Stainless Steel, Austenitic
347 Stainless Steel (S34700)
An austenitic 18/9 chromium-nickel stainless steelin the British Standard Aerospace series of alloys.
S130 is an austenitic chromium-nickel stainless steel. The additional niobium stabilises the alloy overcoming the risk of corrosion common to other stainless steel grades exposed to high welding temperatures. The steel is produced by an electric melting process and also conforms to the requirements of BS S100. The alloy is non-magnetic and cannot be hardened by heat treatment.
Typical applications are within the aerospace industry where corrosive conditions are severe, such as aircraft exhaust stacks, manifolds and ring collectors. It is also used for heavy welded assemblies which cannot be annealed after welding or where the operating conditions cause exposure within the temperature range between 430° to 820°C (800° to 1500°F).
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347 Stainless Steel Related Specifications
| System / Standard | Country / Region | Grade / Designation |
| AISI | USA | 347 |
| UNS | International | S34700 |
| EN / W.Nr. | Europe | 1.4550 |
| EN Name (EN 10088) | Europe | X6CrNiNb18-10 |
| ASTM A240 | USA | 347 (plate, sheet, strip) |
| ASTM A182 | USA | F347 (forgings, flanges, fittings) |
| GB / GB 24511 | China | 06Cr18Ni11Nb |
| JIS | Japan | SUS347 |
| BS | UK | 347S31 |
| AFNOR | France | Z6CNb18-10 |
| KS | Korea | STS347 |
Properties
Chemical Composition
| Chemical Element | % Present |
| Carbon (C) | 0.00 - 0.08 |
| Chromium (Cr) | 17.00 - 19.00 |
| Nickel (Ni) | 9.00 - 11.00 |
| Manganese (Mn) | 0.50 - 2.00 |
| Phosphorous (P) | 0.00 - 0.04 |
| Sulphur (S) | 0.00 - 0.25 |
| Silicon (Si) | 0.00 - 0.75 |
| Niobium (Columbium) (Nb) | 0.00 - 1.10 |
| Molybdenum (Mo) | 0.00 - 1.00 |
| Iron (Fe) | Balance |
Mechanical Properties
Sheet
ASTM A240
| Mechanical Property | Value |
| Proof Stress | 210 MPa |
| Tensile Strength | 540 MPa |
| Elongation A50 mm | 35 % |
| Hardness Brinell | 255 HB |
General Physical Properties
| Physical Property | Value |
| Density | 7.8 g/cm³ |
Applications of 347 Stainless Steel
347 stainless steel (AISI 347 / UNS S34700 / 0Cr18Ni11Nb / SUS347) is a niobium-stabilized austenitic stainless steel designed for welded and high-temperature service, especially where resistance to intergranular corrosion and carbide precipitation is critical.
1. High-Temperature Piping and Pressure Equipment
High-temperature process piping in refineries and petrochemical plants
Superheater, reheater and header tubes in boilers and fired heaters
Manifolds, collectors and headers operating in the 425–870°C (800–1600°F) range
Pressure parts that require resistance to sensitization and intergranular attack after welding
2. Chemical and Petrochemical Processing
Equipment handling hot organic acids, mildly oxidizing or mixed chemical media
Reactors, columns, heat exchangers and transfer lines exposed to elevated temperatures
Components where 304/316 might sensitize due to repeated heating and welding cycles
Tubes, shells and tube sheets in chemical and petrochemical heat exchangers
3. Power Generation and Furnace Components
Boiler tubes, flue gas ducts and high-temperature exhaust sections
Furnace parts, radiant tubes, burners and combustion chamber components
Support hardware and structural pieces in power plant and incinerator gas paths
Parts exposed to cycling between ambient and high temperature where stability against carbide precipitation is required
4. Welded Structures and Fabrications
Large welded tanks, ducting and structures that see elevated service temperatures
Welded manifolds, expansion joints and bellows where intergranular corrosion must be avoided
Fabrications that cannot be post-weld solution annealed but must still resist sensitization
Piping spools and assemblies with many welds in refineries and chemical plants
5. Automotive, Exhaust and Engine-Related Parts
Exhaust manifolds, piping and after-treatment housings operating at high temperature
Turbocharger-related hot-side components where oxidation and thermal cycling are present
Engine and test-rig hardware exposed to hot gases in development and lab environments
Applications needing better high-temperature stability than 304 / 316 under welded conditions
6. Food, Pharmaceutical and Clean-Process Equipment at Elevated Temperature
Sterilization, pasteurization and heat-treatment equipment operating at higher temperatures
Hot-product lines and vessels where both cleanability and intergranular corrosion resistance are required
Welded piping systems subjected to frequent thermal cycling and cleaning-in-place (CIP) regimes
Situations where 304/316 are at risk of sensitization due to repeated high-temperature exposure
Summary
347 stainless steel is mainly used for welded and high-temperature piping, pressure equipment, furnace and exhaust components, and chemical/petrochemical plant hardware that must resist intergranular corrosion and sensitization in the 425–870°C range, providing a more stable alternative to 304/316 for demanding elevated-temperature and welded-service environments.
