Q. What do you understand by Carburization?

Carburization is initiated by the adsorption of carbon in contact with the surface of the metal and may progress by diffusion along the interior, near surface grain boundaries. Stable chromium and iron carbides precipitate within the structure of the metal and cause a reduction in ductility and high temperature strength. Failure is due to brittle fracture at low temperatures or loss of creep strength at high temperatures. Resistance to carburization increases with silicon and nickel content.

For reducing atmospheres, cast HK40 is used frequently in the petrochemical industry, but shows low resistance to carburization unless alloyed with additional Si. Low nickel austenitic stainless steels (e.g. 304, 316, 321, and 347) have relatively poor resistance to carburization. Type 310 stainless steel has better resistance, but not as good as the higher nickel-base Incoloy or Inconel alloys.

For oxidizing atmospheres, selection of an appropriate alloy for service should include consideration of the high temperature properties in the specific atmosphere in question. For example, cobalt-chromium alloys show the most promise for use in a carbon monoxide (CO) environment. Inconel 702, pure nickel and pure copper were the only alloys to resist pitting in lab tests under a very aggressive H₂ / CO atmosphere.