NITRIDING PROCESS EXPLAINED !!

Nitriding is the diffusion of nitrogen into the surface of special alloy steel to give a hard surface and soft core without the need for further treatment. Processing is generally carried out in the temperature range 470ºC to 530ºC in an atmosphere of ammonia, although other processing media can be used, such as salt baths and plasma.

Nitriding is only carried out on special alloy steels containing chromium or aluminium. It is the reaction of the nitrogen with these alloy elements that causes the hardening, so that, unlike carburising and carbonitriding, quenching is not required after processing. The nascent nitrogen is obtained from an atmosphere of ammonia gas, which at 500°C dissociates into its constituent elements, nitrogen and hydrogen. The nitrogen, which is in solution in the iron, diffuses inward and forms aluminium or chromium nitrides producing high hardness in the surface of the nitrided component. A layer of iron nitride and alloy nitrides forms on the surface (the "white layer''). Since this is brittle it is normally removed from bearing surfaces before service. As with carburising the case depth is time and temperature dependent.

Nitriding produces advantages in addition to a freedom from distortion, which is due to the low treatment temperature and the fact that quenching is not required.

Materials that can be nitrided include low carbon steels, which will develop file hardness, alloy steels such as 4130, 4140, 4340 and Nitralloy 135M which are the most common nitriding steels and special application steels including, mold steels (P-20), air hardening tool steels (A-2 and D-2), hot work and shock steels (H-13 and S-7), high speed steels (M-2, M-4 and M-42), and stainless steels (304, 316, and 17-4 PH).

A prime application for nitriding is plastic injection mold components, including screws, tips and barrels. The hardness of the nitrided layer is especially useful in reducing wear from plastic molding, particularly when abrasive plastics like glass filled polymers are extruded.