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Siddhgiri Overseas manufactures SS 347H Tube from our Mumbai facility, an austenitic stainless steel tube grade stabilized with niobium (columbium) specifically to hold up under sustained service between 400°C and 815°C. In this range, standard austenitic grades are prone to sensitization — chromium reacting with carbon to form chromium carbides at the grain boundaries, leaving the material vulnerable to intergranular corrosion at and near weld seams. Niobium bonds with carbon ahead of chromium, forming thermally stable niobium carbides that stay in place through welding rather than burning off, which is why 347H holds its corrosion resistance where it matters most. The deliberately high carbon content (0.04%–0.10%) that gives the grade its "H" designation preserves tensile strength and creep-rupture resistance under continuous high-temperature load. Established in 1962, Siddhgiri Overseas supplies SS 347H tube in seamless and welded construction, tested and certified before dispatch.
SS 347H tube is a niobium-stabilized austenitic stainless steel tube grade engineered for continuous, long-duration exposure to high temperature and pressure — conditions found in superheater bundles, boiler tubing and refinery process tube runs. It belongs to the same family of stabilized grades as SS 321, but uses a different stabilizing element and a different carbon strategy, which is why it exists as its own specification rather than a variant.
The problem SS 347H is built to solve is sensitization: when a standard austenitic stainless is held in the 400°C–815°C (750°F–1500°F) range for extended periods, carbon in the steel reacts with chromium to form chromium carbides at the grain boundaries. This strips chromium away from the areas that maintain the passive protective film, exposing the material to intergranular corrosion — commonly known as weld decay — right at and around weld seams.
Niobium has a stronger chemical affinity for carbon than chromium does, so it reacts with the carbon first, forming fine, evenly dispersed niobium carbides throughout the microstructure instead of at the grain boundaries. Chromium stays where it is needed for corrosion protection. The "H" in the designation signals that carbon is deliberately held high (0.04%–0.10%) rather than minimized — because niobium stabilization already neutralizes the weld-decay risk that high carbon would otherwise create, the extra carbon can instead be put to work improving tensile strength and creep-rupture performance at elevated temperature.
Siddhgiri Overseas produces SS 347H tube in seamless and welded construction, using cold-drawing, precision extrusion and roll-forming processes at our Mumbai facility, with dimensional and mechanical verification carried out on every batch before dispatch.
SS 347H tube is manufactured to a tightly controlled chemistry that balances corrosion resistance, weldability and elevated-temperature strength. Precise, controlled niobium addition — held to a minimum of eight times the carbon content — locks carbon atoms into stable niobium carbides while preserving the chromium distribution needed for corrosion resistance.
|
Carbon (C) |
0.04 – 0.10 (deliberately elevated for strength) |
|
Chromium (Cr) |
17.00 – 19.00 |
|
Nickel (Ni) |
9.00 – 13.00 |
|
Niobium (Nb/Columbium) + Tantalum |
8×C minimum, up to 1.10 max (stabilizer) |
|
Manganese (Mn) |
2.00 max |
The high-carbon "H" strategy carries directly into mechanical performance, giving the grade a real strength advantage over low-carbon stabilized alternatives under sustained heat and load.
|
Tensile Strength |
≥515 MPa (75 ksi) |
|
Yield Strength |
≥205 MPa (30 ksi) |
|
Hardness |
Rockwell B (HRB) ≤95, or Brinell (HBW) ≤217 |
SS 347H tube is manufactured to international standards including ASTM A213 (seamless austenitic alloy-steel tube for boilers, superheaters and heat exchangers), ASTM A249 (welded austenitic tube for the same duty) and their ASME equivalents SA213 and SA249, giving structural safety and interchangeability across projects. Tube is supplied to actual outside diameter and wall thickness — from small-bore instrumentation and capillary sizes up to heavy-wall boiler and superheater tube dimensions — engineered to the pressure and thermal duty of the application, in Plain End, Beveled End or Threaded end configurations and in No. 1 Pickled, 2B Matte, Bright Annealed or Mechanically Polished surface finishes. Solution annealing followed by controlled rapid cooling stabilizes the austenitic grain structure to deliver the high-temperature creep-rupture strength the grade is chosen for. Every batch is verified through hydrostatic, eddy current, ultrasonic and radiographic (100% X-Ray) testing along with positive material identification (PMI), and dispatched with Mill Test Certificates for full traceability.
