Grade 347 (UNS S34700) is a niobium-stabilised austenitic stainless steel that shares the same base composition as Grade 304 but adds niobium (Nb) — also historically referred to as columbium (Cb) in older ASTM designations — at a minimum of 10 times the carbon content. This niobium addition sequesters carbon as stable niobium carbide (NbC) precipitates, preventing the formation of chromium carbide (Cr₂₃C₆) at grain boundaries during exposure to the sensitisation temperature range of 425–870 °C. The result is a grade that is immune to intergranular corrosion and sensitisation in welded fabrications without post-weld heat treatment. Grade 347 also delivers significantly better resistance to polythionic acid stress corrosion cracking (SCC) than titanium-stabilised Grade 321 — a critical advantage in petroleum refinery piping that contacts polythionic acids formed from sulphur deposits and moisture during shutdown and restart cycles. For elevated-temperature service above 550 °C requiring maximum creep strength, Grade 347H (UNS S34709) specifies a controlled carbon minimum of 0.04%, ensuring adequate NbC precipitation for long-term creep rupture resistance.
Tesco Steel & Engineering manufactures 347 and 347H Stainless Steel Flanges across all standard types — Weld Neck (WNRF), Slip-On (SORF), Blind (BFF), Socket Weld (SWRF), Lap Joint (LJRF), Threaded (TRFF), Long Weld Neck (LWN), and Orifice — in sizes from ½" NB to 56" NB across all pressure classes from 150# to 2500# (PN 6 to PN 400). All flanges are forged to ASTM A182 Grade F347 or F347H and machined to ASME B16.5, ASME B16.47, EN 1092-1, DIN, and JIS B2220 dimensional standards. Our ISO-certified facility in Mumbai provides EN 10204 3.1 / 3.2 material certification, PMI testing, and third-party inspection by Lloyd's, Bureau Veritas, TÜV, SGS, or any nominated agency. We hold ready stock of standard 347 flanges and export to 96 countries across 6 continents.
| 347 / 347H SS Flange Specifications at a Glance | |
|---|---|
| Material Standard | ASTM A182 Grade F347 / F347H |
| UNS Numbers | S34700 (347) | S34709 (347H) |
| Stabilising Element | Niobium + Tantalum (Nb+Ta): min 10 × C, max 1.10% |
| Size Range | ½" NB to 56" NB (DN 15 to DN 1400) |
| ANSI Pressure Classes | 150#, 300#, 600#, 900#, 1500#, 2500# |
| DIN Pressure Ratings | PN 6, PN 10, PN 16, PN 25, PN 40, PN 64, PN 100, PN 160, PN 250, PN 320, PN 400 |
| Bore Schedule | STD, XS, XXS, SCH 10, 20, 40, 60, 80, 120, 160 |
| Flange Types | Weld Neck (WNRF), Slip-On (SORF), Blind (BFF), Socket Weld (SWRF), Lap Joint (LJRF), Threaded (TRFF), Long Weld Neck (LWN), Orifice |
| Flange Faces | Raised Face (RF), Flat Face (FF), Ring Type Joint (RTJ) |
| Max Continuous Service Temp | 900 °C (oxidising atmosphere) |
| Sensitisation-Free Range | 425 °C – 870 °C (same advantage as 321) |
| Key Advantage Over 321 | Superior polythionic acid SCC resistance; preferred for nuclear service |
All 347 and 347H flanges are machined to published dimensional tolerances. Bore diameters, bolt circle, number of holes, flange thickness, and raised face height conform to ASME B16.5 (½" to 24" NB) and ASME B16.47 Series A & B (26" NB and above).
| View Full 347 Stainless Steel Flange Dimension Tables → |
The defining addition in Grade 347 is niobium (Nb), specified at a minimum of 10 times the carbon content with a maximum of 1.10%. Niobium forms stable NbC precipitates at temperatures within and above the sensitisation range, permanently tying up the carbon that would otherwise migrate to grain boundaries as Cr₂₃C₆. Unlike titanium (used in Grade 321), niobium transfers effectively across the welding arc, making ER347 a more reliable weld metal stabiliser. Grade 347H specifies a tighter carbon range of 0.04–0.10% to guarantee sufficient NbC for long-term creep resistance above 550 °C, while still maintaining the stabilisation ratio.
