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347/347H Stainless Steel Plate: A Technical Guide to Niobium-Stabilized High-Temperature Performance
Introduction
In industries where equipment operates under sustained thermal load—petrochemical refining, power generation, aerospace exhaust systems, and high-temperature chemical processing—material selection is rarely a matter of preference. It is a matter of metallurgical necessity. Among the austenitic stainless steels engineered for these conditions, grade 347 and its high-carbon variant 347H occupy a distinct position. Stabilized with niobium (also designated columbium), these grades resist a specific failure mechanism that undermines conventional stainless steels at elevated temperatures: intergranular corrosion driven by chromium carbide precipitation.
This article examines the composition, mechanical behavior, standards compliance, and application scope of 347/347H stainless steel plate manufactured to ASTM A240. It is intended for procurement engineers, fabricators, and specifiers who require a precise understanding of when and why these grades outperform alternatives such as 304, 321, and 316.
Chemical Composition and the Role of Niobium
The defining characteristic of 347/347H is the deliberate addition of niobium as a stabilizing element. At temperatures between approximately 425°C and 815°C—the so-called sensitization range—chromium in standard austenitic steels tends to combine with carbon to form chromium carbides at grain boundaries. This depletes the surrounding matrix of chromium, compromising the passive oxide layer that provides corrosion resistance. Niobium has a stronger affinity for carbon than chromium does. By preferentially forming niobium carbides, it leaves chromium in solution and preserves corrosion performance.
The specification requires niobium content of at least eight to ten times the carbon content (Nb + Ta ≥ 10 × C), ensuring sufficient stabilization across the full carbon range.
The table below summarizes the nominal chemical composition per ASTM A240:
| Element | 347 (UNS S34700) | 347H (UNS S34709) |
|---|---|---|
| Carbon (C) | 0.08 max | 0.04 – 0.10 |
| Chromium (Cr) | 17.0 – 19.0 | 17.0 – 19.0 |
| Nickel (Ni) | 9.0 – 13.0 | 9.0 – 13.0 |
| Niobium (Nb) | 10 × C min, 1.0 max | 8 × C min, 1.0 max |
| Manganese (Mn) | 2.00 max | 2.00 max |
| Silicon (Si) | 0.75 max | 0.75 max |
| Phosphorus (P) | 0.045 max | 0.045 max |
| Sulfur (S) | 0.030 max | 0.030 max |
The distinction between 347 and 347H lies primarily in the carbon window. Grade 347 caps carbon at 0.08% to prioritize weldability and corrosion resistance. Grade 347H mandates a minimum of 0.04% carbon, which enhances high-temperature creep and rupture strength—critical for components expected to bear load at temperatures above 500°C over extended service lives.
Mechanical Properties
The minimum mechanical properties required by ASTM A240 for solution-annealed plate are outlined below. These values represent the baseline for acceptance; individual heats frequently exceed them.
| Property | 347 | 347H |
|---|---|---|
| Tensile Strength, min | 515 MPa (75 ksi) | 515 MPa (75 ksi) |
| Yield Strength (0.2% offset), min | 205 MPa (30 ksi) | 205 MPa (30 ksi) |
| Elongation in 50 mm, min | 40% | 40% |
| Hardness (Brinell), max | 201 HB | 201 HB |
| Hardness (Rockwell B), max | 92 HRB | 92 HRB |
At room temperature, the two grades behave similarly. The performance divergence emerges at elevated temperature, where 347H retains a higher fraction of its strength and demonstrates superior resistance to creep deformation—the slow, permanent strain that occurs under constant stress over time. For this reason, ASME Boiler and Pressure Vessel Code applications operating above 525°C typically specify the H grade, which also carries a required minimum grain size (ASTM No. 7 or coarser) to support creep resistance.
Physical Properties
Understanding the physical behavior of 347/347H is essential for thermal design, particularly where differential expansion between joined components must be accounted for.
| Physical Property | Value |
|---|---|
| Density | 7.96 g/cm³ |
| Melting Range | 1400 – 1420 °C |
| Thermal Conductivity (100°C) | 16.3 W/m·K |
| Mean Coefficient of Thermal Expansion (0–100°C) | 16.6 µm/m·°C |
| Specific Heat (0–100°C) | 500 J/kg·K |
| Electrical Resistivity | 0.73 µΩ·m |
| Modulus of Elasticity | 193 GPa |
Corrosion and Oxidation Resistance
The corrosion resistance of 347/347H is comparable to that of 304 in most aqueous environments, offering strong protection against a wide range of atmospheric, chemical, and organic media. Its distinguishing advantage is resistance to intergranular attack after exposure to the sensitization temperature range—a property that makes it especially suitable for welded assemblies that cannot be post-weld annealed.
