Case Hardening, Chemical

the aggregate of technological processes that alter the chemical composition, structure, and properties of a metal’s surface layers without changing the composition, structure, and properties of its core. It is accomplished by the diffusion at elevated temperature of various elements into the surface layers to form a case. The choice of element or combination of elements depends on the surface properties required by a part. The diffusion is accomplished by the addition of carbon (carburizing), nitrogen (nitriding), both nitrogen and carbon together (carbonitriding, cyaniding), various metals (seeDIFFUSION COATING), boron (boronizing), silicon (siliconizing), and other elements.

Depending on the physicochemical state of the medium that contains the diffusing element, chemical case hardening can be carried out from the gas, liquid, solid, or vapor phase (the first two are carried out the most frequently) in gas, vacuum, or tank furnaces on objects made of steel, cast iron, pure metals, and alloys based on nickel, molybdenum, tungsten, cobalt, niobium, copper, aluminium, and other elements.

The physicochemical processes that occur at the surface during case hardening are mainly related to the formation of the diffusing element in an atomic state as a result of a chemical reaction in the diffusion medium or at the boundary between the medium and the metallic surface (when diffusing from the gas or liquid phase) or as a result of the sublimation of the diffusing element (when diffusing from the vapor phase), the subsequent sorption of the atoms of the element by the metallic surface, and the diffusion of the atoms into the surface layers of the metal. Both the concentration of the diffusing element on the metallic surface and the structure and properties of the case depend on the case-hardening method. The depth of the element’s diffusion increases with temperature (according to the exponential law) and with the duration of the process (according to the parabolic law). By altering the structural and energy conditions of the surface, the case formed favorably affects not only the physicochemical properties of the surface but also the volumetric characteristics of the parts.

Chemical case hardening improves resistance to wear, heat, corrosion, and fatigue. (See separate articles on various case-hardening methods.)