Siddhgiri Overseas produces SS 347H tube in seamless and welded construction, with the appropriate type selected according to the system's operating pressure, temperature and structural configuration.
Across all types, final sizing is achieved through high-precision cold-drawing and cold-pilgering passes that tighten geometric tolerances, wall-thickness uniformity and inner-surface finish. Because niobium stabilization already protects against weld-decay in service, SS 347H tube does not require solution annealing after welding to retain its corrosion resistance — though this remains a standard step in our production process to optimize grain structure and mechanical properties. The grade is readily weldable using Gas Tungsten Arc Welding (GTAW/TIG), Gas Metal Arc Welding (GMAW/MIG) and Shielded Metal Arc Welding (SMAW). It is ductile enough for cold bending but work-hardens at a comparatively high rate, so forming is carried out on stable, high-capacity hydraulic equipment. Pickling and passivation after fabrication remove high-temperature scale and restore the self-healing passive oxide layer that gives the tube its long-term corrosion performance.
SS 347H tube is specified in thermal systems that demand sustained creep strength, weld-seam integrity and microstructural stability across the 400°C–815°C service range.
Siddhgiri Overseas supplies SS 347H tube in round, as well as square and rectangular sections for structural frames and support columns in corrosive processing environments, and heavy-wall tubing for chemical injection lines, high-pressure steam headers and radiant power-plant coils — tested, certified and packed for both domestic delivery and export from our Mumbai facility.
Siddhgiri Overseas is ISO certified for quality, occupational health & safety and environment management, and holds the import and export certification required to supply stainless steel pipe and tube to customers worldwide. Certificate copies are available on request for buyers who need them for their own compliance records.
SS 347H is stabilized with niobium and carries a deliberately high carbon content (0.04%–0.10%), giving it resistance to sensitization and weld decay at sustained high temperatures (400°C–815°C) that standard 304 or 316 tube does not have. This makes it the correct choice for boiler tube, superheater tubing and refinery process tube rather than general corrosion-resistant service.
Both are stabilized austenitic grades that resist weld decay, but they use different stabilizing elements. SS 321 uses titanium, while SS 347H uses niobium, which forms more thermally stable carbides and does not volatilize during welding — giving 347H more reliable corrosion resistance right at the weld seam, especially toward the upper end of its temperature range.
The "H" indicates that carbon content is deliberately kept high, at 0.04%–0.10%, rather than minimized. Because niobium stabilization already neutralizes the weld-decay risk that high carbon would normally cause, the extra carbon instead improves tensile strength, creep-rupture strength and resistance to deformation under continuous high heat and pressure.
We manufacture SS 347H tube in Seamless (SMLS) construction for high-pressure boiler and superheater duty, Electric Resistance Welded (ERW) tube for general high-temperature process lines, and Cold Drawn Welded (CDW) tube, which is redrawn over a mandrel to refine the weld-seam grain structure.
SS 347H contains no molybdenum, so it does not offer the pitting and crevice corrosion resistance needed for room-temperature marine or chloride-heavy environments. SS 316 or 316L, which are molybdenum-bearing, are better suited to that duty — SS 347H's strengths are specifically in sustained high-temperature service.
Knife-line attack is a rare, localized form of corrosion that can occur in a narrow band immediately next to a weld under certain repeated-reheat welding conditions. It is manageable with proper welding temperature control and is not a reason to avoid the grade, but it is worth accounting for in welding procedure specifications for niobium-stabilized tube.
Every batch undergoes hydrostatic pressure testing, eddy current and ultrasonic testing, radiography (100% X-Ray where specified), positive material identification (PMI) and dimensional inspection, with Mill Test Certificates issued to document chemical and mechanical conformance before the tube leaves our Mumbai facility.
Our SS 347H tube is manufactured to ASTM A213 (seamless) and ASTM A249 (welded) for boiler, superheater and heat exchanger service, along with their ASME equivalents SA213 and SA249, ensuring dimensional and mechanical interchangeability across international projects.
