Buyer's Guide · 2026

Pressure Tubes for High-Temperature Service: TP304H, TP316H, TP321H, TP347H, HR3C & Super304H Buyer's Guide for USC, PWR, CSP and SMR

A mill-direct procurement playbook for superheater, reheater, steam generator, primary loop and once-through steam generator tubes that must survive 560 °C to 760 °C, 100,000-hour creep life and a 60-year in-service life — with a hard look at the σ-phase, M₂₃C₆ and 475 °C embrittlement risks that quietly shorten austenitic tube life.

1 · What this guide actually covers

Most pressure tubes sold for high-temperature service look identical on a mill test certificate: a 50 mm OD seamless tube, 8 mm wall, solution-annealed, with a heat number. The real differentiation lives in the chemical envelope — especially C, N, B, Ti, Nb, Cu and the Cu/(Ni+Cr) ratio — and in whether the tube can hold 100,000-hour creep rupture strength above 600 °C without long-term sigma-phase (σ) precipitation or M₂₃C₆ sensitization eating the grain boundaries.

This guide is written for:

Power-plant boiler design engineers specifying superheater / reheater material for 600 °C USC and 700 °C AUSC units.
PWR steam-generator (SG) U-tube replacement project buyers comparing TP304, TP316L, TP321, TP347 and Inconel 690 / Inconel 800H alternatives.
CSP (concentrated solar power) receiver tube engineers looking at 565 °C molten-salt service for 25-30 year life.
SMR (small modular reactor) once-through steam generator procurement leads sourcing TP304H, TP316H, HR3C, Super304H for 560-650 °C primary loop.
Refinery ethylene-cracker and steam-methane reformer (SMR) convection-bank tube buyers comparing HP-40, HP-Mod, 25/35 Cr-Ni and 800H/800HT for 800-1100 °C.

We deliberately do not cover waterwall panels, economizer inlet tubes, condensate return lines, or firewater sprinkler risers — those are carbon steel pipe territory covered in a separate guide. And we don't cover finned tube or U-bend tube geometry — those are heat efficiency tubes territory.

2 · The eight high-temperature tube families you actually need to compare

Forget the textbook "18Cr-8Ni" label. For procurement, the eight families that matter at 560-760 °C are:

Standard TP304H / TP316H (18Cr-8Ni / 16Cr-10Ni-2Mo) — the workhorse USC grade.
Stabilized TP321H (18Cr-10Ni-Ti) and TP347H (18Cr-11Ni-Nb) — the weldability- and sensitization-resistant pair.
Super304H (18Cr-9Ni-3Cu-Nb-N) — the 600-625 °C creep booster.
HR3C (25Cr-20Ni-Nb-N) — the 625-700 °C heavy hitter with a 25 % Cr oxidation barrier.
TP310H / TP310HCbN (25Cr-20Ni) — the high-temperature oxidation & sulfidation grade for AUSC and SMR secondary.
Inconel 800H / 800HT (20Cr-30Ni-Ti-Al) — petrochemical reformer, SMR primary, HTGR secondary.
Inconel 690 / Inconel 625 / Alloy 617 — nuclear PWR steam generator and 700 °C+ AUSC.
Modified 9Cr-1Mo (T91/P91) and T92/P92 — ferritic-martensitic boundary, only where differential thermal expansion rules out full austenitic.

The choice between these is not just creep strength — it is also steam-side oxidation rate (which scales roughly with the parabolic rate constant k_p at operating temperature), fireside corrosion allowance in coal/biomass boilers, σ-phase nose (the time-temperature-precipitation window where Cr-rich σ nucleates), and welding repairability after 100,000 h of service.

3 · Why your existing 304H tube failed in year 12, not year 30

A Guangdong Power Grid study on TP304H boiler tubes after long-term high-temperature service in a 600 MW USC unit showed that the dominant failure mode was not creep rupture — it was M₂₃C₆ carbide precipitation on grain boundaries plus σ-phase precipitation in the matrix. Tensile strength and creep rupture life ｄｒｏｐ together as the chromium-depleted zone around the carbides grows.

