ASTM A204 Gr B is a specific grade of carbon-molybdenum (C-Mo) Alloy steel plate designed primarily for moderate and elevated temperature service in pressure vessels and boilers. Here's a breakdown of its key characteristics:
Material Type: Alloy Steel Plate (specifically Carbon-Molybdenum type).
Key Alloying Element: Molybdenum (Mo). The defining characteristic.
Molybdenum content is 0.45% - 0.60% for Grade B. (Grade A has 0.13-0.23%, Grade C has 0.90-1.10%).
Purpose: Designed to offer better elevated temperature strength and creep resistance compared to plain carbon steels like A516. Molybdenum helps maintain strength at higher operating temperatures.
Heat Treatment: Plates are supplied in the normalized condition. This heat treatment refines the grain structure, ensuring uniform mechanical properties and improving toughness.
Mechanical Properties (Typical Minimum Values):
Tensile Strength: 60,000 - 85,000 psi (415 - 585 MPa)
Yield Strength: 35,000 psi min (240 MPa min)
Elongation: 20% min in 8 inches (200 mm)
Toughness Requirement: Impact testing (Charpy V-notch) is required to determine suitability for low-temperature service.
Impact Test Temperature: -50°F (-46°C)
Minimum Average Impact Value: Typically 15 ft-lbf (20 J) at the specified temperature. (The required value depends on the design thickness; thicker plates require higher energy absorption).
Grades within ASTM A204:
Grade A: Lower tensile strength (50-75 ksi / 345-515 MPa), lower Mo.
Grade B: Medium tensile strength (60-85 ksi / 415-585 MPa), medium Mo.
Grade C: Higher tensile strength (70-95 ksi / 485-655 MPa), higher Mo.
Primary Applications:
Boiler Drums and Shells
Pressure Vessels operating at elevated temperatures.
Heat Exchangers and similar components subjected to thermal stresses.
Storage Tanks requiring elevated temperature strength.
Key Standards & Codes:
ASTM A204/A204M: Standard Specification for Pressure Vessel Plates, Alloy Steel, Molybdenum.
ASME Boiler and Pressure Vessel Code (BPVC), Section II: Equivalent material designation is SA-204 Gr B. This is the code-approved version used in pressure vessel construction.
In simple terms: ASTM A204 Gr B is a sturdy steel plate specifically strengthened with molybdenum to handle high pressures and temperatures better than basic carbon steel. It's a workhorse material for critical components in boilers, reactors, and vessels used in power generation, chemical processing, and oil & gas industries, especially when the service temperature makes plain carbon steel too weak.
Key Points to Remember:
Molybdenum Alloy: That's the defining feature (0.45-0.60% Mo).
Elevated Temperature Strength: Better than carbon steel like A516 at temps above ~750°F (400°C).
Pressure Vessel Plate: Designed for welded pressure-containing components.
Normalized: Mandatory heat treatment condition.
Toughness Tested: Must meet minimum impact toughness at -50°F (-46°C).
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