A335 P11 vs P22 vs P91
ASTM A335 P11, P22, and P91 are the three most widely used chrome-molybdenum (Cr-Mo) alloy steel pipe grades for high-temperature service in power plants, refineries, and petrochemical facilities. Each grade increases chromium content and adds microalloying elements to achieve progressively higher creep strength and oxidation resistance, enabling service at higher temperatures with thinner wall sections.
Quick Answer
P11 (1.25Cr-0.5Mo) covers service up to approximately 510°C (950°F). P22 (2.25Cr-1Mo) extends the range to approximately 565°C (1050°F) with better sulfidation resistance. P91 (9Cr-1Mo-VNb) provides dramatically higher creep strength up to 593°C (1100°F) and allows significantly thinner pipe walls, reducing weight and thermal stresses.
Composition and Properties
| Property | P11 (1.25Cr-0.5Mo) | P22 (2.25Cr-1Mo) | P91 (9Cr-1Mo-VNb) |
|---|---|---|---|
| UNS | K11597 | K21590 | K90901 |
| Chromium | 1.00-1.50% | 1.90-2.60% | 8.0-9.5% |
| Molybdenum | 0.44-0.65% | 0.87-1.13% | 0.85-1.05% |
| Vanadium | - | - | 0.18-0.25% |
| Niobium | - | - | 0.06-0.10% |
| Nitrogen | - | - | 0.030-0.070% |
| Tensile strength (min) | 415 MPa (60 ksi) | 415 MPa (60 ksi) | 585 MPa (85 ksi) |
| Yield strength (min) | 205 MPa (30 ksi) | 205 MPa (30 ksi) | 415 MPa (60 ksi) |
| Hardness (max) | 163 HBW | 163 HBW | 250 HBW |
| Heat treatment | Normalize + temper | Normalize + temper | Normalize (1040-1080°C) + temper (730-780°C) |
| Preheat for welding | 150-200°C | 200-250°C | 200-300°C |
| PWHT | 675-730°C | 675-730°C | 730-770°C (critical range) |
| Max service temp | ~510°C (950°F) | ~565°C (1050°F) | ~593°C (1100°F) |
| Matching forging | A182 F11 | A182 F22 | A182 F91 |
| Matching fitting | A234 WP11 | A234 WP22 | A234 WP91 |
| Relative cost | 1.5x CS | 1.8x CS | 2.5-3x CS |
Key Differences
Creep Strength
P91’s microalloying with vanadium, niobium, and nitrogen creates a tempered martensitic microstructure with very fine carbonitride precipitates (MX-type) that pin dislocation movement at elevated temperatures. This provides approximately 2-3x higher allowable stress than P22 at 550°C, enabling wall thicknesses 30-50% thinner for the same design pressure and temperature.
This wall thickness reduction cascades through the entire design: lighter pipe supports, less welding volume, smaller and lighter fittings, and reduced thermal stresses during transient conditions (startup/shutdown).
Oxidation and Sulfidation Resistance
Higher chromium content provides better resistance to high-temperature oxidation and sulfidation. P91 (9% Cr) resists sulfidation in refinery environments much more effectively than P11 (1.25% Cr) or P22 (2.25% Cr). The chromium content determines the position on the McAdams/Couper-Gorman curves used to predict sulfidation rates in hydroprocessing units.
Fabrication Complexity
P91 requires much stricter fabrication controls than P11 or P22:
- Precise preheat and interpass temperature control
- Mandatory PWHT within a narrow temperature window (730-770°C); overheating destroys the optimized microstructure
- Hardness testing after PWHT to verify proper tempering (typically 200-250 HBW)
- Longer PWHT hold times per unit thickness
Selection Guide
| Operating Temperature | Recommended Grade | Rationale |
|---|---|---|
| Up to 427°C | Carbon steel (A106) | Adequate creep life, lowest cost |
| 427-510°C | P11 | Sufficient Cr for mild sulfidation, good economy |
| 510-565°C | P22 | Better creep and sulfidation resistance |
| 540-593°C | P91 | Highest creep strength, thinnest walls |
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