Nitriding

What is nitriding?

In nitriding, nitrogen is diffused into the surface of the material to change its surface properties. Because the nitrogen atom is smaller than the carbon atom, it penetrates the surface more easily. Nitrogen forms very hard nitrides with elements in the steel such as chromium, molybdenum, aluminium and vanadium, so the surface hardens and its wear and fatigue resistance increase.

How is it done?

Nitriding is carried out in a nitrogen-releasing medium at low temperatures where austenite is not present (usually 500–590 °C). Because the part is not taken to its transformation temperature and is not quenched, distortion and dimensional change are much smaller than with other hardening methods; parts usually go into nitriding already machined to final size.

In unalloyed and low-alloy steels, high hardness is reached at diffusion temperatures below 590 °C, but diffusion is slow at these temperatures. Nitrides precipitate finely, and since both the low temperature and the precipitated nitrides slow down nitrogen diffusion, reaching depth takes a long time: reaching 1 mm can take 60–70 hours. This is why a nitrided layer is thinner than a case-hardened or induction-hardened one.

Which steels and parts?

The best results are obtained on alloy steels containing chromium, molybdenum, aluminium or vanadium: quenched and tempered and nitriding steels such as 42CrMo4, 31CrMoV9 and 34CrAlNi7, and hot-work tool steels such as 1.2344 (H13). Quenching and tempering and stress relieving the part beforehand improve its dimensional stability after nitriding. Crankshafts and camshafts, gears, hydraulic parts, screw shafts, and forging, die-casting and extrusion dies are typical applications.

For a quote

State the steel grade, the prior heat treatment condition of the part (e.g. quenched and tempered), the required surface hardness and nitriding depth, and the areas that must not be nitrided in the quote form; attach a drawing if you have one.