SA 516 Grade 70N (SA-516/SA-516M Gr. 70N) is a normalized carbon steel plate specifically designed for welded pressure vessels requiring excellent low-temperature notch toughness. The "N" suffix explicitly indicates the Normalized heat treatment.
Here are its key properties and characteristics:
Core Specification: ASTM A516 / A516M Standard Specification for Pressure Vessel Plates, Carbon Steel, for Moderate- and Lower-Temperature Service.
Heat Treatment: Normalized. Plates are heated above the upper critical temperature (typically around 1600°F / 870°C) and cooled in still air. This process refines the grain structure, enhances toughness (especially at low temperatures), and improves uniformity of mechanical properties.
Key Application: Primarily used in the shell and head sections of pressure vessels operating at moderate to lower temperatures, particularly where good notch toughness is critical to prevent brittle fracture. Common service includes:
Storage tanks (including refrigerated tanks like LPG/LNG)
Boilers
Heat exchangers
Reactors
Process vessels
Mechanical Properties (Typical Minimum Requirements):
Tensile Strength: 70 ksi (485 MPa) min - 85 ksi (585 MPa) max.
Yield Strength: 38 ksi (260 MPa) min. (Note: The "70" refers to Tensile, not Yield). Actual Yield Strength is typically around 50-60 ksi (345-415 MPa).
Elongation:
21% min (in 8 in. / 200 mm)
23% min (in 2 in. / 50 mm)
Impact Toughness (Charpy V-Notch): Mandatory for Gr. 70N.
Test Temperature: -50°F (-45°C)
Minimum Average Value: 20 ft-lb (27 J) for full-size specimens.
Minimum Single Value: 15 ft-lb (20 J) for full-size specimens.
(Higher values are often achieved and specified; this is the minimum requirement proving suitability for low-temperature service).
Chemical Composition (Typical Limits, Weight %):
Carbon (C): 0.27% max (Lower C generally improves weldability and toughness).
Manganese (Mn): 0.85 - 1.20% (Balances strength and toughness).
Phosphorus (P): 0.025% max. (Low P improves toughness).
Sulfur (S): 0.025% max. (Low S improves ductility and through-thickness properties).
Silicon (Si): 0.15 - 0.40% (Deoxidizer).
Other Elements: Limited amounts of residual elements like Copper (Cu), Nickel (Ni), Chromium (Cr), Molybdenum (Mo), Vanadium (V) are permitted but restricted (e.g., max 0.25% Cu, 0.25% Ni, 0.15% Cr, 0.06% Mo, 0.02% V), primarily to control hardenability and weldability.
Crucially, the composition, especially the lower P and S compared to older non-normalized grades, is optimized for the normalized condition to achieve the required toughness.
Key Differentiators from non-N grades (e.g., Gr. 70):
Mandatory Normalizing: Guarantees a consistent, fine-grained microstructure.
Mandatory Low-Temperature Impact Testing: Proven toughness down to -50°F (-45°C) is a fundamental requirement.
Slightly Stricter Chemistry: Typically lower max limits on elements detrimental to toughness (P, S) and residual elements compared to requirements for the same grade in the as-rolled condition.
Higher Toughness: The combination of composition and normalizing yields significantly superior impact properties at low temperatures compared to the same grade in the as-rolled condition.
Weldability & Fabrication:
Generally considered to have very good weldability due to controlled carbon content and CE IIW (Carbon Equivalent, typically below 0.42%).
Standard welding procedures for mild and C-Mn steels are applicable. Precautions like preheating and controlling interpass temperature may still be needed, especially for thicker sections or restrained joints.
Excellent for cold/hot forming due to good ductility.
Important: Severe forming operations near or below the Charpy test temperature (-50°F/-45°C) can reduce the material's toughness. ASME BPVC Section VIII, Div 1, UCS-79 and related Code Cases often mandate post-forming heat treatment (typically stress relieving or re-normalizing) for significant cold forming to restore toughness properties, especially for critical low-temperature service.
In Summary:
SA 516 Gr. 70N is a normalized carbon steel pressure vessel plate renowned for its excellent guaranteed notch toughness down to -50°F (-45°C), combined with good weldability and formability. The mandatory normalizing treatment and impact testing set it apart from non-normalized grades like Gr. 70, making it the preferred choice for critical pressure vessel applications where low-temperature brittle fracture resistance is paramount.
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