ASTM A234 WP91 pipe fittings are buttweld fittings of 9Cr-1Mo-V (Grade 91) steel alloy, which is an addition of Vanadium (0.18–0.25%), Niobium (0.06–0.10%), and Nitrogen (0.03–0.07%) to the plain 9Cr-1Mo (WP9) to produce fine M(C,N) carbides which dramatically improve the creep rupture strength at temperatures where WP22 and WP9 may not. This precipitation strengthening mechanism enables up to 50% reduction in wall thickness, compared to WP22, for the same duty and is the designated wall thickness in modern USC power plants for critical steam piping, including steam headers, superheater outlets and main steam pipes. WP91 fittings meet the following minimum requirements: minimum yield strength of 415 MPa (60 ksi) per ASTM A234 Table 2; and minimum tensile strength of 585 MPa (85 ksi) and upper limit of 760 MPa (110 ksi) per ASTM A234 Table 2, with required PWHT of 730-800°C and hardness testing of 100% to confirm hardness not to exceed 265 HBW after heat treatment. Metinox Overseas is dedicated to offering high-quality ASTM A234 WP91 Pipe Fittings in various types.
Request a Quote now!Table of Contents
- Specifications of ASTM A234 WP91 Pipe Fittings
- Chemical Composition of ASTM A234 WP91 Pipe Fittings
- ASTM A234 WP91 Pipe Fittings Mechanical Properties
- Types of ASTM A234 WP91 Pipe Fittings
- WP91 vs WP22 vs WP9 Comparison
- Pressure Rating Chart
- Applications Usage
- Packaging and Delivery
- Related Products
- Frequently Asked Questions
Specifications of ASTM A234 WP91 Pipe Fittings
| Property | Specification Details |
|---|---|
| Specifications | ASTM A234 / ASME SA234 |
| Dimensions | ASME/ANSI B16.9, ASME B16.28, MSS-SP-43 |
| Size | 1/8" NB to 48" NB (Seamless & 100% X-Ray Welded, Fabricated) |
| Type | Seamless / Welded / Fabricated |
| Bending Radius | R=1D, 2D, 3D, 5D, 6D, 8D, 10D or Custom |
| Thickness | Sch 5s, Sch 10s, Sch 40s, Sch 80s, Sch 160s, Sch XXS |
| PWHT Requirements | 730-800°C (1350-1470°F) |
| Types Available | Type 1 (Normalised + Tempered, standard) / Type 2 (S1 Supplementary Requirement - see Type 1 vs Type 2 section) |
| Hardness | 265 HBW max (Type 1) |
Chemical Composition of ASTM A234 WP91 Pipe Fittings
| Element | Range / Max | Metallurgical Purpose |
|---|---|---|
| Carbon (C) | 0.08-0.12 | Carbide formation and base strength |
| Manganese (Mn) | 0.30-0.60 | Austenite stabilisation |
| Silicon (Si) | 0.20-0.50 | Deoxidation |
| Phosphorus (P) | ≤0.020 | Purity control |
| Sulfur (S) | ≤0.010 | Purity control |
| Chromium (Cr) | 8.00-9.50 | Oxidation and corrosion resistance |
| Molybdenum (Mo) | 0.85-1.05 | Solid-solution creep strengthening |
| Vanadium (V) | 0.18-0.25 | Carbide/nitride precipitation strengthening |
| Niobium / Columbium (Nb/Cb) | 0.06-0.10 | Precipitation strengthening and grain refinement |
| Nitrogen (N) | 0.03-0.07 | Nitride precipitation; essential microalloying element |
| Aluminum (Al) | ≤0.04 | Controlled residual; excess Al inhibits N availability |
| Nickel (Ni) | ≤0.40 | Toughness contribution |
ASTM A234 WP91 Pipe Fittings Mechanical Properties
| Property | Requirement | Notes |
|---|---|---|
| Tensile Strength | 585-760 MPa (85-110 ksi) | Range per ASTM A234 Table 2, Type 1 |
| Yield Strength | 415 MPa (60 ksi) minimum | 0.2% offset method |
| Elongation | 20% minimum | In 2 inches (50 mm) |
| Hardness (Type 1) | 265 HBW maximum | 100% testing mandatory - unique WP91 requirement |
| Impact (optional) | 27 J at RT if specified | Per Supplementary Requirement if ordered |
Types of ASTM A234 WP91 Pipe Fittings
90° Elbow
45° Elbow
Equal Tee
Reducing Tee
Concentric Reducer
Eccentric Reducer
Pipe Cap
Cross
Pipe Bend
WP91 Type 1 vs Type 2 Comparison
Type 1 is the standard normalised and tempered supply condition with hardness limited to 265 HBW. Type 2 is ordered under Supplementary Requirement S1 and is supplied without final tempering, where the buyer or fabricator performs PWHT after assembly.