| Element | 347 (S34700) | 347H (S34709) |
|---|---|---|
| Carbon (C) | 0.08% max | 0.04 – 0.10% |
| Chromium (Cr) | 17.00 – 20.00% | 17.00 – 20.00% |
| Nickel (Ni) | 9.00 – 13.00% | 9.00 – 13.00% |
| Niobium + Tantalum (Nb+Ta) — Stabiliser | 10 × C min, 1.10% max | 10 × C min, 1.10% max |
| Manganese (Mn) | 2.00% max | 2.00% max |
| Silicon (Si) | 1.00% max | 1.00% max |
| Phosphorus (P) | 0.045% max | 0.045% max |
| Sulfur (S) | 0.030% max | 0.030% max |
| Mechanical Property | 347 (F347) | 347H (F347H) |
|---|---|---|
| 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 2" (min) | 35% | 35% |
| Hardness Rockwell (max) | 92 HRB | 92 HRB |
| Hardness Brinell (max) | 201 HBW | 201 HBW |
| Density | 7.96 g/cm³ | 7.96 g/cm³ |
| International Equivalent Grades | |
|---|---|
| UNS (USA) | S34700 (347) / S34709 (347H) |
| EN / DIN (Europe) | 1.4550 / X6CrNiNb18-10 (347) | 1.4961 / X8CrNiNb16-13 (347H) |
| JIS (Japan) | SUS 347 / SUS 347H |
| GOST (Russia) | 08Ch18N12B (347) |
| BS (UK) | 347S31 |
| Old ASTM name | Columbium (Cb) — now standardised as Niobium (Nb) |
347 / 347H SS Weld Neck Flange (WNRF) — The standard choice for high-temperature welded 347 piping systems where sensitisation immunity and polythionic acid SCC resistance are both required. The weld neck's long tapered hub minimises the stress concentration at the flange-to-pipe weld, which is critical in refinery fired heater and superheater circuits subjected to thermal cycling. Available in ASME B16.5 Class 150 through 2500 in both F347 and F347H.
347 / 347H SS Slip-On Flange (SORF) — Used in moderate-pressure 347 piping for heat exchanger secondary piping, refinery process headers in the sensitisation temperature range, and petrochemical intermediate-temperature connections where full weld neck engineering is not required. The two fillet welds in 347 ensure the connection zone remains sensitisation-free even after prolonged high-temperature service.
347 / 347H SS Blind Flange (BFF) — 347 blind flanges seal vessel nozzles and pipeline ends in environments where both sensitisation and polythionic acid SCC are concerns — particularly refinery columns, reactors, and heat exchangers that contain sulphur compounds and are periodically shut down for inspection. Available with raised face (RF), flat face (FF), and ring type joint (RTJ) finishes.
347 / 347H SS Socket Weld Flange (SWRF) — Specified for small-bore instrument, sample, and chemical injection connections (½" to 2" NB) in refinery, petrochemical, and power generation piping. The niobium stabilisation ensures the single fillet weld zone remains corrosion-resistant in both high-temperature service and during polythionic acid exposure at shutdown.
347 / 347H SS Lap Joint Flange (LJRF) — Used with a 347 stub end in high-temperature process systems requiring regular disassembly — such as catalyst bed inlet distributors, heat exchanger floating head connections, and reactor piping that must be opened for scheduled maintenance without disturbing the permanent pipe welds.
347 / 347H SS Threaded Flange (TRFF) — Applied to instrument taps, thermowell connections, and low-pressure utility branches in moderate-temperature 347 piping. Suitable for service up to approximately 260 °C in threaded configurations; above that temperature the thread relaxation risk under thermal cycling requires welded flange types.
347 / 347H SS Long Weld Neck Flange (LWN) — Specified for extended nozzle connections on high-temperature reactors, pressure vessels, and column shells where the nozzle acts as an integral extension of the vessel wall. In 347H, the higher controlled carbon content provides improved creep rupture strength for long-term service above 550 °C, making LWN flanges in F347H a direct specification item in ASME Section VIII high-temperature vessel design.
