How hard is EN40B after nitriding?

Typically about 650–800 HV at the surface (roughly 58–64 HRC), with a hard case a few tenths of a millimetre deep on a tough core.

Why is EN40B used instead of case-hardening steel?

Nitriding needs no quench, so distortion is very small and parts can be finished before treatment. That suits long spindles and screws that would bend if carburised and quenched.

Can EN40B be nitrided after hardening and tempering?

Yes, that is the normal route. The tempering temperature must be higher than the nitriding temperature so the core does not soften.

EN40B nitriding steel: properties, nitriding, uses

EN40B is a 3% chromium-molybdenum steel made to be nitrided. It is supplied hardened and tempered, machined, then nitrided at about 500–530 °C to give a very hard, wear-resistant skin with almost no distortion. It is used for gears, spindles, lead screws, extruder screws and shafts that need high surface wear resistance.

Quick facts

Type: Chromium-molybdenum nitriding steel
Chromium: About 3.0–3.5%
Supply: Hardened and tempered (T condition, about 850–1000 MPa)
Surface after nitriding: About 650–800 HV
Nitriding temperature: About 500–530 °C, below the tempering temperature
Distortion: Very low, because there is no quench

Equivalent grades

Standard	Grade
Old BS 970	EN40B
BS 970	722M24
EN / DIN	31CrMo12 / 1.8515 (close)
IS 1570	25Cr3Mo55 (close)

Equivalents are close, not identical. Name the grade and standard on the drawing.

Typical chemical composition

Element	%
Carbon (C)	About 0.20–0.28
Chromium (Cr)	About 3.0–3.5
Molybdenum (Mo)	About 0.45–0.65
Manganese (Mn)	About 0.45–0.70

Values are typical. The exact limits depend on the standard and supplier; ask for the mill test certificate when it matters.

How it is processed

Bought hardened and tempered
Rough machined, then stress relieved if heavy machining was done
Finish machined or ground, leaving only a very small allowance
Gas, salt-bath or plasma nitrided at about 500–530 °C
Light lapping or polishing only; the hard layer is thin

Typical uses

Gears and splined shafts
Machine tool spindles
Lead screws and ball screw parts
Plastic extruder and injection screws
Crankshafts and cams

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Questions

How hard is EN40B after nitriding?: Typically about 650–800 HV at the surface (roughly 58–64 HRC), with a hard case a few tenths of a millimetre deep on a tough core.
Why is EN40B used instead of case-hardening steel?: Nitriding needs no quench, so distortion is very small and parts can be finished before treatment. That suits long spindles and screws that would bend if carburised and quenched.
Can EN40B be nitrided after hardening and tempering?: Yes, that is the normal route. The tempering temperature must be higher than the nitriding temperature so the core does not soften.

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 18 October 2026.