IBR Approved · Alloy Steel (Grade 91)
ASTM A234 WP91 Buttweld Pipe Fittings
Modified 9Cr-1Mo-V (Grade 91) alloy steel fittings for supercritical and ultra-supercritical power plant main steam and reheater piping above 600°C. Elbows, tees, reducers and caps to ASME B16.9 / B16.28, IBR approved with Form III-C, EN 10204 3.1 / 3.2 traceable.
Grade Overview
ASTM / ASME A234 WP91 — Grade 91, modified 9Cr-1Mo — is a martensitic creep-strength-enhanced ferritic (CSEF) steel microalloyed with vanadium, niobium and nitrogen. This microalloying delivers roughly three times the allowable stress of plain 9Cr-1Mo (WP9), enabling thinner, lighter main steam and reheater piping for supercritical and ultra-supercritical (USC) power plants operating above 600°C.
WP91 is supplied normalized and tempered with hardness controlled to 190–250 HBW, IBR approved with Form III-C for boiler piping. Its EN counterpart is X10CrMoVNb9-1 (material number 1.4903).
Critical: heat treatment & PWHT control
WP91 is unforgiving of procedure deviations. Fittings are supplied normalized & tempered. Weld PWHT is mandatory within a narrow 730–780°C window (kept below Ac1), with final hardness 190–250 HBW. Only a matching Grade 91 (B9) filler — ER90S-B9 / E9015-B9 — may be used, with tight preheat and interpass control. ASME P-No. 15E, Group 1. Every fitting is supplied with heat-treatment and hardness records.
Chemical Composition — A234 WP91
| Element | Requirement | Note |
|---|---|---|
| Carbon (C) | 0.08 – 0.12% | — |
| Chromium (Cr) | 8.00 – 9.50% | Oxidation & creep |
| Molybdenum (Mo) | 0.85 – 1.05% | Creep resistance |
| Vanadium (V) | 0.18 – 0.25% | Creep-strength microalloy |
| Niobium (Nb) | 0.06 – 0.10% | Microalloy |
| Nitrogen (N) | 0.030 – 0.070% | Microalloy |
| Mn / Si / Ni | Mn 0.30–0.60, Si 0.20–0.50, Ni ≤0.40 | — |
Mechanical Properties — A234 WP91
| Property | Requirement |
|---|---|
| Tensile Strength (min) | 585 MPa (85 ksi) |
| Yield Strength (min) | 415 MPa (60 ksi) |
| Elongation (min) | ≥ 20% |
| Hardness | 190 – 250 HBW |
| Heat Treatment | Normalized & Tempered (mandatory) |
Equivalent Grades
| Product Form | Specification |
|---|---|
| Buttweld fittings | ASTM / ASME A234 WP91 |
| Seamless pipe | ASTM A335 P91 |
| Forgings / flanges | ASTM A182 F91 |
| EN designation | X10CrMoVNb9-1 (W.Nr. 1.4903) |
Fitting Types Available in WP91
- Elbows — 45° / 90° / 180°, LR & SR
- Tees — equal & reducing
- Reducers — concentric & eccentric
- Caps — buttweld end closures