Three operational factors accelerate this:

Operating temperature vs design temperature. A superheater designed for 580 °C metal temperature but actually running 605 °C because of slagging & poor heat transfer accelerates σ formation by a factor of 2-3×. Every 10 °C above design roughly halves the time to detectable σ precipitation.

Stress state. Tubes in the final superheater pendant section carry not only internal pressure hoop stress but also dead-weight bending + thermal-gradient stress. Sustained peak stress above 35 MPa at 600 °C pushes the tube into the tertiary creep regime.

Thermal cycling. Daily start-stop cycles (cold start to full load in <4 h) grow oxide spallation on the steam-side, exposing fresh metal to oxidation. The 100,000 h isothermal creep curve in ASME Section II Part D is <conservative> vs cyclic-loaded service.

So the procurement question is not "can this tube reach 600 °C?" — most can. The question is "which tube holds 600 °C with a 6000-8000 start-stop cycle budget for 30 years?" That is where HR3C and Super304H outperform 304H, and where TP347HFG (fine-grain variant) outperforms standard 347H.

4 · Standards cross-reference: ASTM A213 / EN 10216-5 / GB/T 13296 / JIS G3463 / GOST 9941

For procurement, the four major seamless austenitic boiler-tube standards that you will see in any tender are ASTM A213, EN 10216-5, GB/T 13296 and JIS G3463. For Russian/CIS projects add GOST 9941. For nuclear PWR steam generator tubes, ASME BPVC Section II Part B (SB-163, SB-167, SB-407) plus RCC-M Section II (M3301 / M3304 / M3401) are non-negotiable.

Designation	ASTM / ASME	EN	JIS	GB	Max metal temp °C (typical)
TP304H / 18Cr-9Ni-C	A213 TP304H / SA-213	EN 10216-5 1.4948	JIS G3463 SUS304H TB	GB/T 13296 0Cr18Ni9	~600 (sensitization risk 500-700 °C)
TP316H / 16Cr-12Ni-2Mo	A213 TP316H / SA-213	EN 10216-5 1.4918	JIS G3463 SUS316H TB	GB/T 13296 0Cr17Ni12Mo2	~600 (better than 304H in coal-ash corrosion)
TP321H / 18Cr-10Ni-Ti	A213 TP321H / SA-213	EN 10216-5 1.4541	JIS G3463 SUS321H TB	GB/T 13296 0Cr18Ni10Ti	~620 (Ti-stabilized, no IGC after weld)
TP347H / 18Cr-11Ni-Nb	A213 TP347H / SA-213	EN 10216-5 1.4961	JIS G3463 SUS347H TB	GB/T 13296 0Cr18Ni11Nb	~620-650 (Nb-stabilized, post-weld no knife-line attack)
Super304H	A213 S30432 / ASME CC2328	EN 10216-5 1.4904 (T/P)	—	—	600-625 (3 % Cu + Nb-N creep boost, σ risk below 304H)
HR3C	A213 TP310HCbN / ASME CC2329	EN 10216-5 1.4951	—	—	625-700 (25 Cr oxidation barrier, dense Cr₂O₃ scale)
Inconel 800H	B407 / SB-407	EN 10216-5 1.4876	—	GB/T 15062 0Cr20Ni32AlTi	~700-760 (SMR, reformer, HTGR)
Inconel 690 (PWR SG)	B167 / SB-167 (TT-treatment)	EN 10216-5 2.4642	—	—	~350 (PWR SG tubing, primary stress-corrosion cracking)

EN 10216-5 vs A213 procurement note. EN 10216-5 mandates 100 % eddy-current testing (ECT) per EN 12732 / EN 10246-3 and a 1.5× design-pressure hydrostatic test for 10-30 s on every tube. ASTM A213 by default allows either ECT or hydrostatic; the buyer must specify which one. For 600 °C+ USC service, always specify both and reference RCC-M / ASME Section III for nuclear or Section I for utility boiler.