In high-temperature oxidizing atmospheres, 347/347H provides good scaling resistance. It offers reliable performance in continuous service up to approximately 816°C and in intermittent service up to around 870°C. The chromium content sustains a protective oxide scale, while the stabilized structure resists degradation during thermal cycling. In environments containing sulfur-bearing gases or under strongly reducing conditions, performance should be evaluated against the specific atmosphere, as with all austenitic grades.
Comparison with Related Grades
Specifiers frequently weigh 347/347H against 321 (titanium-stabilized), 304/304L, and 316. The following comparison clarifies the selection logic.
| Consideration | 304/304L | 321 | 347/347H |
|---|---|---|---|
| Stabilizing element | None | Titanium | Niobium |
| Intergranular corrosion resistance | Low (304) | High | High |
| High-temperature strength | Moderate | Moderate | Higher (347H) |
| Weld pool behavior | Standard | Ti oxidizes in weld arc | Nb transfers reliably |
| Typical max service temp | ~870°C (scaling) | ~900°C | ~870°C |
Titanium-stabilized 321 competes directly with 347, but titanium tends to oxidize and volatilize in the welding arc, reducing its stabilizing effectiveness in weld metal. Niobium transfers more predictably across the arc, which is why 347 filler and base metal are often preferred for critical welded high-temperature joints. Where maximum creep strength is required, 347H is generally selected over 321.
Fabrication and Welding
347/347H exhibits good formability and can be cold-worked using conventional methods, though it work-hardens more rapidly than carbon steel and may require intermediate annealing for severe forming operations. Machining is comparable to other austenitic grades; positive feeds, rigid tooling, and adequate lubrication yield the best results.
For welding, matching AWS ER347 filler metal is recommended to maintain stabilization in the weld zone. All common fusion welding processes are applicable, including GTAW, GMAW, SMAW, and SAW. Because the grade is stabilized, post-weld heat treatment for corrosion purposes is generally unnecessary—one of its principal advantages over unstabilized grades in field fabrication. Solution annealing, when performed, is carried out at approximately 1010–1120°C followed by rapid cooling.
Common Applications
The combination of high-temperature strength, oxidation resistance, and stabilized corrosion behavior makes 347/347H plate a preferred material across several demanding sectors:
| Industry | Application Examples |
|---|---|
| Petrochemical & Refining | Heat exchangers, reactor internals, transfer lines |
| Power Generation | Boiler components, superheater supports, exhaust ducting |
| Aerospace | Exhaust manifolds, jet engine components, afterburner parts |
| Chemical Processing | Vessels handling nitric acid and organic acids |
| Oil & Gas | High-temperature piping, expansion joints |
Standards and Certification
Plate supplied to ASTM A240 should be accompanied by a mill test certificate (EN 10204 3.1 or equivalent) documenting heat number, chemical analysis, and mechanical test results. This traceability is not merely administrative; in pressure-retaining and safety-critical applications, documented compliance is a condition of code acceptance under ASME and equivalent international frameworks. Buyers should confirm that the H grade, where specified, meets both the carbon minimum and the grain-size requirement, as these are the properties that justify its use in elevated-temperature service.
Wuxi Zhongxin Special Steel Co., Ltd. – One-Stop Global Special Steel Export Service Provider
Wuxi Zhongxin Special Steel Co., Ltd. – One-Stop Global Special Steel Export Service Provider
Wuxi Zhongxin Special Steel Co., Ltd. is located in Xishan District, Wuxi City, nestled in the core special steel distribution hub of the Yangtze River Delta. Benefiting from a complete steel industrial chain and convenient marine logistics with easy access to Shanghai and Ningbo ports, we are an integrated special steel foreign trade enterprise integrating direct mill procurement, spot inventory storage, precision metal processing, and global steel import & export. We hold full self-operated import and export qualifications, and provide one-stop supporting solutions of full-range steel products and non-ferrous metals for global industrial purchasers and overseas engineering projects.
With years of experience in steel export, we have established long-term strategic cooperation as authorized distributors with leading domestic steel manufacturers including TISCO, Baosteel, JISCO and Dongfang Special Steel. All goods are supplied directly from original factories with stable supply, and manufactured in strict compliance with international standards such as ASTM, ASME, EN, DIN, JIS and GB to meet customs clearance and engineering inspection requirements worldwide.
Full Range of Main Export Steel Products
1. Stainless Steel Series: Hot-rolled & cold-rolled stainless steel sheets, coils and strips in grades 304/316L/321/2205 duplex steel; BA bright seamless pipes, EP electrolytic polished seamless pipes, industrial welded pipes; stainless steel round bars, angle steels, flanges and pipe fittings. Custom surface finishes including 8K mirror polishing, wire drawing, matte finishing and embossing are available.