Characteristics of 347 Stainless Steel
347 stainless steel (AISI 347 / UNS S34700 / 0Cr18Ni11Nb / SUS347) is a niobium-stabilized austenitic stainless steel designed to resist intergranular corrosion and carbide sensitization in welded and high-temperature service, particularly in the 425–870°C (800–1600°F) range.
1. Niobium-Stabilized Austenitic Stainless Steel
347 is a chromium–nickel austenitic stainless steel with niobium (and sometimes tantalum) additions.
Niobium ties up carbon as stable Nb carbides, reducing chromium carbide formation at grain boundaries.
The microstructure is fully austenitic in the solution-annealed condition, giving good ductility and toughness.
2. Resistance to Intergranular Corrosion After Welding
The main feature of 347 is its improved resistance to intergranular corrosion compared with unstabilized 304 / 316.
Niobium stabilization prevents chromium depletion at grain boundaries during exposure to 425–870°C.
This greatly reduces the risk of “weld decay” in thick sections and heavily welded fabrications that cannot be solution annealed.
3. High-Temperature Strength and Structural Stability
347 maintains useful mechanical properties at elevated temperatures, typically up to around 870°C (1600°F).
It offers better structural stability than 304 in repeated heating and cooling cycles in this range.
The alloy is widely used where long-term exposure to high temperature would sensitise conventional 18-8 stainless steels.
4. Oxidation and Scaling Resistance
With about 18% Cr and ~10–11% Ni, 347 has good oxidation resistance in air at high temperatures.
It forms a protective chromium-rich oxide scale on furnace, heater, exhaust and flue gas components.
This makes it suitable for tubes, ducts and housings exposed to hot combustion gases and flue streams.
5. Corrosion Resistance Compared with 304 and 316
In general aqueous and atmospheric environments, 347 has corrosion resistance similar to 304.
In chloride-bearing media, pitting and crevice corrosion resistance is generally lower than that of 316 (which contains molybdenum).
347 is chosen not to “outperform” 304/316 in general corrosion, but to provide better weld and high-temperature stability where sensitization is a concern.
6. Non-Hardenable by Heat Treatment, Work-Hardening by Cold Work
347 cannot be hardened by quenching or standard heat treatment; strength is adjusted mainly by cold working.
Cold deformation (rolling, drawing, forming) raises yield strength and hardness while retaining good ductility.
In the annealed condition it combines moderate strength with high toughness, even at low temperatures.
7. Good Weldability and Formability
As an austenitic stainless steel, 347 has good weldability with standard processes (GTAW, GMAW, SMAW).
Niobium stabilization reduces sensitivity to intergranular attack in the heat-affected zone after welding.
It also offers good formability for bending, rolling and fabrication of complex pipework and ducting compared with ferritic or martensitic grades.
8. Magnetic Response and Fabrication Behaviour
347 is essentially non-magnetic in the solution-annealed condition.
Some slight magnetic response can appear after heavy cold work due to strain-induced martensite.
