carburizing. Iron has affinity for carbon near and above its critical temperature. Carbon is absorbed into the metal to form a solid solution with iron, which transforms the outer surface into high carbon steel. Carbon gradually diffuses into the parts. The depth of carburized layer depends on the time and temperature of heat treatment. The method of solid carburizing is to put the parts to be treated into a closed container together with carbon containing materials such as charcoal or coke. This is a long process to produce a hardened layer with a depth of 0.03 to 0.16 inch (0.76 to 4.06 mm).
Low carbon steel with carbon content of about 0.15% is generally used for carburizing, which is not suitable for heat treatment. During the treatment, the outer layer is transformed into high carbon steel with carbon content from 0.9% to 1.2%. The steel with different carbon content has different critical temperature, so special heat treatment is needed. Due to some grain growth in the steel during the long carburizing process, the workpiece should be heated to the critical temperature of the core and then cooled to refine the structure of the core. Then it is reheated to a temperature higher than that of the outer layer and quenched to form a hard and fine structure.
Because it is just higher than the low critical temperature, the hypereutectoid steel is usually austenitized and hardened, so a lower heat treatment temperature is adopted for the outer layer. The third tempering can be used to reduce strain.