Nitriding Calculator — Case Depth, Cycle Time & Process Guide

Nitriding diffuses nitrogen into the surface of a steel at a temperature low enough that the core is never re-austenitised — so, unlike case-hardening by carburising and quenching, there is no phase change to quench from and therefore very little distortion. That is the whole appeal: a very hard, wear- and fatigue-resistant surface produced as the final thermal operation, on a part that has already been hardened, tempered and finish-machined, with dimensions that barely move. It is why nitriding is the go-to for gears, shafts, dies, moulds and valve components where a case-hardened part would distort out of tolerance.

Because the nitrogen has to diffuse inward, the case depth follows a parabolic law: case depth d = K × √t, where t is time at temperature and K is a constant set by the steel and the process conditions. The practical consequence is the one that surprises people quoting a job: depth does not grow linearly with time. Getting twice the case depth takes roughly four times as long. A cycle that reaches a given depth in a working day might need the better part of a week to reach double it — which is why deep nitrided cases are expensive, and why the honest first question is whether the depth on the drawing is genuinely needed.

The steel matters as much as the clock. Nitriding works by forming hard, stable nitrides with alloying elements — chromium, molybdenum, vanadium and especially aluminium — so the alloy steels developed for it (the Cr-Mo grades like EN19 and EN24, and the aluminium-bearing "Nitralloy" types like EN40B) take a much harder, more useful case than a plain carbon steel, which has little to react with. Tool steels such as H13 nitride to a very high surface hardness and are routinely nitrided for die and mould life. Stainless steels can be nitrided too, though hardening the surface trades away some corrosion resistance. The calculator carries a representative growth constant and a typical surface hardness for each of the common grades.

Two things every nitrided drawing should account for. First, a compound (white) layer forms right at the surface — a few microns to a few tens of microns of iron nitrides. It is extremely hard and wear-resistant but can be brittle, so for fatigue-critical or highly loaded parts it is often thinned or removed by a light finishing operation, and the spec should say which. Second, nitriding causes a small, predictable dimensional growth — the surface expands slightly as it takes up nitrogen — so critical features are finish-machined marginally undersize, or lapped afterwards, to land on size. Neither is large, but both are the sort of thing that scraps a batch if it is discovered after the fact rather than allowed for on the drawing.

On process routes: conventional gas nitriding in ammonia is the long, deep-case workhorse; plasma (ion) nitriding runs at lower temperatures with tight control and easy masking of areas that must stay soft; and salt-bath nitrocarburising (the Tufftride / Tenifer family) is a much shorter cycle that puts a thin, tough wear-and-corrosion skin on a part in hours rather than days. Which one fits depends on the case depth, the distortion budget and whether selective hardening is needed. If you are specifying a nitrided part and want the case depth and route sanity-checked against the feature it protects — before it is on the drawing and in the quote — that is exactly the conversation to have with us and the treatment house together.