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 IV (Part D) (formerly Form III-C), EN 10204 3.1 / 3.2 traceable.
Where WP91 Is Used
- ✓ Main steam and hot reheat lines in supercritical and ultra-supercritical power plants
- ✓ High-pressure steam piping in heat recovery steam generators (HRSGs)
- ✓ High-temperature refinery and petrochemical lines
WP91 Compared With Related Grades
| Grade | Chemistry | Key difference |
|---|---|---|
| A234 WP22 | 2¼Cr-1Mo | Lower creep strength; thicker walls at the same conditions |
| A234 WP9 | 9Cr-1Mo | Not the same as WP91: no vanadium, niobium or nitrogen additions |
| A234 WP91 | 9Cr-1Mo-V (modified) | High creep strength; tight control of heat treatment and hardness |
| A234 WP92 | 9Cr-2W | Tungsten added for higher creep strength than WP91 |
How to Specify WP91 Fittings on a Purchase Order
A complete order line avoids clarification rounds. Example:
Elbow 90° LR, 10" SCH 120, ASTM A234 WP91, ASME B16.9, normalised and tempered, hardness report, PMI, EN 10204 3.1
- ✓ Ask for the heat treatment record and hardness results on every piece (190–250 HBW)
- ✓ Ask for PMI that distinguishes WP91 from WP9 and WP92
- ✓ Add IBR Form IV (Part D) for Indian boiler piping, and 3.2 witnessed inspection if your specification requires it
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 IV (Part D) 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) |
EN 10253 equivalent: see WP91 pipe fittings on arshyapipefittings.com, and the European grade EN 1.4903 (X10CrMoVNb9-1).
Fitting Types Available in WP91
- Elbows — 45° / 90° / 180°, LR & SR
- Tees — equal & reducing
- Reducers — concentric & eccentric
- Caps — buttweld end closures
Related grades: WP92 · WP22 · WP9 · IBR Form IV (Part D) guide · IBR grades list · WP91 for HRSGs
Testing & Inspection — WP91
- Chemical analysis of each heat, including V, Nb and N — the microalloying elements that give Grade 91 its creep strength.
- Hardness checked after normalizing & tempering against the 190–250 HBW band. Low hardness can point to over-tempering or ferrite; high hardness to untempered martensite — both reduce service life.
- Tensile test to ASTM A234 (585 MPa min. tensile, 415 MPa min. yield).
- Positive material identification (PMI) to confirm 9Cr-1Mo-V and keep WP91 from being mixed with plain WP9.
- Dimensional and visual inspection to ASME B16.9; NDE (MT / UT / RT) as specified in your purchase order.
Dimensions & Size Range — ASME B16.9
WP91 fittings are made to ASME B16.9 (long-radius) and ASME B16.28 (short-radius elbows) dimensions, from 1/2" to 48" NB and above, in schedules SCH 20 to SCH 160 / XXS. Wall thickness matches the connecting pipe schedule. Centre-to-end dimensions for the most-ordered sizes:
| NPS | Outside diameter (mm) | 90° LR elbow “A” (mm) | 45° LR elbow “B” (mm) | Equal tee “C” (mm) |
|---|---|---|---|---|
| 1/2" | 21.3 | 38 | 16 | 25 |
| 1" | 33.4 | 38 | 22 | 38 |
| 2" | 60.3 | 76 | 35 | 64 |
| 3" | 88.9 | 114 | 51 | 86 |
| 4" | 114.3 | 152 | 64 | 105 |
| 6" | 168.3 | 229 | 95 | 143 |
| 8" | 219.1 | 305 | 127 | 178 |
| 10" | 273.1 | 381 | 159 | 216 |
| 12" | 323.9 | 457 | 190 | 254 |
| 16" | 406.4 | 610 | 254 | 305 |
| 20" | 508.0 | 762 | 318 | 381 |
| 24" | 610.0 | 914 | 381 | 432 |
Long-radius elbows have a centre-line radius of 1.5 × NPS; short-radius (ASME B16.28) 1.0 × NPS. Full tables to NPS 48, reducing tees, reducers and caps are available with every quotation. For EN 10253 dimensions see arshyapipefittings.com.
Export Documentation & Packing
We ship WP91 fittings to projects outside India with the documents EPC contractors and inspection agencies expect:
- Mill test certificate — EN 10204 3.1 as standard; 3.2 with third-party witness
- Third-party inspection — Bureau Veritas, TÜV, Lloyd’s Register, SGS, DNV or the client’s own inspector
- IBR Form IV (Part D) — for boiler piping on Indian Boiler Regulations projects
- Traceability — every fitting marked with grade, size, schedule and heat number, matching the certificate
- Packing — seaworthy wooden cases or pallets, bevel ends protected with end caps
- Shipping documents — commercial invoice, packing list and certificate of origin; FOB, CFR or CIF terms
Send your BOQ through the RFQ form (Excel or PDF attachments accepted) or on WhatsApp — quotations are usually issued within 24 hours.
WP91 Fittings — Frequently Asked Questions
What is the difference between WP9 and WP91?
WP9 is plain 9Cr-1Mo. WP91 is modified 9Cr-1Mo with vanadium, niobium and nitrogen added, which gives much higher creep strength above about 550°C. The two are not interchangeable, and WP91 needs much tighter heat-treatment and PWHT control. See P9 vs P91 explained.
What hardness should WP91 fittings have?
190–250 HBW after normalizing & tempering, and the same range is checked again after weld PWHT (730–780°C). Readings outside the band are a sign of incorrect heat treatment.
Should I choose WP91 or WP92?
WP92 adds tungsten for still higher creep strength and is used in ultra-supercritical main steam lines; WP91 is the established choice for most supercritical and HRSG piping. Comparison: P91 vs P92 fittings.