Carburizing

http://circleplastics.co.uk/assembly-fabrication/?gclid=EAIaIQobChMIwPu5t4qs3AIVAQAAAB0BAAAAEAAYACAAEgJVzfD_BwE Carburizing 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.

http://sunsationalhomeimprovement.com/2019 This manufacturing process can be characterized by the following key points: It is applied to low-carbon workpieces; workpieces are in contact with a high-carbon gas, liquid or solid; it produces a hard workpiece surface; workpiece cores largely retain their toughness and Ductility; and it produces case hardness depths of up to 0.25 inches (6.4 mm). In some cases it serves as a remedy for undesired decarburization that happened earlier in a manufacturing process.