Machinability is similar to other austenitic stainless steels: not free-cutting, but workable with appropriate tooling, cutting data and coolant.
Summary
347 stainless steel is a niobium-stabilized austenitic stainless steel that combines 304-type general corrosion resistance with significantly improved resistance to intergranular corrosion and sensitization in welded and high-temperature service, along with good high-temperature strength, oxidation resistance, weldability, formability and non-magnetic behaviour in the annealed condition.
Additional Information
Weldability
Weldability of 347 Stainless Steel
347 stainless steel (AISI 347 / UNS S34700 / 0Cr18Ni11Nb / SUS347) is an austenitic, niobium-stabilized stainless steel with good weldability. It is specifically designed to resist sensitization and intergranular corrosion in the heat-affected zone (HAZ), making it very suitable for welded, high-temperature service.
1. General Weldability Characteristics
347 can be welded using standard austenitic stainless-steel procedures
Niobium stabilization greatly reduces the risk of intergranular corrosion in the HAZ after welding
No hardening by welding; the alloy remains austenitic (non-hardening) and tough in the welded condition
Often chosen instead of 304/316 when components are heavily welded and cannot be solution annealed afterwards
2. Suitable Welding Processes
All common fusion welding processes can be used, including:
GTAW (TIG) for high-quality, precision welds
GMAW (MIG / MAG) for production and pipe welding
SMAW (manual metal arc) with suitable stainless electrodes
FCAW and SAW where appropriate consumables are available
Process choice depends on thickness, joint design and required weld quality
3. Filler Metal Selection
Matching or near-matching fillers (such as 347 or 347Si type filler metals) are typically used
347-type filler contains niobium to maintain stabilization and resistance to sensitization in the weld metal
In some applications, 308 or 308L filler may be used for general 18-8 compatibility, but 347 filler is preferred where high-temperature and intergranular corrosion resistance are critical
For dissimilar joints (e.g. carbon steel to 347), over-alloyed fillers may be used depending on service conditions
4. Preheat, Interpass Temperature and Heat Input
Preheating is normally not required for 347 stainless steel, except to remove moisture and chill on very thick or highly constrained parts
Interpass temperatures are typically kept moderate (similar to other austenitic stainless steels) to avoid excessive heat input
Controlled heat input helps limit distortion and maintains a refined, stable austenitic microstructure in the HAZ
Avoid very high heat input and overheating, which can increase distortion and reduce mechanical properties
5. Post-Weld Heat Treatment
Post-weld solution annealing is usually not necessary, which is one of the main advantages of 347
The niobium stabilization allows the material to resist intergranular corrosion even without post-weld solution treatment
If post-weld heat treatment is specified (for special service conditions), it is generally for stress relief or homogenization rather than for avoiding sensitization
Care must still be taken to avoid prolonged exposure in critical temperature ranges if service conditions are very severe
6. Effect of Welding on Corrosion Resistance
Compared with unstabilized 304/316, 347 has better resistance to weld decay in the HAZ
Proper welding practice maintains good resistance to intergranular corrosion in hot service and cyclic temperature conditions
Smooth, well-contoured weld beads with full penetration and low levels of defects give the best corrosion performance
Cleaning of welds (removal of slag, heat tint and contamination) is essential to restore the passive film and minimise localized attack
7. Distortion, Residual Stress and Design Considerations
Like other austenitic steels, 347 has relatively high thermal expansion and can distort during welding
Good joint design, balanced welding sequences and adequate fixturing help control distortion
Residual stresses from welding can contribute to stress-corrosion cracking in very severe environments; sensible design and, where necessary, stress-relief treatments can mitigate this
Use generous radii, avoid sharp notches, and design joints to minimise severe restraint and shrinkage stresses
Summary
347 stainless steel has good weldability and is specifically optimized for welded, high-temperature service: it can be welded with standard austenitic techniques using 347-type fillers, usually without post-weld solution annealing, while maintaining resistance to intergranular corrosion and providing stable performance in high-temperature, welded piping and structural applications.
