A387 Grade 11 Class 1 is a chromium-molybdenum alloy steel plate specified under the ASTM A387 standard, widely used in the fabrication of pressure vessels and boiler components that operate at elevated temperatures. It belongs to the family of low-alloy steels containing a moderate amount of chromium and molybdenum, which imparts good strength, creep resistance, and resistance to hydrogen attack in high-temperature service environments. Class 1 indicates a normalized and tempered heat treatment condition, resulting in a refined microstructure that balances toughness and strength for reliable performance under thermal and mechanical stress. This grade is commonly employed in oil refineries, petrochemical plants, and power generation facilities where equipment is exposed to high-pressure and high-temperature fluids.
Tensile Requirements for ASTM A387 Grade 11 Alloy Steel Plates Class 1 Plates
| Designation: | Requirement: | Grade 11 |
| A387 Grade 11 | Tensile strength, ksi [MPa] | 75 to 100 [515 to 690] |
| Yield strength, min, ksi [MPa]/(0.2% offset) | 43 [310] | |
| Elongation in 8 in. [200mm], min % | 18 | |
| Elongation in 2 in. [50mm], min, % | 22 | |
| Reduction of area, min % | ––– |
Chemical Requirements for ASTM A387 Grade 11 Alloy Steel Plates
| Element | Chemical Composition (%) | |
| A387 Grade 11 | ||
| Carbon: | Heat Analysis: | 0.05 - 0.17 |
| Product Analysis: | 0.04 - 0.17 | |
| Manganese: | Heat Analysis: | 0.40 - 0.65 |
| Product Analysis: | 0.35 - 0.73 | |
| Phosphorus: | Heat Analysis: | 0.035 |
| Product Analysis: | 0.035 | |
| Sulphur (max): | Heat Analysis: | 0.035 |
| Product Analysis: | 0.035 | |
| Silicon: | Heat Analysis: | 0.50 - 0.80 |
| Product Analysis: | 0.44 - 0.86 | |
| Chromium: | Heat Analysis: | 1.00 - 1.50 |
| Product Analysis: | 0.94 - 1.56 | |
| Molybdenum: | Heat Analysis: | 0.45 - 0.65 |
| Product Analysis: | 0.45 - 0.70 |
processing
1. Steelmaking and Melting (Killed Steel)
As per ASTM standards, A387 Grade 11 must be produced as killed steel.
Deoxidation: Silicon or aluminum is added to remove oxygen, preventing gas evolution and ensuring a uniform chemical composition throughout the slab.
Impurity Control: Modern mills often use vacuum degassing to lower phosphorus and sulfur levels, which minimizes internal defects and improves weldability.
2. Rolling and Forming
The steel slabs are formed into plates using the Hot Rolling (HR) process.
Heating: Slabs are heated to approximately 1700°F (925°C) to make the metal malleable.
Reduction: The heated slab passes through a series of rollers to achieve the final specified thickness (ranging from 4 mm to 400 mm) and width.
3. Heat Treatment (Mandatory)
Heat treatment is the most critical phase for defining the mechanical properties of Grade 11 Class 1.
Annealing: Heating the plate above the transformation range and cooling slowly in a furnace to produce a soft, uniform structure.
Normalizing & Tempering (N+T): This is the most common condition. The plate is heated to 900–950°C (1650–1740°F) and air-cooled to refine grain size, followed by tempering.
Tempering Temperature: For Grade 11, the minimum tempering temperature must be 1150°F (620°C). This relieves internal stresses and ensures the specific tensile strength required for Class 1 (60–85 ksi).
4. Fabrication and Welding Processes
Due to its high chromium-molybdenum content, this steel is susceptible to hardening and cracking during welding.
Preheating: Prior to welding, the material typically requires preheating to at least 250°F (121°C) to slow the cooling rate of the weld and prevent the formation of brittle martensite.
Post-Weld Heat Treatment (PWHT): Welded structures are usually subjected to PWHT (often at temperatures similar to tempering, around 620°C) to reduce residual stresses and improve the toughness of the heat-affected zone (HAZ).
