GlossaryKind of term: Manufacturing termNitriding
What is nitriding?
Nitriding diffuses nitrogen into the surface of steel, giving a hard, wear-resistant case without quenching and with little distortion.
The treatment is thermochemical, with nitrogen from a gas or a plasma, and works on cast iron as well as steel; salt-bath processes are in practice mostly nitrocarburising. A thin compound layer, also called the white layer, forms at the surface, with a diffusion zone beneath it in which hardness falls gradually to the core hardness. The nitrided depth usually lies between a few hundredths and a few tenths of a millimetre.
What does nitriding depend on?
The treatment typically runs at roughly 480 to 580 °C, depending on process and steel; plasma nitriding can also run lower. Since the part is not quenched, and quenched and tempered steel stays below its tempering temperature provided it was tempered high enough, distortion is small but not zero: stresses from machining can be released at that temperature, and the surface grows slightly, depending on the compound layer thickness. Hardness and depth depend on the grade and the treatment time: steels with nitride formers such as chromium, molybdenum, vanadium or aluminium reach a higher hardness than non-alloy steel. Nitriding steels and quenched and tempered grades such as 42CrMo4 are common; stainless steel needs an adapted process.
What does the standard say about nitriding?
The drawing usually gives a surface hardness in HV with its test load, and a nitriding hardness depth, sometimes also a limit on compound layer thickness. In the usual definition, found among others in ISO 18203, nitriding hardness depth is the depth at which hardness is still 50 HV above core hardness. The indication of heat treatment on drawings is covered by ISO 15787. Without a test load, a hardness requirement on a thin case is ambiguous: too high a load measures through the case.
How does nitriding work in practice?
Because the dimensional change is small, nitriding usually comes after finish machining: the part is machined to size with the expected growth allowed for, then given its hard case. Long or thin parts often get a stress relief between roughing and finishing first, to limit distortion during nitriding. Threads and faces still to be machined are masked where they must stay free of the case. Nitriding is usually subcontracted; the heat treater often charges per load or per kilogram, with a minimum, and the cycle is long, so load size and lead time weigh on price and delivery date.
What is nitriding often confused with?
Nitriding is not case hardening: case hardening adds carbon at a higher temperature and quenches, giving a deeper case and more distortion. Nitrocarburising is related but not the same: nitrogen and carbon diffuse together, usually in a shorter cycle, with the emphasis on the compound layer. And nitriding is not a coating: no layer of another material is added; the steel itself changes at the surface.
How do you see nitriding in Blake?
If you subcontract nitriding, it gets its own subcontract line in the Blake estimate, priced from your subcontract library. Until a price is there, the line stays open. More on estimating.
Frequently asked questions
Is grinding needed after nitriding?
Usually not. Distortion is small and growth predictable enough to machine to size beforehand. Very tight fits are sometimes lightly ground or lapped afterwards; that removes the thin compound layer first, so plan the allowance accordingly.
Why is steel often quenched and tempered before nitriding?
Because the nitrided case is thin and needs a strong core under load so it does not collapse. Quenching and tempering provides that core. If the tempering temperature lies above the nitriding temperature, the core keeps its strength during nitriding.
How do I keep threads free of the nitrided case?
By masking the zone, for example with a paste or an electroplated layer in gas nitriding, or mechanically in plasma nitriding. State on the drawing which faces must not be nitrided, otherwise the whole surface is treated.