What is the difference between nitriding and carburising?

Carburising adds carbon at about 900 °C and needs quenching, so parts distort and are often ground afterwards. Nitriding adds nitrogen at about 500–580 °C with no quench, so distortion is very small, but the case is thinner.

Can nitrided parts be ground?

Only lightly: the hardest layer is thin, so heavy grinding removes it. Parts are normally finished to size before nitriding.

What is QPQ treatment?

Quench-Polish-Quench: salt-bath nitriding followed by polishing and an oxidising bath. It gives a hard, black, corrosion-resistant surface.

Nitriding job work: gas, salt bath and QPQ explained

Nitriding is a surface hardening treatment that diffuses nitrogen into finished steel parts at a fairly low temperature. It gives a very hard, wear- and fatigue-resistant surface with very little distortion, so parts can usually be machined to final size first.

Quick facts

Temperature: About 500–580 °C (below the hardening range)
Types: Gas nitriding, salt-bath (liquid) nitriding, QPQ, nitrocarburising
Case depth: Gas: about 0.2–0.6 mm (0.0079–0.0236 inch); salt bath: thinner, about 0.1–0.3 mm (0.0039–0.0118 inch)
Surface hardness: Depends on the steel; nitriding steels reach about 900–1,100 HV
Distortion: Very low
Usually priced: Per kg or per batch

Types of nitriding

Type	What it gives	Typical uses
Gas nitriding	Deeper hard case, long cycle	Crankshafts, spindles, gears, dies
Salt-bath (liquid) nitriding	Thin hard layer, shorter cycle	Shafts, pins, small parts
QPQ	Salt-bath nitriding plus polish and oxidise: hard, black, corrosion resistant	Hydraulic rods, pins, gun parts
Nitrocarburising (FNC)	Thin wear-resistant layer, often black	Auto parts, bushes

Which steels suit nitriding

Nitriding steels with aluminium and chromium (EN41B) reach the highest hardness
Alloy steels such as EN19 and EN24 nitride well, usually hardened and tempered first
Hot-work die steel H13 for dies
Plain carbon steels gain wear resistance but a lower hardness

What to tell the heat treater

Steel grade and its present condition (e.g. hardened and tempered)
Type of nitriding, case depth and surface hardness wanted
Areas to keep soft (masked)
Quantity and part weight

Need nitriding done?

Post your requirement free. Workshops that take your kind of work see it and get in touch. You only talk to the ones that fit.

Get started

Related job work

Machines

Induction hardening machine

Materials

Questions

What is the difference between nitriding and carburising?: Carburising adds carbon at about 900 °C and needs quenching, so parts distort and are often ground afterwards. Nitriding adds nitrogen at about 500–580 °C with no quench, so distortion is very small, but the case is thinner.
Can nitrided parts be ground?: Only lightly: the hardest layer is thin, so heavy grinding removes it. Parts are normally finished to size before nitriding.
What is QPQ treatment?: Quench-Polish-Quench: salt-bath nitriding followed by polishing and an oxidising bath. It gives a hard, black, corrosion-resistant surface.

The figures on this page are typical values for guidance, written by Diemeco from standard engineering practice. They are not a specification: confirm grades and properties with the relevant standard or the supplier's mill test certificate, and tolerances with your workshop. Last checked 9 October 2026.