A234 WP11 vs WP22
ASTM A234 WP11 and WP22 are chromium-molybdenum (Cr-Mo) alloy steel grades for buttweld pipe fittings used in high-temperature service. WP11 contains 1.25% Cr and 0.5% Mo. WP22 contains 2.25% Cr and 1.0% Mo. The higher chromium and molybdenum content of WP22 provides superior oxidation resistance and creep strength at elevated temperatures.
Chemical Composition
| Element | WP11 (%) | WP22 (%) |
|---|---|---|
| Carbon | 0.05-0.15 | 0.05-0.15 |
| Manganese | 0.30-0.60 | 0.30-0.60 |
| Chromium | 1.00-1.50 | 2.00-2.50 |
| Molybdenum | 0.44-0.65 | 0.87-1.13 |
| Silicon | 0.50-1.00 | max 0.50 |
| Phosphorus | max 0.030 | max 0.030 |
| Sulfur | max 0.030 | max 0.030 |
Mechanical Properties
| Property | WP11 (Class 1) | WP22 (Class 1) |
|---|---|---|
| Min yield strength | 205 MPa (30 ksi) | 205 MPa (30 ksi) |
| Min tensile strength | 415 MPa (60 ksi) | 415 MPa (60 ksi) |
| Min elongation (2”) | 20% | 20% |
| Hardness (max) | 197 HBW | 197 HBW |
Both grades share the same minimum mechanical properties in Class 1 condition. The difference lies entirely in their high-temperature performance, driven by the alloy content.
Temperature Service Range
| Parameter | WP11 | WP22 |
|---|---|---|
| Maximum continuous service temp | ~540 degrees C (1000 degrees F) | ~600 degrees C (1100 degrees F) |
| Oxidation resistance | Good up to 540 degrees C | Better; good up to 600 degrees C |
| Creep strength at 500 degrees C | Adequate | Superior |
| Hydrogen attack resistance | Moderate | Better (higher Cr) |
| Matching pipe | ASTM A335 P11 | ASTM A335 P22 |
| Matching forged flange | ASTM A182 F11 | ASTM A182 F22 |
When to Use Each Grade
The piping material specification (pipe class) dictates the grade selection. WP11 and WP22 are not interchangeable; each matches a specific pipe and flange grade:
| Service Condition | Recommended Grade | Reason |
|---|---|---|
| Steam piping up to 540 degrees C | WP11 | Sufficient Cr-Mo for this temperature range |
| Steam piping 540-600 degrees C | WP22 | Higher Cr-Mo needed for oxidation/creep |
| Hydroprocessing reactors | WP22 | Better hydrogen attack resistance |
| Catalytic reformer piping | WP22 | High-temperature hydrocarbon service |
| Boiler feed water (high temp) | WP11 | Adequate for typical BFW temperatures |
| General refinery high-temp service | WP11 or WP22 | Per pipe class specification |
Welding Considerations
Both grades require preheat and post-weld heat treatment (PWHT):
| Welding Parameter | WP11 | WP22 |
|---|---|---|
| Preheat temperature | 150-200 degrees C | 200-250 degrees C |
| PWHT temperature | 680-720 degrees C | 680-730 degrees C |
| PWHT hold time | 1 hour per 25 mm thickness (min) | 1 hour per 25 mm thickness (min) |
| Filler metal | ER80S-B2 / E8018-B2 | ER90S-B3 / E9018-B3 |
| Interpass max temp | 300 degrees C | 300 degrees C |
Failure to perform proper PWHT on Cr-Mo fittings leads to excessive hardness in the heat-affected zone, increasing susceptibility to stress corrosion cracking. PWHT is mandatory per ASME B31.3 for both grades.
Availability and Cost
WP11 fittings are more widely stocked than WP22, as 1.25Cr-0.5Mo piping is more common in general refinery service. WP22 is typically ordered with longer lead times (8-16 weeks vs 4-8 weeks for WP11). Both carry a significant premium over carbon steel A234 WPB fittings.
For higher-temperature applications requiring 9Cr-1Mo alloy, see ASTM A234 WP91 in the BW fittings materials guide.
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