5 · The five procurement traps we keep seeing in 2025-2026

5.1 Specifying TP304 when the design temperature is 600 °C+

Some buyers still write "TP304 / 1.4301" because it is the cheapest. But standard TP304 (with C ≤ 0.08 %) is a low-carbon water-service grade, not a high-temperature creep grade. At 600 °C, the 100,000 h stress-rupture strength is roughly 50 % of TP304H (with C 0.04-0.10 %). Always specify the H suffix for any tube that operates above 525 °C continuously.

5.2 Reading "1.4948" as equivalent to "TP304H" without the full chemistry check

EN 10216-5 1.4948 has a tighter N specification and a different Ti/Nb balance than ASTM A213 TP304H. The "equivalent" label is approximate. For nuclear, ASME BPVC Section III requires the chemistry to be reported as heat-by-heat and the equivalence assessed by a Q-List reviewer, not by the trader.

5.3 Ordering TP347H without specifying the HFG (fine-grain) variant for USC final-stage superheater

TP347HFG (fine-grain, ASTM grain size 7 or finer) has 20-30 % better 100,000 h creep rupture strength than standard TP347H at 625 °C. If the spec sheet just says "TP347H", the mill will deliver coarse-grain. For final-stage superheater / reheater always write "TP347HFG" or "ASME CC2327" and add "ASTM grain size ≥ 7 per E112".

5.4 Missing the σ-phase limit when re-ordering against a previous 10-year-old spec

A spec from 2014 may not have referenced the ASME Section II Part D 2015 revision on long-term σ-phase data for HR3C and Super304H. If you are ordering for a 30-year life, the procurement specification must include the latest long-term creep / aging data tables — not the 100,000 h point, but the 200,000 h extrapolation envelope.

5.5 Forgetting that MTC 3.1 ≠ MTC 3.2 for nuclear

EN 10204 3.1 cert is signed by the mill's quality representative, independently of production. 3.2 cert is countersigned by an independent third-party inspector (SGS, Bureau Veritas, TÜV, DNV, etc.). For nuclear-class 1 / 2 / 3 pressure-retaining components, ASME BPVC Section III and RCC-M Section II both require 3.2. For utility boiler Section I, 3.1 is the default. Mixing the two up-front means re-issuing MTCs and a 4-6 week delay in delivery.

6 · MTC and NDT matrix for high-temperature pressure tubes

A high-temperature tube that is going to live at 600 °C for 30 years needs at least these eight items in its MTC and NDT dossier:

Eight must-have items for each heat / lot

Heat-number traceable chemistry (C, Mn, Si, P, S, Cr, Ni, Mo, N, Ti, Nb, Cu, B, Co) — per ASTM A213 / A262 Practice A or EN 10216-5 6.1.
Solution-anneal condition (1040-1100 °C for 304H/316H, 1100-1180 °C for HR3C, 1150-1200 °C for 800H) — record soak time and quench medium.
Room-temperature tensile (Rm, Rp0.2, A%, HB / HRB) — per ASTM A370 / EN ISO 6892-1.
Elevated-temperature tensile at design temperature (e.g. 600 °C, 650 °C) — per ASTM E21 / EN ISO 6892-2 for critical AUSC and SMR service.
Hardness per ASTM E18 (HRB for austenitic, HRC for Inconel) — to confirm no sensitization (HRB ≤ 90 for 304/316 H-grade, HRC ≤ 22 for 690 TT).
Intergranular corrosion test (ASTM A262 Practice A or E) — for any tube that will see a weld repair in service.
100 % eddy-current test (ECT) per ASTM E213 / EN 10246-3 — record calibration notch depth (typically 5 % of nominal wall for austenitic, 3 % for ferritic).
Hydrostatic test at 1.5× design pressure for 10-30 s (or as specified) — per ASTM A1015 / EN 10216-5 8.4.

Add to that, for the first article / qualification lot only:

ASTM E112 grain size (number ≥ 6 for HFG variants, ≥ 3 for standard austenitic).
ASTM E562 ferrite-scope or ASTM E1245 (for 304H / 316H weld-overlay compatibility check).
Creep-rupture test at design temperature (one of three specimen lots, 1,000 h minimum, extrapolated to 100,000 h per ISO 204 / ASTM E139).
σ-phase susceptibility test (ASTM G48 modified or ASTM A923 Method A/B for duplex, but for HR3C the more relevant test is the 700 °C × 10,000 h aging + room-temp impact per ASME CC2329).

