ASTM A387 Grade 11 Class 2 vs. ASTM A387 Grade 22 Steel
ASTM A387 Grade 11 Class 2 is a 1.25% chromium, 0.5% molybdenum alloy steel plate supplied in the normalized and tempered condition. It offers good high-temperature strength and oxidation resistance up to about 540°C (1000°F), making it suitable for moderately severe service in petroleum refinery vessels, power boilers, and components exposed to hydrogen. Its primary advantage is a favorable balance of performance and cost where full post-weld heat treatment of the final vessel is impractical.
ASTM A387 Grade 22 Class 2 is a 2.25% chromium, 1% molybdenum alloy steel plate also supplied normalized and tempered. With significantly higher chromium and molybdenum content, it delivers superior high-temperature strength, creep resistance, and resistance to hydrogen attack at temperatures exceeding 540°C (1000°F). It is the material of choice for the most severe service in hydrocrackers, high-pressure reactors, and critical power equipment where Grade 11's capabilities are exceeded.
1. Chemical Composition: The Core Difference
| Element | A387 Grade 11 (Class 2) | A387 Grade 22 (Class 2) |
|---|---|---|
| Chromium (Cr) | 1.00 – 1.50% | 2.00 – 2.50% |
| Molybdenum (Mo) | 0.45 – 0.65% | 0.90 – 1.10% |
| Common Name | 1.25Cr-0.5Mo | 2.25Cr-1Mo |
Key Takeaway: Grade 22 has significantly more Chromium and nearly double the Molybdenum. This directly translates to superior high-temperature performance.
2. Properties & Performance Comparison
| Property | A387 Grade 11 (Class 2) | A387 Grade 22 (Class 2) | Implication |
|---|---|---|---|
| High-Temp Strength | Good | Superior | Grade 22 has higher allowable stress values at temperatures above ~450°C (840°F) per ASME codes. |
| Creep Resistance | Good | Excellent | Higher Mo content in Grade 22 provides much better long-term resistance to deformation under stress at high temps. |
| Oxidation Resistance | Good (to ~540°C / 1000°F) | Better (to ~600°C / 1110°F) | Higher Cr forms a more protective scale, allowing use at higher metal temperatures. |
| Hydrogen Attack Resistance | Good for moderate service | Superior | Grade 22 is the standard for severe high-pressure hydrogen service (e.g., hydrocrackers). Its higher Cr/Mo better resists decarburization and methane blistering. |
| Toughness & Ductility | Good | Good (but requires more careful PWHT control) | Both are tough when properly heat-treated. Grade 22 is more susceptible to temper embrittlement if cooled slowly through 375-575°C range. |
| Weldability | Good, with standard preheat/PWHT | More demanding | Grade 22 requires stricter control of preheat (often 200-250°C), interpass temperature, and PWHT due to higher hardenability. |
3. Heat Treatment (Class 2 for both)
Both are supplied in the Normalized and Tempered condition. However, their specific heat treatment parameters differ due to alloy content:
Grade 11: Typical tempering range ~650-720°C (1200-1325°F).
Grade 22: Typical tempering range ~690-750°C (1275-1380°F). The higher alloy content requires higher tempering temperatures to achieve optimal ductility and toughness.
4. Applications: Where Each is Specified
| A387 Grade 11 Class 2 | A387 Grade 22 Class 2 |
|---|---|
| • Moderate-temperature reactors (e.g., catalytic reformers, certain hydrotreaters). • Power plant boilers (steam drums, headers) for subcritical units. • Heat exchangers and vessels in less severe hydrogen service. • Applications where cost-effectiveness is key and service conditions are within its limits. |
• Severe-service reactors (e.g., high-pressure hydrocrackers, hydrodesulfurization units). • High-temperature/high-pressure sections of advanced power boilers and heat recovery systems. • Critical vessels where maximum resistance to hydrogen attack and creep is required (per API 941 "Nelson Curve"). • Petrochemical reactors with aggressive chemistries. |
5. Cost & Fabrication Considerations
Cost: Grade 22 is more expensive due to its higher alloy content.
Fabrication: Grade 22 requires more stringent welding procedures (WPS/PQR), higher preheat, and often longer PWHT times. The risk of cracking is higher if procedures aren't followed meticulously.
1. How does the "Class 2" condition affect the welding procedure for A387 Grade 11?
This question explores the practical fabrication implications of using the pre-tempered Class 2 material, focusing on the specific adjustments needed for preheat, interpass temperature, and Post-Weld Heat Treatment (PWHT) compared to Class 1.
2. What are the main disadvantages of specifying A387 Grade 11 Class 2 over Class 1?
This question investigates the potential trade-offs, such as higher cost, less flexibility for subsequent hot forming, or the risk of property degradation if improper PWHT is applied to the already heat-treated plate.
3. In what specific applications is A387 Grade 11 Class 2 the mandatory or preferred choice?
This question seeks to define the engineering scenarios where the higher as-delivered strength and pre-treated microstructure of Class 2 provide a critical advantage, such as in repair projects or for components where full vessel PWHT is impossible.
4. What controls are necessary during the PWHT of a Class 2 material to prevent over-tempering?
This question addresses a key technical risk: ensuring that the mandatory post-weld stress relief does not lower the yield and tensile strength of the base metal below the minimum specified values for Class 2.
5. How does the impact toughness of Class 2 compare to Class 1 after proper fabrication?
This question compares the final service performance of the two classes, examining whether the different manufacturing routes result in significant differences in fracture resistance at operating temperatures.
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.
