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347 and 347H are austenitic stainless steels stabilized with niobium (often reported as Nb + Ta). While very similar, 347H is the high-carbon version specifically designed for enhanced high-temperature strength performance.
Chemical Composition
Typical chemical composition ranges for both grades (approximate standard values):
347 Stainless Steel (UNS S34700)
Element
Composition (%)
Carbon (C)
≤ 0.08%
Manganese (Mn)
≤ 2.00%
Silicon (Si)
≤ 0.75%
Phosphorus (P)
≤ 0.045%
Sulfur (S)
≤ 0.030%
Chromium (Cr)
17.0 - 19.0%
Nickel (Ni)
9.0 - 13.0%
Columbium + Tantalum (Cb + Ta)
10 × C% min, up to about 1.0% typically
Molybdenum (Mo)
Usually not intentionally added (residual typically <0.75%)
Iron (Fe)
Balance
347H Stainless Steel (UNS S34709)
Element
Composition (%)
Carbon (C)
0.04 - 0.10% (higher than 347)
Manganese (Mn)
≤ 2.00%
Silicon (Si)
≤ 0.75%
Phosphorus (P)
≤ 0.045%
Sulfur (S)
≤ 0.030%
Chromium (Cr)
17.0 - 19.0%
Nickel (Ni)
9.0 - 13.0%
Columbium + Tantalum (Cb + Ta)
8 × C% min, typically up to around 1.0%
Molybdenum (Mo)
Generally not a deliberate addition
Iron (Fe)
Balance
Key Technical Characteristics
Relation to 304/321 Types
347/347H grades share similar chromium and nickel content with 304/304H, but are stabilized with niobium (instead of titanium like 321) to prevent chromium carbide precipitation and intergranular corrosion in the sensitization range (approximately 450-850°C).
Significance of "H" Grade
The "H" designation (347H) indicates higher carbon content, which provides:
Enhanced creep strength
Superior high-temperature strength performance
Ideal for boilers, heaters, superheaters, and refinery/chemical plant tubing applications
Maintained stabilization with niobium to prevent sensitization issues despite higher carbon content
Stabilization Mechanism with Niobium (Nb)
Niobium forms niobium carbides rather than allowing chromium carbides to form at grain boundaries. This mechanism:
Preserves chromium in solid solution
Maintains corrosion resistance after welding or elevated temperature service
Protects against intergranular corrosion
Important Note: For exact composition limits required for design, procurement, or certification purposes, always refer directly to the relevant standards (such as ASTM A240, A182, A213, A312, etc.) and the specific product's mill test certificate (MTC). Different product forms and standards may include slightly different ranges or additional controlled elements.