347 / 347H SS Orifice Flange — Paired orifice flanges in Grade 347 are used for flow measurement in high-temperature sulphur-containing refinery process streams, where resistance to polythionic acid attack during shutdown and restart protects the accuracy of the orifice bore and differential pressure taps over the life of the installation.
Grade 347 is most frequently evaluated alongside Grade 321 because both solve the same sensitisation problem by different stabilisation mechanisms. The table below compares the properties most relevant to high-temperature and chemically aggressive service.
| Property | 347 (S34700) | 347H (S34709) | 321 (S32100) | 304 (S30400) | 316L (S31603) |
|---|---|---|---|---|---|
| Stabiliser | Nb+Ta (niobium) | Nb+Ta (niobium) | Ti (titanium) | None | None |
| Sensitisation Immunity | Yes | Yes | Yes | No | Partial (low C) |
| Polythionic Acid SCC Resistance | Superior | Superior | Moderate | Poor | Poor |
| Nuclear Service Preference | Preferred | Preferred | Acceptable | N/A | N/A |
| Weld Metal Stabilisation | Reliable (Nb transfers across arc) | Reliable | Variable (Ti oxidises) | — | — |
| Max Continuous Temp | 900 °C | 900 °C | 900 °C | 870 °C | 870 °C |
| Creep Resistance >550 °C | Good | Superior | Good | Moderate | Moderate |
| Chloride Resistance | Moderate | Moderate | Moderate | Moderate | Good (Mo) |
| ASTM Flange Grade | A182 F347 | A182 F347H | A182 F321 | A182 F304 | A182 F316L |
| Best For | Refinery, nuclear, high-temp welded | Boilers, superheaters, creep service | Heat exchangers, exhaust | General service | Marine, chemical, chloride |
Key decision point: choose 347 over 321 when polythionic acid SCC risk exists (refinery sulphur service), when nuclear service is involved, or when reliable weld metal stabilisation is critical. Choose 321 when titanium stabilisation is the project or procurement standard and polythionic acid SCC is not a concern.
Tesco Steel & Engineering supplies 347 / 347H flanges with Mill Test Certificates to EN 10204 Type 3.1 as standard, or Type 3.2 (third-party witnessed) on request.
| Standard | Scope |
|---|---|
| ASTM A182 / ASME SA182 | Material standard for forged alloy and stainless steel flanges — Grade F347 and F347H |
| ASME B16.5 | Pipe flanges and flanged fittings — ½" to 24" NB, Class 150 to 2500 |
| ASME B16.47 | Large diameter flanges — 26" to 60" NB (Series A: MSS SP-44, Series B: API 605) |
| ASME B16.36 | Orifice flanges — Class 300 to 2500 |
| EN 1092-1 | European flanges — PN 6 to PN 400, material 1.4550 (347) / 1.4961 (347H) |
| DIN 2631 – 2638 | German DIN flanges — PN 6 to PN 160 |
| JIS B2220 | Japanese flanges — 5K to 63K, material SUS 347 / SUS 347H |
| BS 3605 / BS 4504 | British Standards stainless flanges |
| GOST 12820 / 12821 | Russian standard flanges — material 08Ch18N12B |
| MSS SP-44 | Steel pipeline flanges — large diameter |
| ASME Section VIII / B31.3 | F347H listed with elevated-temperature allowable stresses for pressure vessel and process piping design |
| API RP 582 | Welding guidelines for the petroleum and chemical industries — covers 347 stabilised grades in refinery service |
Grade 347 is one of the most reliable austenitic stainless steels to weld in terms of consistent weld metal corrosion resistance, because niobium — unlike titanium in Grade 321 — transfers effectively across the welding arc and reliably stabilises the deposited weld metal.
Filler Metal: ER347 (AWS A5.9) wire is the standard filler for GTAW (TIG) and GMAW (MIG) welding of Grade 347 flanges to pipe and fittings. E347-15/16/17 (AWS A5.4) electrodes are used for SMAW. ER347 filler contains niobium that stabilises the weld metal against sensitisation. ER347 is also the recommended filler for welding Grade 321 base metal where ER321 is considered unreliable. Never use unstabilised 308 or 304 fillers for 347 in high-temperature or corrosive service — the weld metal would be susceptible to sensitisation and polythionic acid SCC.
