Carbides in low-temperature-carburized stainless steels

A novel, low-temperature (470 deg. C) gas-phase carburization treatment, developed by the Swagelok Company, increases the surface hardness of 316-type austenitic stainless steels from ∼200 to ∼1000HV25 via a colossal supersaturation of up to 12 at.% carbon in solid solution. Upon extended treatment, carbide precipitation does eventually occur. Transmission electron microscopy of carburized bulk, foil, and powder samples has revealed two different carbides: M5C2 ('Haegg' or 'χ' carbide) and M7C3. The particles of both carbides have the shape of long needles or laths, contain a high density of planar defects normal to the particle axis, and adopt special topotaxial orientation relationships with the austenite matrix