What is the difference between A387 Grade 22 Class 2 and A387 Grade 11 Class 2 ?
ASTM A387 Grade 22 Class 2 is a 2.25% Chromium-1% Molybdenum (2¼Cr-1Mo) alloy steel plate supplied in the normalized and tempered condition. It offers superior high-temperature strength, exceptional resistance to hydrogen attack (per the Nelson Curve), and excellent creep resistance above 540°C (1000°F), making it the standard for severe-service applications like hydrocracker reactors and high-pressure power boilers where a full vessel post-weld heat treatment is not feasible.
ASTM A387 Grade 11 Class 2 is a 1.25% Chromium-0.5% Molybdenum (1¼Cr-½Mo) alloy steel plate, also supplied normalized and tempered. It provides good high-temperature strength and moderate hydrogen resistance for less severe services, such as catalytic reformers and medium-pressure vessels, at a lower cost than Grade 22. It is chosen when the operating conditions fall within its capability and the higher initial strength of the pre-tempered condition is required.
The core difference between A387 Grade 22 Class 2 and A387 Grade 11 Class 2 lies in their alloy composition and the resulting high-temperature performance, not their heat treatment condition (both are supplied Normalized and Tempered). They represent two distinct tiers of chromium-molybdenum steels.
| Aspect | A387 Grade 11 Class 2 | A387 Grade 22 Class 2 |
|---|---|---|
| Alloy Composition | 1.25% Chromium, 0.5% Molybdenum (1¼Cr-½Mo) |
2.25% Chromium, 1.0% Molybdenum (2¼Cr-1Mo) |
| Key Performance Driver | Good elevated temperature strength; cost-effective for moderate service. | Superior high-temperature strength & creep resistance; excellent hydrogen attack resistance. |
| High-Temp Strength | Good up to ~540°C (1000°F). | Superior, especially above 540°C (1000°F); higher allowable stress values in ASME codes. |
| Hydrogen Service | Resists hydrogen attack up to moderate temperatures & pressures (Nelson Curve). | Far superior resistance; used for severe high-pressure hydrogen service (e.g., hydrocrackers). |
| Oxidation Resistance | Good to ~540°C. | Better, effective to ~595°C (1100°F) due to higher Cr content. |
| Primary Applications | • Catalytic reformers • Medium-severity hydrotreaters • Power boiler components (sub-critical) |
• Hydrocracker reactors • Severe hydrotreating reactors • High-pressure/high-temperature power boilers |
| Cost & Fabrication | Lower cost (less alloy). Easier to weld (lower preheat ~150-200°C). |
Higher cost. Requires stricter welding controls (higher preheat ~200-250°C). |
Summary: How to Choose
Choose A387 Grade 11 Class 2 for moderate-temperature, moderate-pressure hydrogen service where conditions fall within its capability on the Nelson Curve and where cost-effectiveness is a priority.
Choose A387 Grade 22 Class 2 for severe-service, high-temperature, high-pressure hydrogen environments (especially above 540°C / 1000°F) where maximum resistance to hydrogen attack and creep is critical, such as in hydrocracker reactors.
In essence: While both are "Class 2" (pre-tempered), Grade 22 is a higher-alloy, higher-performance, and more expensive material used when the service severity exceeds the limits of Grade 11. The choice is fundamentally dictated by process conditions, design codes (ASME, API), and the Nelson Chart for hydrogen service.
1. What is the key risk during the Post-Weld Heat Treatment (PWHT) of A387 Grade 22 Class 2 material?
This question focuses on the critical fabrication challenge of applying the mandatory PWHT to an already-tempered material, where exceeding specific time-temperature parameters can lead to over-tempering and a drop in yield strength below the minimum specification.
2. Why would a designer specify Class 2 instead of Class 1 for a Grade 22 reactor?
This question explores the primary engineering rationale, often related to the impracticality of performing a full furnace heat treatment on a massive or complex finished vessel, making the pre-tempered Class 2 plate the necessary choice.
3. How does the Charpy impact toughness of Class 2 compare after welding and local PWHT?
This question addresses a key service performance concern, investigating whether the localized heating from welding and subsequent PWHT can degrade the notch toughness in the heat-affected zone compared to the base metal.
4. What are the main cost and lead-time implications of choosing Grade 22 Class 2 over Class 1?
This question examines the project trade-offs beyond technical specs, as Class 2 plate typically commands a higher price and may have longer procurement lead times due to its additional mill heat treatment step.
Full specification and details are available on request. The above information is provided for guidance purposes only. For specific design requirements please contact our technical sales staff.
