SA387 Grade 11 Class 2 (often written as SA387 Gr11 Cl2 or 11.2) is a specific grade of Chromium-Molybdenum (Cr-Mo) Alloy steel plate designed primarily for welded pressure vessels and boilers that operate at elevated temperatures.
Here's a breakdown of its key characteristics:
Standard: Defined by the ASTM International standard A387 / A387M - Standard Specification for Pressure Vessel Plates, Alloy Steel, Chromium-Molybdenum.
Grade 11: This indicates the chemical composition range:
Carbon (C): 0.05 - 0.17%
Manganese (Mn): 0.40 - 0.65%
Phosphorus (P): ≤ 0.025%
Sulfur (S): ≤ 0.025%
Silicon (Si): 0.50 - 0.80%
Chromium (Cr): 1.00 - 1.50% (Nominal 1.25%)
Molybdenum (Mo): 0.45 - 0.65% (Nominal 0.55%)
Other Alloys (Cu, Ni, etc.): Typically have low maximum limits.
Class 2: This indicates the required heat treatment condition:
Normalized and Tempered: The plates are heated above the upper transformation temperature (Austenitized) and cooled in air (Normalized) to refine the grain structure. They are then reheated to a specific temperature below the transformation range (Tempered) to achieve the required combination of strength and toughness. This state provides good mechanical properties and microstructure stability for high-temperature service.
Key Mechanical Properties:
Tensile Strength: 415 MPa (60 ksi) min. - Usually around 450-585 MPa (65-85 ksi)
Yield Strength: 205 MPa (30 ksi) min. - Usually around 240-310 MPa (35-45 ksi)
Elongation: 20% min. (in 8in / 200mm)
Impact Toughness: Typically requires Charpy V-notch impact testing at specific temperatures (e.g., -30°C / -20°F) to ensure adequate notch toughness for pressure vessel applications. Minimum absorbed energy values are usually specified in the purchase order.
Why Chromium-Molybdenum?
High-Temperature Strength & Creep Resistance: Mo provides excellent strength and resistance to deformation ("creep") at elevated temperatures (typically up to ~550°C / 1000°F).
Oxidation/Corrosion Resistance: Cr forms a stable oxide layer on the surface, providing significantly better resistance to oxidation (scaling) in steam and sulfur-containing atmospheres compared to carbon steels. It also offers improved resistance to hydrogen attack.
Microstructure Stability: The Cr-Mo combination helps maintain stable mechanical properties during long-term exposure to heat.
Primary Applications:
Boiler Drums and Headers
Pressure Vessels in Petrochemical Refineries
Reactors in Chemical Plants
Heat Exchangers Shells operating at elevated temperatures
Sections requiring good elevated temperature properties and corrosion resistance beyond what carbon steel can offer.
Why Class 2?
The Normalized and Tempered condition (Class 2) is specified for most moderate to heavy thickness pressure vessel sections where the combination of strength and toughness is critical. It offers better microstructural uniformity and toughness compared to Class 1 (Supplied in the as-rolled condition - rarely used for critical vessels now).
In summary: SA387 Gr11 Cl2 is a normalized and tempered chromium-molybdenum alloy steel plate widely used in the construction of welded pressure vessels and boilers designed to operate at high temperatures where standard carbon steel plate would lack sufficient strength, creep resistance, and oxidation/corrosion resistance.
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