Polythionic Acid SCC Prevention: Polythionic acid (H₂SₓO₆, where x = 2–5) forms when residual sulphur compounds on internal pipe surfaces react with moisture and oxygen during shutdowns and start-ups of refinery units. It causes inter-granular stress corrosion cracking in sensitised stainless steel. Grade 347's niobium stabilisation prevents the grain boundary chromium depletion that makes steel susceptible to this attack. API RP 582 and NACE RP0170 specify Grade 347 (or 321) as the correct material for austenitic stainless piping in polythionic acid SCC risk environments — and 347 is generally preferred over 321 because niobium provides more complete stabilisation under severe conditions.
Preheat & Interpass Temperature: No preheat is required for standard 347 welding. Keep interpass temperature below 175 °C. Back-purging with argon is required for GTAW root passes in corrosion-service piping.
Post-Weld Heat Treatment (PWHT): PWHT is not required for Grade 347 or 347H in sensitisation-sensitive service. This is the purpose of the niobium stabilisation. If solution annealing is required for other reasons (residual stress relief in heavy sections), anneal at 1040–1120 °C and water quench. For 347H, a stabilising anneal at 900–925 °C following solution annealing optimises NbC precipitation and maximises creep properties for long-term elevated-temperature service.
Nuclear Service Welding: For nuclear applications using 347, weld procedures must be qualified to ASME Section III rules. The preference for 347 over 321 in nuclear service stems from the significantly lower neutron activation of niobium compared to titanium. Activated titanium (Ti-51, half-life ~5.8 min) produces more problematic activation products than niobium (Nb-94, though long-lived, is managed in nuclear waste streams under established protocols). Project-specific nuclear activation calculations should be reviewed with the nuclear safety authority.
Grade 347 flanges are specified across industries where sensitisation immunity, polythionic acid SCC resistance, and reliable weld performance in high-temperature service are simultaneously required.
| Industry | Specific Application |
|---|---|
| Petroleum Refining | Fired heater piping, atmospheric distillation unit (ADU) nozzles, vacuum distillation unit (VDU) flanges, hydrogen reformer outlet headers — all sulphur-service environments with polythionic acid SCC risk during shutdown |
| Nuclear Power | Reactor coolant system nozzles, primary circuit flanges, waste heat exchanger connections, spent fuel pool piping — niobium preferred over titanium for lower neutron activation |
| Power Generation | Superheater and reheater outlet header flanges (347H), steam turbine valve body nozzles, high-temperature attemperator piping connections |
| Petrochemical | Ethylene cracker quench tower nozzles, steam cracker piping flanges, sulphur recovery unit (SRU) piping connections |
| Heat Exchangers & Boilers | Shell-side nozzle flanges, tube sheet connection rings, steam drum nozzles, evaporator inlet/outlet flanges |
| Aerospace | Jet engine combustor section flanges, auxiliary power unit (APU) exhaust connections, high-temperature test rig piping |
| Chemical Processing | Nitric acid plant absorber nozzles, sulphuric acid converter piping, high-temperature reactor inlet flanges |
| Cryogenic & LNG | 347 is used in some cryogenic applications for its good low-temperature toughness combined with high-temperature stabilisation capability in systems with wide temperature swings |
| Pulp & Paper | Black liquor evaporator nozzle flanges, recovery boiler connections, digester inlet piping in sulphur-containing process streams |
Specify Grade 347 (F347) when: The primary concerns are sensitisation immunity in welded assemblies AND polythionic acid SCC resistance in refinery sulphur service. 347 is also the preferred stabilised grade for nuclear service. It is the go-to specification for refinery fired heater piping, atmospheric and vacuum distillation unit nozzles, and any stainless steel piping that contacts sulphur-bearing media and is subject to shutdown and restart cycles.
