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Alloy Steel · Pipes · ASTM A335 P91

ASTM A335 P91 Pipes

In short: ASTM A335 P91 pipe is the 9Cr-1Mo-V creep-strength-enhanced steel for main steam, hot reheat and header piping in supercritical and combined-cycle plants, allowing much thinner walls than P22.

P91
Designation
7.77
density, g/cm³
3.1 / 3.2
EN 10204 certificates
5
P91 product forms
Quote ASTM A335 P91 →Sizes and weights
P91 pipe weight by sizeCalculated at 7.77 g/cm³: ½" 1.26 kg/m; ¾" 1.67 kg/m; 1" 2.48 kg/m; 1¼" 3.35 kg/m; 1½" 4.00 kg/m; 2" 5.38 kg/m; 2½" 8.55 kg/m; 3" 11.18 kg/m; 4" 15.91 kg/m; 6" 27.97 kg/m; 8" 42.11 kg/m; 10" 59.69 kg/m; 12" 78.91 kg/m. P91 pipe weight by size7.77 g/cm³ ½": 1.26 kg/m½"¾": 1.67 kg/m¾"1": 2.48 kg/m1"1¼": 3.35 kg/m1¼"1½": 4.00 kg/m1½"2": 5.38 kg/m2"2½": 8.55 kg/m2½"3": 11.18 kg/m3"4": 15.91 kg/m4"6": 27.97 kg/m6"8": 42.11 kg/m8"10": 59.69 kg/m10"12": 78.91 kg/m12" 78.9 NPS, Sch 40 · kg/m
Calculated values for estimating; actual weights vary within tolerances.

Why this combination

Why main-steam lines moved to P91

Creep-strength-enhanced ferritic (CSEF) steelMuch higher creep strength than P22: thinner wallsStrict normalise-and-temper and PWHT controlType 1 / Type 2 designations in recent A335/A213 editions

Thinner walls mean lighter piping, lower thermal stress during start-ups and smaller supports. P91's creep strength at around 565–600 °C is what makes that possible.

Its properties depend on normalising and tempering; specify Type 1 or Type 2 and check hardness and heat-treatment records on arrival.

Specification

How to specify ASTM A335 P91

NPS ⅛" to 48" and larger (DN 6 – DN 1200+), schedules 5S to XXS, single random, double random and cut lengths

P91 pipe cross-section, NPS 6 Sch 40 Engineering drawing showing outside diameter 168.28 mm and wall thickness 7.11 mm (ASTM A335 P91 P91). Wall thickness is exaggerated for clarity. OD 168.28 mm WT 7.11 mm NPS 6SCH 40 ASTM A335 P91 P91
P91 pipes: ASTM A335 P91
Specification
ASTM A335 P91
Type
Type 1 or Type 2 (recent A335 editions)

Sizes and weights

ASTM A335 P91 sizes, weighed at 7.77 g/cm³

Other sizes on request. For general pipes information, such as manufacturing routes and end finishes, see the pipes guide.

P91 pipe: wall (mm) / weight (kg/m) at 7.77 g/cm³, ASME B36.10M
NPSOD mmSch 40Sch 80Sch 160XXS
½"21.342.77 / 1.263.73 / 1.604.78 / 1.937.47 / 2.53
¾"26.672.87 / 1.673.91 / 2.175.56 / 2.877.82 / 3.60
1"33.403.38 / 2.484.55 / 3.206.35 / 4.199.09 / 5.39
1¼"42.163.56 / 3.354.85 / 4.426.35 / 5.559.70 / 7.69
1½"48.263.68 / 4.005.08 / 5.357.14 / 7.1710.15 / 9.44
2"60.333.91 / 5.385.54 / 7.418.74 / 11.0111.07 / 13.31
2½"73.035.16 / 8.557.01 / 11.309.53 / 14.7714.02 / 20.20
3"88.905.49 / 11.187.62 / 15.1211.13 / 21.1315.24 / 27.40
4"114.306.02 / 15.918.56 / 22.0913.49 / 33.2017.12 / 40.61
6"168.287.11 / 27.9710.97 / 42.1218.26 / 66.8721.95 / 78.40
8"219.088.18 / 42.1112.70 / 63.9823.01 / 110.1322.23 / 106.82
10"273.059.27 / 59.6915.09 / 95.0228.58 / 170.5525.40 / 153.55
12"323.8510.31 / 78.9117.48 / 130.7233.32 / 236.3025.40 / 185.04

Chemistry

P91 composition

Density
7.77 g/cm³
P91 Alloy Steel (Modified 9% chromium-1% molybdenum-vanadium) chemical composition, % (ASTM A335 (P91))
ElementLimit, %Why it matters
C0.08 – 0.12
Mn0.30 – 0.60
P0.020 max
S0.010 max
Si0.20 – 0.50
Cr8.00 – 9.50
Mo0.85 – 1.05
V0.18 – 0.25
Nb0.06 – 0.10
N0.030 – 0.070
Ni0.40 max
Al0.02 max

Limits per ASTM A335 (P91); other product forms and specifications differ slightly. Your order's specification and edition govern.

0%5%10% Cr Mo
Main alloying elements of P91 Alloy Steel (Modified 9% chromium-1% molybdenum-vanadium): bars show the permitted range (ASTM A335 (P91)).

Questions

About ASTM A335 P91

Why is P91 pipe hardness checked?

Hardness outside the specification window can indicate wrong heat treatment, which shortens creep life.

Is P91 pipe available IBR-certified?

Yes, from IBR-recognised mills where required.

From enquiry to dispatch

How a ASTM A335 P91 order runs

  1. 01 EnquiryGrade, specification, size, quantity
  2. 02 ReviewChecked against your standard
  3. 03 InspectPMI, dimensions, TPI if required
  4. 04 CertifyEN 10204 3.1 / 3.2, heat traceable
  5. 05 ShipPacked and documented for the destination

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