| Property | WP22 (2.25Cr-1Mo) | WP9 (9Cr-1Mo) | WP91 (9Cr-1Mo-V) |
|---|---|---|---|
| Chromium | 2.00-2.50% | 8.00-10.00% | 8.00-9.50% |
| Vanadium / Niobium | None | None | V 0.18-0.25% + Nb 0.06-0.10% |
| Max service temperature | ~580°C | ~600°C | 650°C+ (ultra-supercritical) |
| Tensile min | 415 MPa | 415 MPa | 585 MPa |
| Yield min | 205 MPa | 205 MPa | 415 MPa |
| Creep strength vs WP22 at 600°C | Baseline | Moderate improvement | 50-70% higher - enables thinner walls |
| PWHT mandatory | Yes (675°C min) | Yes (730-790°C) | Yes (730-800°C; 100% HT) |
| Typical application | Steam piping to 580°C | Refinery heaters, moderate USC | Ultra-supercritical boilers, reheaters, HP headers |
Pressure Rating Chart of ASTM A234 WP91 Pipe Fittings
| Nominal Diameter (in) | Nominal Diameter (mm) | OD (in) | OD (mm) | Thickness (in) | Thickness (mm) | Pipe & Reducers (psi) | Pipe & Reducers (kPa) | LR Elbows (psi) | LR Elbows (kPa) | Mitred Elbows (psi) | Mitred Elbows (kPa) | Tees (psi) | Tees (kPa) | Laterals (psi) | Laterals (kPa) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 4 | 100 | 4.500 | 114 | 0.078 | 2.0 | 505 | 3480 | 450 | 3105 | 380 | 2620 | 250 | 1725 | 155 | 1070 |
| 4 | 100 | 4.500 | 114 | 0.083 | 2.1 | 535 | 3690 | 485 | 3345 | 405 | 2790 | 265 | 1825 | 165 | 1140 |
| 4 | 100 | 4.500 | 114 | 0.109 | 2.8 | 710 | 4895 | 640 | 4415 | 535 | 3690 | 355 | 2450 | 220 | 1515 |
| 4 | 100 | 4.500 | 114 | 0.120 | 3.0 | 790 | 5445 | 710 | 4895 | 595 | 4105 | 390 | 2690 | 245 | 1690 |
| 4 | 100 | 4.500 | 114 | 0.125 | 3.2 | 825 | 5690 | 740 | 5100 | 620 | 4275 | 410 | 2825 | 255 | 1760 |
| 5 | 125 | 5.563 | 141 | 0.078 | 2.0 | 405 | 2790 | 365 | 2515 | 305 | 2105 | 200 | 1380 | 125 | 860 |
| 5 | 125 | 5.563 | 141 | 0.109 | 2.8 | 570 | 3930 | 515 | 3550 | 425 | 2930 | 285 | 1965 | 175 | 1205 |
| 5 | 125 | 5.563 | 141 | 0.125 | 3.2 | 660 | 4550 | 595 | 4105 | 490 | 3380 | 325 | 2240 | 200 | 1380 |
| 6 | 150 | 6.625 | 168 | 0.078 | 2.0 | 340 | 2345 | 305 | 2515 | 260 | 1795 | 168 | 1160 | 105 | 725 |
| 6 | 150 | 6.625 | 168 | 0.109 | 2.8 | 475 | 3275 | 430 | 2965 | 365 | 2515 | 235 | 1620 | 145 | 1000 |
| 6 | 150 | 6.625 | 168 | 0.125 | 3.2 | 550 | 3790 | 495 | 3415 | 420 | 2895 | 275 | 1895 | 170 | 1170 |
| 8 | 200 | 8.625 | 219 | 0.078 | 2.0 | 260 | 1795 | 230 | 1585 | 200 | 1380 | 125 | 860 | 80 | 550 |
| 8 | 200 | 8.625 | 219 | 0.109 | 2.8 | 365 | 2515 | 325 | 2240 | 280 | 1930 | 180 | 1240 | 110 | 760 |
| 10 | 250 | 10.750 | 273 | 0.125 | 3.2 | 335 | 2310 | 300 | 2070 | 260 | 1795 | 160 | 1105 | 105 | 725 |
| 12 | 300 | 12.750 | 324 | 0.125 | 3.2 | 280 | 1930 | 250 | 1725 | 220 | 1515 | 130 | 895 | 85 | 585 |
Applications Usage of ASTM A234 WP91 Pipe Fittings
- Power Generation Industry