Specify Grade 347H (F347H) when: Long-term creep rupture strength above 550 °C is a design requirement in addition to sensitisation immunity. 347H's controlled carbon minimum (0.04%) ensures sufficient NbC precipitation for maximum creep resistance. It is the standard specification for power plant superheater and reheater outlet headers, high-temperature pressure vessel nozzles under ASME Section VIII, and long-term elevated-temperature reactor piping designed to ASME B31.3 high-temperature allowables.
Prefer 347 over 321 when: (1) Polythionic acid SCC is a documented or probable risk — refinery sulphur service, pulp and paper processes with sulphur compounds; (2) nuclear service is involved; (3) weld metal stabilisation reliability is critical, because niobium transfers across the arc more reliably than titanium. In most other high-temperature, non-sulphur environments, 321 and 347 are functionally interchangeable, with selection driven by project specification and stock availability.
Consider 310S over 347H when: Operating temperatures exceed 900 °C. Grade 310S (24–26% Cr, 19–22% Ni) is required for continuous service up to 1100 °C where both 347 and 347H would exceed their oxidation limits.
What is a 347 stainless steel flange?
A 347 stainless steel flange is a pipe fitting manufactured from Grade 347 (UNS S34700), a niobium-stabilised austenitic stainless steel. The addition of niobium (Nb) at a minimum of 10 times the carbon content prevents sensitisation — intergranular chromium carbide precipitation — during welding and high-temperature service between 425–870 °C. Flanges are forged to ASTM A182 F347 and machined to ASME B16.5 or B16.47. They are used in petroleum refinery piping, nuclear service, power generation, and any high-temperature welded assembly where intergranular corrosion and polythionic acid SCC resistance are required.
What is the difference between 347 and 321 stainless steel flanges?
Both grades prevent sensitisation through stabilisation, but they use different elements: Grade 347 uses niobium (Nb), Grade 321 uses titanium (Ti). The key practical differences are: (1) Grade 347 provides superior resistance to polythionic acid SCC — a critical failure mode in refinery sulphur service during shutdowns; (2) Grade 347 is preferred in nuclear applications because niobium has lower and more manageable neutron activation products than titanium; (3) ER347 filler metal is more reliable as a weld metal stabiliser because niobium transfers across the welding arc consistently, whereas titanium in ER321 tends to oxidise and may leave the weld metal under-stabilised. For non-refinery, non-nuclear applications, both grades are broadly interchangeable.
What is polythionic acid stress corrosion cracking (SCC) and why does 347 resist it?
Polythionic acids (H₂SₓO₆) form when sulphur deposits on the internal surfaces of refinery equipment react with atmospheric moisture and oxygen during maintenance shutdowns. They cause inter-granular SCC in sensitised austenitic stainless steel — attacking the chromium-depleted grain boundaries created during exposure to 425–870 °C in service. Grade 347's niobium stabilisation prevents grain boundary chromium depletion entirely, removing the susceptible microstructure that polythionic acids attack. This is the primary reason 347 is specified over 321 in sulphur-bearing refinery service, and it is specifically addressed in NACE RP0170 and API RP 582.
What is the difference between 347 and 347H stainless steel flanges?
Both grades have the same base composition and niobium stabilisation. The only difference is carbon content. Grade 347 allows up to 0.08% carbon with no defined minimum. Grade 347H specifies 0.04–0.10% carbon — the controlled minimum ensures adequate NbC precipitation for superior creep rupture strength above 550 °C. For flanges in heat exchangers and general process piping below 550 °C, standard 347 is sufficient. For power plant superheaters, reformer systems, and long-term elevated-temperature pressure vessels, 347H is specified for its better creep properties and is directly listed in ASME Section II Part D with higher allowable stresses at elevated temperatures.
Why is niobium called columbium (Cb) in some older standards?
Columbium (Cb) is the historical American name for the element now officially designated as niobium (Nb) by IUPAC. ASTM standards historically used the designation "Cb" — which is why you still see "10×C Cb min" in older A182 editions and why the Grade 347 composition table column was labelled "Cb" in legacy documents. Modern editions of ASTM A182 and all current international standards (EN, JIS, ISO) use niobium (Nb). The element is identical; only the name differs by convention.
