STAINLESS STEEL GRADE 347H (UNS S34709) — COMPREHENSIVE TECHNICAL DATASHEET

Stainless Steel Grade 347H (UNS S34709) is a high-carbon, niobium-stabilized austenitic stainless steel optimized for service environments characterized by elevated thermal exposure and highly corrosive process streams. As the high-carbon variant of the standard Grade 347 alloy, Grade 347H is specifically engineered to maintain long-term structural stability, enhanced creep resistance, and exceptional high-temperature tensile durability.

The primary metallurgical advantage of Grade 347H is its resistance to sensitization—the precipitation of chromium carbides ($Cr_{23}C_{6}$) at grain boundaries inside the critical thermal window of 427°C to 816°C (800°F to 1500°F). The stoichiometric addition of niobium preferentially forms fine, dispersed niobium carbides ($NbC$) within the grains, actively trapping carbon atoms and preserving chromium solid-solution levels to ensure continuous passivity along the grain boundaries.

The carbon limits of Grade 347H are strictly controlled between 0.04% and 0.10%. Dual certification with standard Grade 347 is achieved when the carbon content falls precisely between 0.04% and 0.08%, allowing elements to meet both low-temperature corrosion boundaries and premium high-temperature creep thresholds.

* Note: Silicon limit is relaxed to 1.00% maximum for structural tubing and forged configurations under ASTM A182.

For principal piping designers, boiler specialists, and procurement officials requiring comprehensive engineering metrics, finite element strain logs, and code-compliance certifications, the unified datasheet must be accessed.

Due to its highly alloyed FCC matrix, Grade 347H exhibits low thermal conductivity (roughly 30% that of mild structural carbon steel) paired with elevated coefficients of thermal expansion. These parameters dictate transient thermal stresses.

The non-linear tracking of thermophysical properties across an escalating thermal service range is summarized below:

At ambient limits, the solution-treated matrix balances robust yield performance with excellent cold-deforming reserves and fracture resilience.

These values guide stress equations during transient, non-equilibrium pressure excursions across processing plants before creep runtime limits take over.

At continuous structural exposure above 600°C, vacancy diffusion paths dominate. Fine, coherent intra-granular niobium carbonitride ($Nb(C,N)$) precipitates act as pinning centers that lock dislocation glide, significantly lowering the secondary minimum creep rate.

The performance gap in allowable working stress illustrates the higher temperature capacity achieved by the niobium stabilization chemistry of Grade 347H over titanium-stabilized alternatives in the creep-governed design regime.

• Polythionic Acid Stress Corrosion Cracking (PTA-SCC): Outstanding resistance in sulfur-bearing refinery streams. Niobium binding prevents intergranular carbide precipitation during continuous processing, eliminating localized chromium-depleted zones. This allows 347H to completely resist PTA-SCC during moist downtime blocks without mandatory post-weld chemical stabilization.
• Isothermal vs. Cyclic Gas Oxidation: Safe up to 850°C (1562°F) in continuous dry air, and up to 750°C (1382°F) in continuous steam runs. Thermal cycling induces interfacial shear stresses due to the high expansion mismatch ($\alpha \approx 20.5 \times 10^{-6}/\text{K}$ at 1000°C), which can fracture the protective chromia film.
• Halide/Chloride Pitting Restrictions: Lacking molybdenum additions, the alloy's pitting resistance equivalent number is limited ($\text{PREN} \approx 18$). Prone to localized crevice activation and stress corrosion cracking in chloride-bearing fluids at service environments exceeding 60°C (140°F). Avoid reducing mineral acids like active sulfuric lines.

Grade 347H exhibits excellent fusion weldability, but fully austenitic weld zones are highly susceptible to hot cracking (micro-fissuring) due to low impurity solubility along liquid boundaries. Heat inputs must be kept low, and interpass temperatures must remain below 150°C (302°F) to ensure the weld metal solidifies with a protective delta-ferrite content within a 5 to 13 FN (Ferrite Number) range.

• Grain Size Enforcement Boundary: When requesting material for creep-critical application lines, purchase specs must mandate an actual carbon content of 0.04% to 0.10% and require a certified ASTM E112 grain size of No. 7 or coarser. Dual-certified 347/347S items must be audited to verify they meet these criteria; otherwise, grain boundary vacancy sliding will cause premature creep collapse.
• Mitigating SRC via PWHT: For thick, highly constrained joints, mitigate stress relaxation cracking by enforcing a rapid post-weld heat treatment. Weldments must be heated quickly through the critical precipitation window ($500^\circ\text{C} - 800^\circ\text{C}$) to a peak stress-relief range of 900°C to 950°C, allowing bulk residual stress relaxation to occur before coherent intra-granular strain aging locks the matrix. Alternatively, specify ductile AWS E16.8.2 weld consumables to absorb shrinkage strain safely.
• Processing Safety Limits: The work-hardening rate is high ($\approx 14 \text{ MPa per } 1\%$ reduction of area). Severe cold bending must be completed in stages with intermediate annealing steps. Machining parameters must employ continuous positive feeds; tool riding or dwell will cause rapid surface glazing, resulting in premature tool failure.
• Smelting Optimization: For high-pressure utility boiler loops or aerospace exhaust manifolds exposed to extreme pressure and corrosive salts, specify secondary Electro Slag Remelting (ESR) or Protective Atmosphere Electro Slag Remelting (PESR) processing to minimize inclusion density and ensure elemental homogeneity.