P91 vs P22 — why the change?

At 600 °C P91 has roughly double the allowable stress of P22 (2.25Cr-1Mo), so steam lines can be built with much thinner walls — less weight, less welding, better thermal flexibility.

Why is P91 heat treatment so critical?

Its strength comes from a tempered martensite structure stabilised by V/Nb carbonitrides. A missed or short temper leaves hard, brittle martensite that has caused in-service failures — PWHT records are always checked.

Grades · Pipe & Tube Steel · P91 / X10CrMoVNb9-1

P91 / X10CrMoVNb9-1

Data verified

P91 is the 9Cr-1Mo-V-Nb creep-strength-enhanced ferritic steel that made modern ultra-supercritical power stations possible. Nitrogen and niobium carbonitrides hold its creep strength at 600 °C far beyond the older 2.25Cr and 12Cr grades, so main-steam pipework runs thinner and lighter.

DIN1.4903

AISIASTM A335 P91 / A213 T91

GB10Cr9Mo1VNb (GB/T 5310)

CategoryPipe & Tube Steel

Chemical composition

Element	Content (%)
C	0.08–0.12
Si	0.20–0.50
Mn	0.30–0.60
P	≤0.020
S	≤0.010
Cr	8.00–9.50
Mo	0.85–1.05
V	0.18–0.25
Nb	0.06–0.10
N	0.03–0.07
Ni	≤0.40

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

Properties

Yield strength	≥415 MPa
Tensile strength	585–760 MPa
Density	7.85 g/cm³
Creep service	Power plant steam to ~600 °C

Heat treatment

Supplied normalized (1040–1080 °C) and tempered (730–780 °C). The temper is not optional — untempered P91 is brittle, and post-weld heat treatment at 730–760 °C is mandatory after any welding.

Equivalent grades

Standard	Designation
din	1.4903
aisi	ASTM A335 P91 / A213 T91
gb	10Cr9Mo1VNb (GB/T 5310)

Compare side by side: P91 / X10CrMoVNb9-1 vs P92 / X10CrWMoVNb9-2

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

Applications & Processing

Applications: Main and hot-reheat steam lines, superheater/reheater tubing (T91), headers, high-energy piping in USC and CCGT plant.

Processing: A code-controlled material: weld with matching B9 consumables, strict preheat/interpass control and recorded PWHT — the Nasteel buyer should treat the mill heat-treatment certificate as part of the product. Cold bending limits and hardness caps apply.

FAQ

P91 vs P22 — why the change?: At 600 °C P91 has roughly double the allowable stress of P22 (2.25Cr-1Mo), so steam lines can be built with much thinner walls — less weight, less welding, better thermal flexibility.
Why is P91 heat treatment so critical?: Its strength comes from a tempered martensite structure stabilised by V/Nb carbonitrides. A missed or short temper leaves hard, brittle martensite that has caused in-service failures — PWHT records are always checked.

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Source: EN 10216-2 / ASTM A335 published ranges · Last updated 2026-11-03. Data is indicative — verify critical values against the governing standard and mill certificate before use.