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EN 10253 Type B Specialists
ASTM A234 WP11 EN 13CrMoSi5-5 · 1.7337 P11 · 1¼Cr-½Mo-Si To 540°C

ASTM A234 WP11 (P11) Pipe Fittings

1¼Cr-½Mo silicon-bearing alloy steel buttweld pipe fittings for boiler headers, superheater piping, and refinery service to 540°C. The elevated silicon content (0.50–1.00%) improves scale and oxidation resistance over WP12 in high-temperature oxidising atmospheres.

Chemical Composition — ASTM A234 WP11

ElementWP11 Cl.1 %WP11 Cl.2/3 %
Carbon (C)0.05–0.150.05–0.15
Manganese (Mn)0.30–0.600.30–0.60
Silicon (Si)0.50–1.000.50–1.00
Chromium (Cr)1.00–1.501.00–1.50
Molybdenum (Mo)0.44–0.650.44–0.65
Phosphorus (P)≤0.045≤0.045
Sulphur (S)≤0.045≤0.045

The elevated silicon content (0.50–1.00%) and higher chromium (1.00–1.50%) are the distinguishing features of P11 vs P12 — they improve scale and oxidation resistance at service temperatures approaching 540°C.

Mechanical Properties — ASTM A234 WP11

PropertyClass 1Class 2
Tensile Strength (min)415 MPa485 MPa
Yield Strength (min)205 MPa275 MPa
Elongation (min)30%20%
Hardness (max)197 HBW241 HBW
Heat TreatmentNormalised + Tempered or Annealed
PWHT RequiredYes — 690–750°C

WP11 vs WP12 — Key Differences

PropertyWP11WP12
Cr content1.00–1.50%0.80–1.25%
Mo content0.44–0.65%0.44–0.65%
Si content0.50–1.00%0.15–0.50%
EN W.Nr.1.73371.7335
Max service temp540°C540°C
Oxidation resistanceBetter (higher Si)Standard
Weld fillerER80S-B2 (Si-matched)ER80S-B2

Applications & PWHT Requirements

  • ›Boiler headers and steam drums — high-temperature steam service to 540°C where oxidation resistance is a selection criterion
  • ›Superheater and reheater piping — coal-fired and gas-fired power boilers
  • ›Refinery fired heater coils — oxidising flue-gas atmospheres where silicon scale resistance matters
  • ›Heat exchangers — high-temperature process service with oxidising atmosphere
PWHT: All WP11 fittings require post-weld heat treatment at 690–750°C. Hardness after PWHT must be ≤197 HBW (Class 1) or ≤241 HBW (Class 2). PMI by XRF or OES is recommended to confirm Si content and distinguish WP11 from WP12 in the field.

Request WP11 (P11) Fitting Quote

Available in elbows, tees, reducers, and caps. PWHT certificates and hardness reports included. EN 10204 3.1 standard. See also ASTM A234 WP12.

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Supplying an Indian boiler or ASME B16.9 project? The equivalent grade is listed on our group site ibrfittings.com: ASTM A234 WP11 fittings.