ASTM A387 Gr.11 Class2 High Strength Chrome Moly Plate

ASTM A387 Grade 11 Class 2 (ASME SA-387 Gr. 11 Cl. 2) is a high-strength 1.25% Cr – 0.5% Mo chromium-molybdenum alloy steel plate for pressure vessels. Supplied in a high-strength condition-with a tensile strength of 515–690 MPa and a minimum yield strength of ≥310 MPa-it exceeds Class 1 (tensile 415–585 MPa / yield ≥240 MPa) by approximately 100 MPa. This higher yield capability allows designers to specify thinner plate thicknesses under identical design pressures, significantly reducing overall equipment deadweight and welding volume.

This grade is dual-certified under both ASTM A387/A387M and ASME SA-387/SA-387M (ASME BPVC Section II, Part A), with general delivery requirements complying with ASTM A20/A20M and designated under UNS K11789.

Key Alloy Elements: Why Chromium-Molybdenum Steel?

Chromium (Cr: 1.00–1.50%): Enhances elevated-temperature strength and oxidation resistance while significantly improving resistance to hydrogen sulfide (H₂S) and sulfur-bearing media. This makes Cr-Mo steel the premier choice for sour service applications in refining and petrochemical plants. Chromium also increases hardenability, ensuring uniform mechanical properties through-thickness following quenching and tempering.

Molybdenum (Mo: 0.45–0.65%): Serves as the primary creep-resistant element in the Cr-Mo system. Molybdenum provides solid-solution strengthening and stabilizes carbides, effectively suppressing dislocation movement and grain boundary creep at elevated temperatures. It is critical for maintaining dimensional stability and strength retention during long-term service between 350°C and 550°C.

What is the continuous high-temperature service range for ASTM A387 Gr.11 Cl.2?

The typical continuous operating temperature range is 350°C to 550°C (650°F to 1020°F), positioning it as a mid-temperature Cr-Mo steel. While ASME allowable stress tables list SA-387 Gr. 11 up to approximately 590°C (1100°F), allowable stress values drop sharply above 550°C. Increasing plate thickness to compensate becomes economically unviable, making its engineering use rare above this threshold.

What happens if the design/operating temperature is exceeded?

• Precipitous Drop in Allowable Stress: Required code-calculated wall thickness increases exponentially, losing cost efficiency and potentially exceeding design code limits.
• Accelerated Creep Rate: A temperature rise of just 10–15°C can double the creep rate, drastically reducing equipment design life (typically calculated at 100,000 hours) due to irreversible cumulative creep damage.
• Severe High-Temperature Oxidation: Heavy scaling and spalling reduce the effective wall thickness while risking downstream contamination and piping blockages.
• Microstructural Degradation: Prolonged overheating causes carbide coarsening and matrix degradation, leading to a permanent drop in both ambient and elevated-temperature tensile strength that cannot be reversed by returning to normal temperatures.
• Aggravated Environmental Degradation: In environments containing hydrogen or sulfides, over-temperature conditions significantly increase the risk of High-Temperature Hydrogen Attack (HTHA) and sulfidation corrosion, leading to premature cracking.

Engineering Conclusion: Operating above design temperature presents severe safety hazards. Equipment must strictly operate within designated temperature limits. If operating conditions continuously exceed this range, material specifications should be upgraded to Grade 22 (2.25Cr-1Mo) or Grade 91.

ASTM A387 Gr.11 Class2 High Strength Steel Plate Typical Applications & Equipment

• Boiler Equipment: Steam drums, headers, and elevated-temperature pressure shells for power plant and industrial boilers.
• Refining Units: Mid-temperature columns, towers, and vessel shells for Atmospheric/Vacuum Distillation Units (CDU/VDU), Fluid Catalytic Cracking (FCC), Delayed Coking, and high-temperature unit piping.
• Heat Exchanger Equipment: Shells, tube sheets, and heavy-duty pressure flanges for elevated-temperature shell-and-tube exchangers.
• Power Generation Support: Steam drums, headers, feedwater heater shells, and tube sheets.

Why Choose GNEE Steel?

Certification & Quality Assurance: EN 10204 3.1 / 3.2 Material Test Certificates (MTC) with full heat traceability, including verified chemical composition and mechanical test results.

Value-Added Processing: One-stop processing including custom cutting, beveling, drilling, cut-to-length, leveling, shot blasting, and shop priming per fabrication drawings to reduce client secondary processing.

Technical Consultation: Engineering selection reviews, temperature boundary analysis, equivalent grade cross-referencing, and welding/PWHT recommendations during the inquiry stage.

FAQ

What is the main difference between A387 Grade 11 and Grade 12?

The main difference between ASTM A387 Grade 11 and Grade 12 is their chemical composition of chromium. Grade 11 is a 1.25% chromium and 0.5% molybdenum alloy steel (1.25Cr-0.5Mo), whereas Grade 12 contains slightly less chromium at 1.0% alongside 0.5% molybdenum (1.0Cr-0.5Mo).

What is the ASTM A387 material?

ASTM A387 is a chromium-molybdenum (chrome-moly) alloy steel plate specification. It is engineered for weldable industrial boilers and pressure vessels designed for high-temperature service. The added chromium and molybdenum provide high tensile strength, creep resistance, and strong protection against corrosion and oxidation.

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