Why manganese and nitrogen instead of more nickel?

21-4N stabilises the austenite with 7.5–9.5% Mn plus 0.35–0.50% N, keeping nickel at only 3.25–4.50%. That was the original cost-saving design of the grade, and the nitrogen also raises hot strength for exhaust-valve duty.

Grades · Stainless Steel · SUH35 (21-4N)

SUH35 (21-4N)

Data under review

SUH35 is the JIS G4311 heat-resistant valve steel known worldwide as 21-4N: an austenitic Cr-Mn-Ni-N steel whose nitrogen content gives high hot strength without the cost of high-nickel alloys. It is the classic exhaust-valve material for automotive and motorcycle engines.

DIN1.4871 (X53CrMnNiN21-9)

AISI21-4N (UNS S63008)

JISSUH35 (JIS G4311)

CategoryStainless Steel

Chemical composition

Element	Content (%)
C	0.48–0.58
Si	≤0.35
Mn	7.50–9.50
P	≤0.040
S	≤0.030
Cr	20.00–22.00
Ni	3.25–4.50
N	0.35–0.50

Weight percent, per the governing standard ranges. Actual values per heat are on the mill certificate.

Properties

Structure	Austenitic, nitrogen-strengthened
Service use	Engine exhaust valves
Heat resistance	Valve service to ≈800 °C
Magnetic	Essentially non-magnetic (austenitic)

Heat treatment

Solution anneal 1100–1150 °C, water quench; aging ≈750 °C is applied for some valve specifications.

Equivalent grades

Standard	Designation
din	1.4871 (X53CrMnNiN21-9)
aisi	21-4N (UNS S63008)
jis	SUH35 (JIS G4311)
gb	5Cr21Mn9Ni4N (near)

Equivalents overlap in chemistry and use but are not always identical — see our equivalents guide before substituting in certified designs.

Applications & Processing

Applications: Engine exhaust valves (automotive, motorcycle, small industrial engines), high-temperature valve and fastener parts.

Processing: Hot forge and machine in the solution-treated condition; valves are typically friction-welded to a martensitic stem grade (SUH3/SUH11 class) and finish-ground. Not a general-purpose stainless — it is a valve-specific grade.

FAQ

Why manganese and nitrogen instead of more nickel?: 21-4N stabilises the austenite with 7.5–9.5% Mn plus 0.35–0.50% N, keeping nickel at only 3.25–4.50%. That was the original cost-saving design of the grade, and the nitrogen also raises hot strength for exhaust-valve duty.

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Source: JIS G4311 published ranges · Last updated 2026-11-03. Data is indicative — verify critical values against the governing standard and mill certificate before use.