2. Carbon Steel & Seamless Pipe Series: ASTM A106 Gr.B seamless carbon steel pipes in SCH40/SCH80, API 5L oil & gas casings, hot/cold rolled carbon steel sheets & coils, medium plates, checkered plates, round bars, H-beams, channel steels and angle steels for construction engineering.
3. Electrical Silicon Steel Series: Grain-oriented & non-oriented silicon steel sheets and coils, ultra-thin silicon steel specially for transformers, suitable for export orders of motors, transformers and new energy electrical equipment.
4. Coated Anti-Corrosion Steel: Hot-dip galvanized sheets & coils, color coated steel and weathering steel, widely used in overseas factory buildings, photovoltaic power stations and steel structure infrastructure projects.
5. Special Alloys & Non-Ferrous Metals: Nickel-based alloys, copper and aluminum sheets, pipes and bars to satisfy supporting demands of high-end chemical and new energy industries.
Supporting Precision Export Processing Capacity
We own a 10,000-square-meter-level warehousing and processing center equipped with imported laser cutting machines, waterjet cutting equipment, high-precision slitting & leveling lines and CNC polishing production lines. We provide one-stop deep processing services including pipe beveling, plate cutting, bending and mirror polishing to fixed lengths. All delivered goods can be supplied with EN 10204 3.1 Material Test Certificates, SGS/BV third-party inspection reports, together with hydrostatic testing, non-destructive testing and full heat number traceability documents.
Core Advantages of Foreign Trade Export
1. Mass Spot Inventory: Thousands of tons of conventional specifications are kept in stock for quick order arrangement of standard 4m/6m/12m fixed-length pipes and various sheets & coils. Special steel grades and non-standard sizes can be scheduled directly from steel mills, supporting both small-batch inquiries and bulk supply for large engineering projects.
2. Mature Global Sales Network: Our professional multilingual foreign trade team exports products to over 40 countries covering the Middle East, Southeast Asia, Europe, Africa, South America and Australia, serving customers from petrochemical, pressure vessel, photovoltaic new energy, food & pharmaceutical, electric power, overseas infrastructure, engineering machinery and other sectors.
3. One-Stop Quotation under All Trade Terms: We offer complete quotations on FOB China port, CFR and CIF overseas destination port basis, supporting full container load and less-than-container load shipments. We can issue certificates of origin, inspection certificates and a full set of customs clearance documents, and track marine logistics throughout the whole process.
4. Standardized Seaworthy Export Packaging: Steel pipes are fitted with plastic end caps, bundled with steel straps and wrapped in waterproof films; steel sheets are covered with anti-rust films. Each bundle is clearly marked with specification, wall thickness, material grade, heat number and country of origin to realize anti-rust protection during long-distance ocean transportation.
5. Transparent & Comprehensive Commercial Services: Quotation validity period, payment terms, loading/unloading fees, inspection charges, bank commissions and other extra costs are fully listed without hidden charges. Our technical team provides 24-hour online support for material selection, standard interpretation and delivery schedule follow-up, along with long-term after-sales guarantee.
Upholding the business philosophy of Quality First, Sincerity for Win-Win, Global-Oriented Service, we strive to be your long-term trusted partner for special steel export with a complete product portfolio, stable original factory supply, professional processing supporting capacity and mature foreign trade delivery system.
Q1：Do you offer sample orders？
A：Yes, samples are available for quality evaluation. Sample cost is charged but refundable upon bulk order.
Q2：What is your MOQ？
A：Our standard MOQ is 100 kg per specification. For common grades and standard sizes, we accept small batch orders. Mixed orders for different products are also welcome.
Q3：Can you provide custom sizes？
A：Yes, we can produce any length, width, thickness, or diameter based on your drawings. Cutting, slitting, and other processing services are available with tolerance as required.
Q4：What certifications do you provide？
A：Each shipment comes with EN 10204 3.1 mill test certificate (MTC). Third-party inspection reports (SGS/BV/TÜV) can be arranged upon request.
Q5：What is your lead time？
A：7–15 working days for stock items. 20–30 days for custom orders, depending on quantity and specification.
Q6：What surface treatments are available？
A：Various surface treatments are available upon request, including pickled, polished, coated, oiled, or as required.
Q7：What payment terms do you accept？
A：T/T (30% deposit + 70% against BL copy), L/C at sight, Western Union, etc.
Q8：What about packaging for export？
A：Standard seaworthy packaging with waterproof protection, steel straps, corner protectors, and ISPM 15 fumigation certification. Custom packaging is available as required.
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