7 · Mill-direct vs stockist: the procurement-economics reality

For standard austenitic grades (TP304H, TP316H, TP321H, TP347H) with a 6-12 month lead-time allowance, the mill-direct economics stack up roughly like this:

Cost item	Mill-direct (sourced from a mill with full MTC, 3.1 / 3.2)	Stockist / trader
Unit price (TP304H, 50 OD × 8 mm)	Mill list price 100 %	Stockist list 120-150 %
MOQ (mill heat)	5-20 t per size, 20-50 t per grade	1 t or single tube
MTC fee (3.1 / 3.2)	0.3-0.6 % of tube value (incl. in mill quote)	1.0-1.5 % additional if re-issued
Lead time (in-stock)	4-6 weeks from China mill	1-2 weeks ex-warehouse
Lead time (custom heat / size)	8-12 weeks standard austenitic; 10-14 weeks HR3C / Super304H	14-20 weeks (mill re-route through stockist)
Logistics saving on full package (5-8 grades × 20-50 t)	8-15 % of total package via bundled mill schedule	0-5 % (no control over production lot)
Risk: wrong chemistry or missing heat-number traceability	Low (mill-controlled, single-mill single-MTC)	High (mix of mill batches, occasional sub-standard material)

For HR3C, Super304H, Inconel 800H/800HT, Inconel 690, the lead time math flips: most stockists don't carry these in inventory, and the mill-direct 10-16 week wait is unavoidable. The right move is to commit to a forward order 6-9 months before erection starts, accept the mill-direct MOQ of 5-15 t, and combine HR3C + 800H + 690 in a single RFQ to amortize the mill's campaign-change cost.

8 · Selecting by application: which tube for which service

8.1 USC 600 °C superheater / reheater (coal-fired utility)

Spec: TP347HFG or Super304H, OD 38-51 mm, wall 6-10 mm, solution-annealed, 100 % ECT + 1.5× hydrostatic, MTC 3.1, ASTM grain size ≥ 7. Avoid 304H in final-stage superheater; 304HFG can be used in primary superheater where metal temperature stays under 580 °C. For AUSC 700 °C pilot units, use HR3C for the highest-temperature section and Super304H for the intermediate.

8.2 PWR steam-generator U-tube (nuclear)

Spec: Inconel 690 TT (thermally-treated) per ASME SB-167 or RCC-M M3301/M3304, OD 15.9-22.2 mm, wall 1.0-1.5 mm, Mill-Anneal (MA) or Stress-Relief-Anneal (SRA) condition depending on whether the unit is Westinghouse (MA) or Framatome / KEPCO (SRA). 100 % ECT + helium leak test, MTC 3.2 with independent third-party inspection (BIS, SGS, BV, TÜV, or French nuclear-authorized body). For older PWR unit retubing, sometimes TP304 / TP316 NG (Nuclear Grade) is still specified for compatibility with the original SG design.

8.3 CSP molten-salt receiver (concentrated solar power)

Spec: Inconel 800H or 800HT per SB-407, OD 30-60 mm, wall 2-4 mm, solution-annealed at 1150-1200 °C, MTC 3.1, ECT + 1.5× hydrostatic, with extra requirement: no sulfur-bearing lubricant in cold-drawing (residual S must be ≤ 0.005 % post-pickling) because molten nitrate salt attacks residual S at 565 °C. Specify "low-S drawing lubricant" in the RFQ.

8.4 SMR once-through steam generator (NuScale, BWRX-300, ACP100)

Spec: TP304H or TP347H per A213 + ASME Section III Subsection NB, OD 16-25 mm, wall 1.5-2.5 mm, solution-annealed, 100 % ECT, MTC 3.2, with extra requirement: elevated-temperature tensile at 525 °C and 575 °C per ASTM E21 included in first-article qualification. For higher-temperature SMR designs (Xe-100, IMSR, MMR), HR3C and Inconel 617 enter the picture and the lead time jumps to 16-24 weeks.

