What are the properties and applications of A387 Grade 11 Class 1 Steel?
ASTM A387 Grade 11 Class 2 is the same chromium-molybdenum alloy composition but supplied in the fully normalized and tempered (or quenched and tempered) heat treatment condition. This provides higher as-delivered strength and a more refined microstructure, making it suitable for applications where the final PWHT by the fabricator is impractical or where higher initial mechanical properties are required for the finished component.
Properties of A387 Grade 11 Class 2 Steel:
A387 Grade 11 Class 2 is a 1.25% Chromium-0.5% Molybdenum alloy steel plate supplied in the normalized and tempered (or quenched and tempered) heat treatment condition. Its key properties include:
Higher as-delivered strength compared to Class 1, with a minimum yield strength of 240 MPa (35 ksi) due to the mill tempering process.
Good high-temperature strength, creep resistance, and oxidation resistance (similar to Class 1).
Enhanced microstructural stability from the mill heat treatment, providing a uniform tempered bainitic structure.
Good weldability, but requires strict preheating (~150–200°C / 300–400°F) and mandatory Post-Weld Heat Treatment (PWHT) after fabrication. Crucially, PWHT must be controlled to avoid over-tempering and softening the base metal below specified strength levels.
Reduced fabrication flexibility for hot forming, as subsequent re-normalizing and tempering may be required to restore properties.
Applications of A387 Grade 11 Class 2 Steel:
Class 2 is specified for demanding high-temperature pressure vessel applications where its pre-tempered condition offers distinct advantages:
Petroleum refineries: Critical reactors (e.g., hydrocrackers, hydrotreaters) and vessels where full PWHT of the final assembly is impractical due to size, complex internals, or lack of furnace capacity.
Power generation: High-pressure boiler sections, steam drums, and heat exchangers requiring higher as-received strength.
Chemical/petrochemical plants: Equipment in hydrogen service where the stabilized temper condition is beneficial.
Repair, replacement, or modular components that must match the higher strength of existing Class 2 equipment without undergoing full vessel heat treatment after installation.
ASTM A387 Grade 11 Class 2 Chemical Composition (%)
| Grade / Material | C | Mn | Si | P (max) | S (max) | Cr | Mo |
|---|---|---|---|---|---|---|---|
| A387 Grade 11 Class 2 (A387 Gr11 Cl2) | 0.05–0.17 | 0.40–0.65 | 0.15–0.40 | 0.035 | 0.035 | 1.00–1.50 | 0.45–0.65 |
Key alloy data highlights:
Chromium (Cr): 1.00–1.50% → improves oxidation & corrosion resistance
Molybdenum (Mo): 0.45–0.65% → enhances creep strength & hydrogen resistance
ASTM A387 Grade 11 Class 2 Mechanical Properties
| Grade / Material | Tensile Strength | Yield Strength (min) | Elongation (min) |
|---|---|---|---|
| MPa | MPa | % | |
| A387 Grade 11 Class 2 (A387 Gr11 Cl2) | 515–690 MPa | ≥ 310 MPa | ≥ 18% |
Compared with Class 1 (415–585 MPa tensile / 240 MPa yield),
Class 2 offers significantly higher strength, making it suitable for more demanding pressure and temperature conditions.
Why Choose Class 2 Over Class 1?
Class 2 is selected when the final fabricated vessel cannot undergo a full PWHT or when the design explicitly requires the higher initial strength and microstructural consistency of a pre-tempered plate. It is the "performance-ready" grade, ideal for applications where the final heat treatment by the fabricator is limited to local PWHT or stress relief only. Proper welding and PWHT controls are essential to preserve its optimized properties.
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.
