Deutsche Version: Formstücke aus P91 (1.4903)
X10CrMoVNb9-1 · Modified 9Cr-1Mo · EN 10253-2 Type B
The dominant grade for supercritical and ultra-supercritical power plant steam systems and HRSG high-pressure circuits. 3× the allowable stress of original P9. V+Nb+N microalloying is critical — this grade demands strict weld procedure compliance.
P91 is one of the most procedure-sensitive grades in industry. PWHT at 745–775°C is mandatory — below 745°C, the tempered martensite microstructure is not fully developed and long-term creep properties are compromised. Filler metal must be ER90S-B9 (GTAW) or E9015-B9 (SMAW). Using P9 filler (ER80S-B8) is a serious error that produces inadequate weld metal properties.
| Property | P9 (1.7386) | P91 (1.4903) |
|---|---|---|
| EN designation | X11CrMo9-1 | X10CrMoVNb9-1 |
| Vanadium (V) | None | 0.18–0.25% |
| Niobium (Nb) | None | 0.06–0.10% |
| Nitrogen (N) | None | 0.030–0.070% |
| ASME P-Number | P-No.5B | P-No.15E |
| Allowable at 600°C | ~40 MPa | ~114 MPa (3× higher) |
| Filler (GTAW) | ER80S-B8 | ER90S-B9 (different) |
| PWHT temperature | 720–760°C | 745–775°C |
| Primary use | Legacy/CANDU plant | Supercritical power, new HRSG |
| Element | Min | Max | Note |
|---|---|---|---|
| Carbon (C) | 0.08 | 0.12 | — |
| Chromium (Cr) | 8.00 | 9.50 | Oxidation resistance |
| Molybdenum (Mo) | 0.85 | 1.05 | Creep resistance |
| Vanadium (V) | 0.18 | 0.25 | Key microalloying addition |
| Niobium (Nb) | 0.060 | 0.10 | Grain refinement |
| Nitrogen (N) | 0.030 | 0.070 | Strengthening |
| Nickel (Ni) | — | 0.40 | — |
| Aluminium (Al) | — | 0.040 | Controlled low — Al ties up N |
Aluminium must be controlled low (<0.040%) — high Al ties up the free nitrogen, eliminating the strengthening effect of the N addition.
| EN designation | X10CrMoVNb9-1 |
| Material number | 1.4903 |
| ASTM equivalent | A234 WP91 / Grade 91 |
| ASME P-Number | P-No.15E (not 5B) |
| Max temperature | ≤650°C (sustained) |
| PWHT | 745–775°C (mandatory) |
| Filler (GTAW) | ER90S-B9 only |
| Standard | EN 10253-2 Type B |
| Certificate | EN 10204 3.1 standard |
PWHT 745–775°C. Time-temperature chart + hardness on EN 10204 3.1. TÜV / Lloyd's 3.2 available. PMI on every order.
Get QuoteDN15–DN600 and larger. Outside diameters follow the EN 10220 / EN ISO 1127 pipe series; wall thickness is matched to your connecting pipe.
| Size | OD (mm) |
|---|---|
| DN 15 | 21.3 |
| DN 20 | 26.9 |
| DN 25 | 33.7 |
| DN 32 | 42.4 |
| DN 40 | 48.3 |
| DN 50 | 60.3 |
| DN 65 | 76.1 |
| DN 80 | 88.9 |
| DN 100 | 114.3 |
| DN 125 | 139.7 |
| DN 150 | 168.3 |
| DN 200 | 219.1 |
| DN 250 | 273.0 |
| DN 300 | 323.9 |
| DN 350 | 355.6 |
| DN 400 | 406.4 |
| DN 450 | 457.0 |
| DN 500 | 508.0 |
| DN 600 | 610.0 |
P9 (1.7386) is the original 9Cr-1Mo grade without microalloying additions. P91 (1.4903) is a modified 9Cr-1Mo grade containing vanadium (0.18–0.25%), niobium (0.06–0.10%), and nitrogen (0.03–0.07%). These additions produce a tempered martensitic microstructure with approximately 3× the allowable stress of P9 at 600°C. P9 is ASME P-No.5B; P91 is ASME P-No.15E. They have different filler requirements (ER80S-B8 for P9, ER90S-B9 for P91) and different PWHT temperatures. They are NOT interchangeable.
P91 requires ER90S-B9 for GTAW/GMAW and E9015-B9 for SMAW. Using ER80S-B8 (the P9 filler) on P91 weld joints is incorrect and will result in a weld metal with insufficient allowable stress and potentially incorrect microstructure. The V and Nb content of the filler must match the base metal.
Post-weld heat treatment is mandatory for P91, in the 745–775°C range. Below 745°C the tempered martensite is not fully developed and long-term creep properties suffer.
ASTM A234 WP91 (Grade 91). In ASME terms P91 is P-No. 15E, not P-No. 5B like P9.
Supplying an Indian boiler or ASME B16.9 project? The equivalent grade is listed on our group site ibrfittings.com: ASTM A234 WP91 fittings.