8.5 Refinery ethylene-cracker & steam-methane reformer convection bank

Spec: Inconel 800HT or HP-Mod (35Cr-45Ni-Mo-W) per ASTM B407 / B829, OD 60-114 mm, wall 6-12 mm, solution-annealed 1200-1240 °C for 800HT, with extra: creep-rupture test at 900 °C / 100 MPa, target life ≥ 100,000 h. Reformer tubes in ethylene crackers run 1050-1100 °C external, which is well above the envelope of any 18Cr-8Ni grade. Don't try to substitute 310S or 321H here; the creep strength drops an order of magnitude.

For the lower-temperature convection section (preheat, mixed-feed, steam superheat), 800H and 800HT dominate. For the bottom-of-convection steam-generation tubes, stainless steel pipe such as TP304H or 347H may suffice and reduce cost by 30-40 %.

9 · Five frequently asked questions from project buyers

Q1. Can I substitute TP321H for TP347H?

For 600 °C+ continuous service, no — TP347H (Nb-stabilized) holds 100,000 h rupture strength roughly 15-20 % better than TP321H (Ti-stabilized) because NbC is more stable than TiC at the grain boundary. For intermittent 540 °C service (e.g. chemical-process dryer tubes), TP321H is acceptable and 8-12 % cheaper. Always document the substitution with a Material Substitution Request (MSR) signed by both the design authority and the third-party inspector.

Q2. What is the real difference between Super304H and HR3C?

Super304H is an 18Cr-9Ni steel with 3 % Cu and a small Nb-N addition that boosts creep strength by fine Cu-rich and Nb(C,N) precipitates. It is the best choice for 600-625 °C. HR3C is a 25Cr-20Ni steel with Nb-N and roughly 2.5× the oxidation resistance of 304H at 650-700 °C because of the dense Cr₂O₃ layer. The trade-off: HR3C is roughly 2.5-3× the price per kg, harder to weld, and requires 1100-1180 °C solution anneal. Use Super304H as default for 600-625 °C; escalate to HR3C only where metal temperature exceeds 625 °C continuously.

Q3. Is Inconel 690 always required for PWR steam generator U-tubes?

For new-build PWR units since the 1990s, yes — 690 TT (thermally treated at 715 °C for 10-15 h after mill anneal) replaced Inconel 600 and Incoloy 800 because of stress-corrosion cracking (SCC) resistance in primary water. For SG retubing of older units, the choice is between 690 TT (best SCC, ~3-4× cost of 800) and 800NG (nuclear-grade, lower cost, acceptable SCC for moderate water chemistry). Some Korean and Chinese operators specify 690 NG (nuclear grade) with extra Ti + Mn control.

Q4. We need 80 t of TP347H in 10 weeks. Realistic?

It depends on the mill campaign. For a mill running TP347H already, 80 t can be delivered in 8-10 weeks (4 weeks production + 4-6 weeks sea-freight ex-China + 1-2 weeks port + inland). For a mill that needs to switch from 304H to 347H, add 3-4 weeks for campaign change. The honest answer: commit the RFQ 12-14 weeks before delivery date and the mill-direct schedule holds. Last-minute RFQs to stockists often result in 14-18 weeks anyway, with no chemistry control.

Q5. Can I bundle the order with related items to save logistics cost?

Yes — and you should. A typical USC or SMR secondary-side package includes: superheater / reheater tubes, economizer inlet tubes, heat efficiency tubes for the economizer, header pipe fittings (elbows, returns), pipe flanges (ASME B16.5 Class 600-1500), industrial valves (ASME B16.34 Class 600-1500), gasket stud bolt nut (ASME B16.20 + B16.21 + A193 + A194). A single bundled mill-direct RFQ that consolidates 5-7 of these categories typically saves 8-15 % on the total package vs ordering each separately. EZ Steel Industrial supplies all seven categories from the same mill yard with a single combined MTC and a single shipping window.

