GAS NITRIDING is a case-hardening process whereby nitrogen is introduced into the surface of a solid ferrous alloy by holding the metal at a suitable temperature (below Ac1, for ferritic steels) in contact with a nitrogenous gas, usually ammonia. Quenching is not required for the production of a hard case. The nitriding temperature for all steels is between 495 and 565 ¡ãC (925 and 1050 ¡ãF).
PLASMA, OR ION, NITRIDING, is a method of surface hardening using glow discharge technology to introduce nascent (elemental) nitrogen to the surface of a metal part for subsequent diffusion into the material. In a vacuum, high-voltage electrical energy is used to form a plasma, through which nitrogen ions are accelerated to impinge on the workpiece. This ion bombardment heats the workpiece, cleans the surface, and provides active nitrogen. Ion nitriding provides better control of case chemistry and uniformity and has other advantages, such as lower part distortion than conventional (gas) nitriding. A key difference between gas and ion nitriding is the mechanism used to generate nascent nitrogen at the surface of the work.
FERRITIC NITROCARBURIZING processes are those thermochemical treatments which involve the diffusional addition of both nitrogen and carbon to the surface of ferrous materials at temperatures completely within the ferrite phase field. The primary object of such treatments is usually to improve the anti-scuffing characteristics of ferrous engineering components by producing a "compound layer" on the surface which has good tribological properties (Fig. 1). In addition, the fatigue characteristics of the material can be considerably improved, particularly when nitrogen is retained in solid solution in the "diffusion zone" beneath the compound layer. This is normally achieved by quenching into oil or water from the treatment temperature, usually 570 ¡ãC (1060 ¡ãF).
Fig. 1 Mild steel after 3 h gaseous nitrocarburizing in an ammonia/endothermic gas mixture at 570 ¡ãC (1060 ¡ãF) followed by oil quenching.