ASTM A376 TP347H Stainless Steel Seamless Pipe Pickled Annealed for High Temperature

ASTM A376 TP347H (UNS S34709, W.-Nr. 1.4961) is a niobium-stabilised austenitic stainless steel seamless pipe produced to ASTM A376 / ASME SA376 for high-temperature central-station service, and supplied in the pickled and solution-annealed condition. It is the standard material for main steam piping, superheater and reheater tubes, boiler headers and high-pressure feed lines in fossil-fuel power plants, where continuous metal temperatures of 540–650 °C (1000–1200 °F) are combined with sustained internal pressure.

The grade is specified because it solves two failure modes that cost operators real money. First, the addition of niobium (Nb + Ta = 8×C to 1.00 %) ties up carbon as niobium carbides, so chromium remains in solid solution and the pipe keeps its corrosion resistance inside the 427–816 °C sensitisation band — the exact reason standard 304H piping suffers intergranular attack on weld heat-affected zones. Second, its higher carbon range (0.04–0.10 %) delivers creep and stress-rupture strength well above standard TP347, which is what keeps wall thickness — and total project weight — within design limits at 600 °C and above.

What separates this supply from commodity A312 pipe is the process discipline written into A376: solution annealing at a minimum of 1065 °C (1950 °F) with rapid cooling, and a mandatory 100 % hydrostatic or non-destructive electric test on every single length. Every shipment leaves our mill with a heat-number-traced EN 10204 3.1 mill test certificate, third-party inspection (SGS, BV, TÜV, Intertek or class society) welcomed at the mill, and dimensional and PMI verification recorded against your purchase order.

The grade is normally selected when the design metal temperature exceeds the practical ceiling of TP316H/TP321H, when cyclic thermal loading accelerates sensitisation, or when steam-side oxidation scaling has to be controlled over a 100,000-hour design life. Typical design envelopes are 540–650 °C continuous metal temperature, internal pressure up to ASME B31.1 / EN 13480 schedules, and steam, flue gas, hydrogen-rich or hydrocarbon process media.

• Superheater, reheater and economiser tube bundles in subcritical and supercritical boilers.
• Main steam and hot reheat piping, boiler headers, manifolds and attemperator lines.
• Heat exchanger tube bundles in refinery and petrochemical service, including hydroprocessing, steam methane reforming and hydrogen units.
• High-temperature chemical and petrochemical process piping where intergranular corrosion resistance after welding is mandatory.
• Power generation auxiliary systems, nuclear support circuits, aerospace ground support equipment, shipbuilding exhaust and boiler systems.
• Industrial furnace radiant and convection sections, and high-temperature flue ducting.

TP347H is a stabilised grade and must not be substituted by unstabilised 304H or by low-carbon 304L where sensitisation resistance is required. For ASME Section I and Section VIII Div. 1 construction, the allowable stress values of ASTM A376 TP347H apply up to 815 °C; above that temperature the material is no longer covered by the specification and an engineering evaluation is required. Cold bending and welding are permitted, but post-weld heat treatment is generally not applied to stabilised austenitic grades — solution annealing is the correct route if the specification demands it.

1. State the design metal temperature, design pressure and process medium — these three values decide whether TP347H, TP347HFG or a nickel alloy is technically justified.
2. Fix the governing standard: ASTM A376 for high-temperature central-station seamless pipe, ASTM A312 or A213 where lower-temperature corrosion resistance is the driver.
3. Give the size combination — OD, wall schedule, length pattern and end finish — and confirm whether pickled or pickled-and-polished surface is required.
4. Define the inspection level: MTC EN 10204 3.1, PMI on 100 % of lengths, hydrostatic or eddy-current test, ASTM A262 Practice E intergranular corrosion test, UT or third-party witness.
5. Send the data sheet and required quantity for a firm FOB quotation with confirmed lead time and delivery schedule.

Quality control runs from incoming raw material to final marking. The inspection and test plan below is the default for A376 TP347H orders and can be extended to any third-party witness level requested in the purchase order.

• Incoming billet and round bar verification by optical emission spectrometry, with heat number recorded against the production route.
• Chemical analysis of each heat, reported on the mill test certificate with the actual values, not only the specification limits.
• 100 % hydrostatic pressure test to A376 test pressure, or non-destructive electric (eddy current) test where the specification calls for it.
• Mechanical testing per heat: tensile, yield, elongation, plus flattening or flaring test on tube sizes.
• Metallographic examination of grain size and niobium carbide distribution; ASTM A262 Practice E intergranular corrosion test available on request.
• Dimensional, straightness, wall-thickness and surface inspection — no residual scale, no laps, no cracks after pickling.
• Positive material identification (PMI) on 100 % of finished lengths to confirm the ordered grade before packing.
• Third-party inspection by SGS, BV, TÜV, Intertek or class society surveyors is welcome at our works; we book the inspector on your behalf.

Documentation per shipment: EN 10204 3.1 mill test certificate, packing list, heat-number traceability sheet, non-destructive test report, dimensional report and, where applicable, PED declaration of conformity and third-party inspection release note.

5. Frequently Asked Questions

What is the MOQ for ASTM A376 TP347H seamless pipe?

500 kgs per size for stock schedules and 1 ton for non-standard wall thickness or special cut lengths. Mixed sizes can be combined into one order to reach an economical FOB lot.

How is TP347H different from TP321H and TP347?

TP347H and TP321H are both stabilised, but 347H uses niobium and a higher carbon range, which gives better creep and stress-rupture strength above 540 °C. Standard TP347 has a lower carbon range (0.08 % max) and therefore lower elevated-temperature strength than the H variant.

What is the maximum service temperature?

ASTM A376 covers TP347H for high-temperature service up to 815 °C under ASME allowables; in practice continuous metal temperatures of 540–650 °C are the normal design window for superheater and main steam duty, with excursions to 750 °C tolerated for short periods.

Can you supply the pipe pickled and polished, and with beveled ends?

Yes. Pickled, pickled-and-polished and bright-annealed finishes are available, with plain ends, 30° beveled ends or threaded ends, and protective end caps fitted before packing.

Which documents come with the shipment?

EN 10204 3.1 mill test certificate with actual chemical and mechanical values, packing list, heat-number traceability sheet, non-destructive test report and dimensional report. 

Do you accept third-party inspection?

Yes — SGS, BV, TÜV, Intertek and class society surveyors are welcome at our mill. We arrange the inspection booking and provide a witness report with the shipping documents at no extra charge.