What is the maximum temperature for 347 stainless steel flanges?
Grade 347 flanges can be used in continuous service up to 900 °C in oxidising atmospheres. Grade 347H extends creep life at temperatures above 550 °C due to its controlled carbon content. For continuous service above 900 °C, Grade 310S (UNS S31008) should be specified. When determining allowable pressure at elevated temperature, consult ASME B16.5 Table 2-1.1 for F347 pressure-temperature ratings in each pressure class, noting that allowable pressure decreases significantly with rising temperature.
What filler metal is used to weld 347 stainless steel flanges?
ER347 (AWS A5.9) wire is the standard filler for GTAW and GMAW, and E347-15/16/17 (AWS A5.4) for SMAW. ER347 is also the recommended filler for welding Grade 321 base metal where ER321 filler is considered unreliable due to titanium's tendency to oxidise during arc welding. Never use unstabilised 308 or 304 fillers for 347 in service above 425 °C — the weld metal would be susceptible to sensitisation and polythionic acid SCC.
Can 347 flanges be used in nuclear power plants?
Yes. Grade 347 is one of the preferred austenitic stainless grades for nuclear service, specifically because niobium has lower and more manageable neutron activation characteristics compared to titanium in Grade 321. For ASME Section III nuclear construction, 347 flanges must be supplied to SA-182 F347 with qualified material certifications, heat traceability, and nuclear N-stamp or equivalent documentation. The specific activation profile of niobium in the neutron flux of the reactor design must be evaluated by the nuclear safety authority for each project.
What certifications are available with 347 flanges from Tesco Steel?
Standard supply includes EN 10204 Type 3.1 Mill Test Certificates confirming chemical analysis (with Nb/Ta stabilisation ratio), mechanical properties, heat number, and heat treatment records. On request: EN 10204 Type 3.2 (third-party witnessed), PMI reports, hydrostatic test certificates, ultrasonic (UT) or radiographic (RT) inspection reports, NACE MR0175 / API RP 582 compliance declarations, and ASME Section II Part D elevated-temperature allowable stress confirmation for F347H. Third-party inspection by Lloyd's, Bureau Veritas, TÜV, SGS, or any nominated agency.
What is the price of 347 stainless steel flanges in India?
The price of 347 stainless steel flanges depends on size (NB), pressure class, schedule, flange type, grade (347 or 347H), quantity, and prevailing nickel and niobium raw material costs. Grade 347 typically carries a modest premium over 304 due to the niobium addition and more controlled manufacturing process. 347H commands a further premium for the tighter carbon specification and additional creep qualification testing. Tesco Steel & Engineering offers competitive ex-works pricing from Mumbai with full export documentation. Request a current price list and stock availability here.
Who manufactures 347 stainless steel flanges in India?
Tesco Steel & Engineering, based in Mumbai, India, is an ISO-certified manufacturer and exporter of 347 and 347H Stainless Steel Flanges to ASTM A182, ASME B16.5, B16.47, EN 1092-1, DIN, JIS B2220, and GOST standards. The company supplies to EPC contractors, refineries, nuclear plant operators, power plant builders, and process plant operators across 96 countries worldwide.
Tesco Steel & Engineering maintains ready stock of 347 Stainless Steel Flanges in the most commonly specified sizes, pressure classes, and types for fast delivery on urgent requirements. Custom sizes, 347H grade, dual-certified material, special schedules, and nuclear-qualified supply chains are available on order with full material traceability from melt to finished product.
To receive a current price list, stock confirmation, and lead time, submit an enquiry with: grade (347 or 347H), flange type, size (NB), pressure class or PN rating, quantity, delivery port, and any certification or inspection requirements. Our technical sales team responds within 24 hours.
Global Export: Tesco Steel & Engineering exports 347 & 347H Stainless Steel Flanges to customers across the UAE, Saudi Arabia, USA, UK, Germany, Australia, Canada, Singapore, Malaysia, Oman, Qatar, Kuwait, Bahrain, South Africa, Nigeria, Kenya, Indonesia, Thailand, Vietnam, Brazil, and 76 other countries worldwide.