10 · EZ Steel Industrial — mill-direct supply for high-temperature pressure tubes

EZ Steel Industrial was founded in 1994 in Changsha, Hunan, China, and operates a 480,000+ t/year capacity covering carbon and carbon-alloy steel, stainless steel, copper and nickel alloy, heat-efficiency tubes, pipe fittings, pipe flanges, gaskets / stud bolts / nuts, and industrial valves. The mill is API / EN / ASME certified, ISO 9001 laboratory accredited, and ASME + AWS authorized.

For the high-temperature pressure-tube scope described in this guide, EZ Steel supplies:

Carbon & alloy steel pressure tubes: ASTM A106 / A192 / A210 / A213 (T11, T12, T22, T91, T92) / A335 (P11, P12, P22, P91, P92) / EN 10216-2 (16Mo3, 13CrMo4-5, 10CrMo9-10, X10CrMoVNb9-1) / GB 5310 / JIS G3461 STB340 / STB410 / STBA12 / STBA22 / STBA24 / GOST 8732 / GOST 550.
Stainless pressure tubes: ASTM A213 TP304 / TP304H / TP304L / TP316 / TP316H / TP316L / TP321 / TP321H / TP347 / TP347H / TP347HFG / S30432 (Super304H) / TP310HCbN (HR3C) / TP310S / EN 10216-5 1.4301 / 1.4306 / 1.4401 / 1.4404 / 1.4541 / 1.4571 / 1.4904 / 1.4951 / GB/T 13296 / JIS G3463 STB340-S / STB340-C / SUS304H TB / SUS316H TB / SUS321H TB / SUS347H TB / GOST 9941.
Nickel alloy for reformer / SMR primary: Inconel 800H / 800HT (UNS N08810 / N08811) per ASTM B407 / B829, Inconel 600 / 601 (N06600 / N06601), Inconel 625 (N06625), Inconel 690 (N06690) per ASTM B167 / B163, Monel 400 (N04400) per ASTM B165, Inconel 617 (N06617).
Heat efficiency tubes for boiler economizer / air-preheater / heat-recovery steam generator (HRSG) — U-bend tubes per U bend tubes and finned tubes in the same heat-number families.
Project-bundled carbon steel pipe + stainless steel pipe + fittings + flanges + gaskets + valves — single-mill, single-MTC, single-shipping-window delivery for the entire secondary-side package.

Each shipment leaves the mill with a 3.1 MTC (or 3.2 for nuclear per RCC-M / ASME Section III), a 100 % ECT calibration record, the hydrostatic test log, and a heat-by-heat chemistry certificate. Independent third-party inspection (SGS / Bureau Veritas / TÜV / DNV) can be arranged on request for nuclear or high-pressure utility tenders.

RFQ checklist — send this to your mill

Before you contact the mill, make sure your RFQ has these ten items, or the response will be slow and the quote will be rough.

10 must-have items in a high-temperature pressure-tube RFQ

Tube designation + standard (e.g. "TP347HFG per ASTM A213" or "S30432 Super304H per ASME CC2328").
OD × wall × length, with quantity in t or pieces.
Design metal temperature °C and design pressure MPa.
Service life target (h, start-stop cycles per year).
Delivery condition: solution-annealed, cold-drawn, hot-finished, or as-welded (for welded grades).
MTC: EN 10204 3.1 or 3.2; for nuclear add RCC-M / ASME Section III reference.
NDT: 100 % ECT per ASTM E213 / EN 10246-3, hydrostatic at 1.5× design for 10-30 s.
Special chemistry limits (low S, low Co, low Cu, controlled ferrite) if applicable.
Third-party inspection requirement (SGS, BV, TÜV, DNV) and witness points.
Bundled scope: just the tubes, or the tubes + fittings + flanges + gaskets + valves as a single mill-direct package.

For a 2026 USC, AUSC, PWR-retube, CSP-receiver, SMR-primary, or ethylene-cracker project, send the RFQ with the ten items above to:

EZ STEEL INDUSTRIAL
199 Xiangfu Road, Yuhua District, Changsha, Hunan, China
Tel: +86 731 8870 6116 | Email: export@ezsteelpipe.com
Website: https://www.ezindustrialtube.com/