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 American Standard 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.
Leave a Comment
Have a question or feedback? Send us a message.