Carburizing

The mild steel is enclosed in a cast iron box having a material rich in carbon, such like small pieces of charcoal, heated to 900 to 950^oC and permitted it to remain at that temperature for sufficient time so that the carbon is absorbed to the needed depth. Then the steel is allowed to cool gradually in the box. The outer skin of the article is changed into high carbon steel having about 0.8 to 1.2% carbon. Activated charcoal varying among 3.5 and 10 mm in diameter mixed with semi- coke and peat coke along with soda ash (Na₂CO₃) and barium carbonate (BaCO₃) to the extent of 40% of weight of charcoal make the carburizing pack. The depth to which this hardening occurs depends upon the time permitted to the metal to remain in contact with charcoal at the elevated temperature. The usual depth of 0.8 mm to 1.6 mm is attained in 3 to 4 hrs. The part of the surface not needed to be hardened can be saved from carburizing by coating with appropriate paste or by plating with 0.1 mm of copper.