5. Supplemental Testing
To verify quality for critical service (like sour gas or hydrogen environments), plates may undergo:
Simulated PWHT: Testing a sample that has undergone the same heat cycles the final vessel will experience during manufacturing.
Ultrasonic Testing (UT): Ensuring internal soundness and freedom from lamination.
HIC/NACE Testing: Verifying resistance to Hydrogen-Induced Cracking for refinery use.
applications
Pressure vessels and boilers
Used in the fabrication of pressure vessels, boilers, and related components that operate under high temperatures and pressures, where good creep resistance and strength retention are required.
Oil refinery equipment
Applied in refinery units such as reactors, heat exchangers, and piping systems that handle hot hydrocarbons and process fluids.
Petrochemical and chemical processing
Utilized in reactors, columns, and heat exchangers within petrochemical and chemical plants, particularly in services involving elevated temperatures and hydrogen-containing environments.
Power generation systems
Found in components of power plants, including boilers, steam generators, and associated pressure parts, where resistance to high-temperature oxidation and creep is essential.
Other high-temperature service equipment
Used in various industrial applications requiring steel plates with good elevated-temperature strength and resistance to hydrogen attack, such as in certain process heaters and furnace components.
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What non-destructive testing (NDT) methods are commonly used for A 387 Gr 11 CL 1?
Common NDT methods include ultrasonic testing (UT) for internal defects, magnetic particle testing (MT) or liquid penetrant testing (PT) for surface defects, and radiographic testing (RT) for weld joint inspection.
What is the elongation requirement of A 387 Gr 11 CL 1?
The minimum elongation in 50 mm (2 inches) gauge length is 22%, ensuring good ductility for forming and fabrication.
How does temperature affect the mechanical properties of A 387 Gr 11 CL 1?
At elevated temperatures (up to 593°C/1100°F), it retains excellent tensile strength, creep resistance, and fatigue strength compared to carbon steel. Beyond this temperature, its properties gradually degrade.
What is the difference between A 387 Gr 11 CL 1 and A 387 Gr 12 CL 1?
The main difference is the molybdenum content: Gr 11 CL 1 has 0.45-0.65% Mo, while Gr 12 CL 1 has 0.87-1.13% Mo. Gr 12 CL 1 offers better high-temperature strength and creep resistance but is more expensive.
What are the main applications of A 387 Gr 11 CL 1?
It is widely used in the manufacture of pressure vessels, boilers, heat exchangers, and petrochemical equipment that operate at elevated temperatures, such as refinery reactors, steam generators, and catalytic crackers.
What is the maximum service temperature for A 387 Gr 11 CL 1?
Its maximum continuous service temperature is approximately 593°C (1100°F). Beyond this temperature, its creep and oxidation resistance decline significantly.
What is the minimum yield strength of A 387 Gr 11 CL 1?
The minimum yield strength is 205 MPa (30,000 psi) for plates with thickness up to 50 mm (2 inches); it may slightly decrease for thicker plates.
Can A 387 Gr 11 CL 1 be replaced with A 516 Gr 70?
Only for low-temperature or ambient-temperature applications. A 516 Gr 70 is carbon steel with poor high-temperature properties, so it cannot replace A 387 Gr 11 CL 1 in elevated-temperature service.
Can A 387 Gr 11 CL 1 be welded?
Yes, it is weldable. However, preheating (typically 150-260°C/300-500°F) and post-weld heat treatment (PWHT) are required to prevent cold cracking and reduce residual stresses, ensuring weld joint integrity.
What welding processes are suitable for A 387 Gr 11 CL 1?
Common suitable welding processes include shielded metal arc welding (SMAW), gas tungsten arc welding (GTAW/TIG), gas metal arc welding (GMAW/MIG), and submerged arc welding (SAW).
What is the certification requirement for A 387 Gr 11 CL 1 plates?
Plates must be certified in accordance with ASME SA-387, including material test reports (MTRs) with chemical composition, mechanical properties, and heat treatment records. Third-party inspection (e.g., ABS, DNV) may be required for critical applications.