- Thermal Power Plants
- Nuclear Power Plants
- Oil & Gas Industry
- Petrochemical Industry
- Refineries
- Chemical Processing Industry
- Boiler Systems
- Superheater & Reheater Systems
- Heat Exchanger Systems
ASTM A234 WP91 Pipe Fittings Packaging and Delivery
Packaging is very important in making sure that the ASTM A234 WP91 pipe fittings are safely transported from our company to the client. There are several types of packaging that Metinox Overseas can offer, depending on the specific product and the specific requirement of the client. However, if there are no packing instructions in the purchase order, we will use the standard export packing procedures that we offer.
The ASTM A234 WP91 pipe fittings are packed with caps at each end, custom fitted at the customer’s request. All the products are securely packed inside wooden crates, boxes, or bundles, all marked clearly to distinguish them.
Apart from the packaging, at Metinox Overseas, we also offer all the necessary shipping documents. This way, we can guarantee safe delivery of the products from dispatch to the arrival point.
Frequently Asked Questions
ASTM A234 WP91 pipe fittings are high-strength chromium-molybdenum alloy steel fittings used in high-temperature and high-pressure piping systems.
WP91 is equivalent to ASTM A335 P91 alloy steel, both having similar 9Cr-1Mo-V composition and properties.
They are used in power plants, refineries, and steam piping systems operating under extreme heat and pressure.
WP91 fittings are covered under ASTM A234 and manufactured as per standards like ASME B16.9 and ASME B16.28.
Yes, WP91 fittings are specifically designed for high-pressure and high-temperature service conditions.
PWHT at 730-800°C (1350-1470°F) is mandatory. Without PWHT, the weld and HAZ remain in an untempered martensitic condition, hardness exceeds 265 HBW, and the fitting will fail creep requirements. ASTM A234 requires 100% hardness testing on all WP91 fittings as a unique code requirement.
Type 1 is the standard supply condition: normalised and tempered by the manufacturer, hardness ≤265 HBW. Type 2 (Supplementary Requirement S1) is supplied without final tempering; the buyer or fabricator is responsible for PWHT of the assembled system. Specify Type 1 for standard projects; specify Type 2 / S1 only when field PWHT of the complete piping system is planned.
Both are 9Cr-1Mo grades but WP91 adds Vanadium (0.18-0.25%), Niobium (0.06-0.10%), and Nitrogen (0.03-0.07%). These microalloying elements form stable M(C,N) carbides that provide precipitation-strengthened creep resistance at temperatures above 580°C where plain WP9 is inadequate. WP9 does not have these additions.
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