Carburizing,
carburising (chiefly British English), or
carburization is a
heat treatment process in which
iron or
steel absorbs
carbon liberated when the metal is heated in the presence of a carbon bearing material, such as
charcoal or
carbon monoxide, with the intent of making the metal harder. Depending on the amount of time and temperature, the affected area can vary in carbon content. Longer carburizing times and higher temperatures typically increase the depth of carbon diffusion. When the iron or steel is cooled rapidly by
quenching, the higher carbon content on the outer surface becomes hard via the transformation from
austenite to
martensite, while the core remains soft and tough as a
ferritic and/or
pearlite microstructure.