Fabrication
Fabrication of 347 Stainless Steel
347 stainless steel (AISI 347 / UNS S34700 / 0Cr18Ni11Nb / SUS347) is a niobium-stabilized austenitic stainless steel with good weldability and formability, designed for welded and high-temperature service. Fabrication is similar to 304/316, but with extra attention to high-temperature exposure and cleaning for best corrosion performance.
1. General Fabrication Approach
347 is supplied mainly in the solution-annealed condition.
Most forming, bending, rolling and welding operations are done in this state.
If heavy cold work is applied, a final solution anneal may be used to restore ductility and corrosion resistance.
Because it is austenitic and non-hardenable by quenching, fabrication focuses on controlling work hardening, distortion and cleanliness rather than hardness changes.
2. Forming and Cold Working
347 has formability similar to 304, with good ductility and elongation.
Suitable for deep drawing, bending, rolling, spinning and complex fabrication of pipework and ducting.
It work-hardens during deformation, so tools and presses must allow for rising forming loads.
Intermediate anneals can be used after heavy cold work to restore ductility and reduce forming forces.
Generous radii, proper lubrication and smooth tooling help avoid galling and surface damage.
3. Hot Working and Heat Treatment in the Fabrication Route
Hot working (forging, hot rolling) is carried out at typical austenitic stainless steel temperatures.
After hot working, parts are usually solution annealed and rapidly cooled (typically water quench) to obtain a fully austenitic, corrosion-resistant structure.
Solution annealing after heavy hot work or extensive cold work helps:
Remove strain hardening
Maximize resistance to intergranular corrosion
Restore toughness and formability for further processing
4. Machining
Machinability is comparable to 304/316: not free-cutting, but manageable with correct practice.
347 tends to work-harden under the tool; use:
Sharp, rigid tooling (carbide preferred for production)
Moderately low cutting speeds with adequate feed to cut below the work-hardened layer
Plenty of coolant to control heat and improve tool life
Good planning is to rough-machine before any final high-temperature exposure, then do finish machining after the last solution anneal if tight tolerances are needed.
5. Welding as Part of Fabrication
347 is specifically chosen for welded high-temperature service and has excellent weldability.
Common processes: GTAW/TIG, GMAW/MIG, SMAW and FCAW with appropriate stainless fillers.
347-type filler metals (stabilized with Nb) are preferred where high-temperature or intergranular corrosion resistance is critical.
Preheat is generally not required; interpass temperatures are kept moderate, as with other austenitic grades.
Post-weld solution annealing is usually unnecessary, which is a key advantage over unstabilized 304 in many high-temperature welded fabrications.
6. Surface Finishing, Cleaning and Pickling
For best corrosion performance, fabricated 347 components should be thoroughly cleaned:
Remove weld scale, heat tint and oxides by pickling, grinding or blasting.
Eliminate spatter, slag and embedded iron from tooling or contamination.
Use suitable passivation or cleaning procedures to restore a uniform passive film.
Smooth, polished surfaces are more resistant to pitting and crevice corrosion, especially in hot or chloride-bearing environments.
7. Distortion and Dimensional Control
As an austenitic steel, 347 has relatively high thermal expansion and low thermal conductivity.
During welding and high-temperature operations, it is prone to distortion if not properly fixtured.
Good practice includes:
Balanced weld sequences and symmetrical heating where possible
Proper clamping and fixturing of long or thin components
Allowance for some movement in design, especially in large ducting and piping fabrications
Summary
347 stainless steel is easy to fabricate by the standards of high-temperature alloys: it offers good cold and hot formability, weldability comparable to 304/316, workable machinability and excellent tolerance of welded high-temperature service, provided that work hardening, distortion and post-fabrication cleaning are properly managed to preserve its resistance to intergranular corrosion and general corrosion.
Hot Working
Hot Working of 347 Stainless Steel
347 stainless steel (AISI 347 / UNS S34700 / 0Cr18Ni11Nb / SUS347) is a niobium-stabilized austenitic stainless steel that can be hot worked using methods similar to 304/316. Correct temperature control and cooling are important to maintain a stable, corrosion-resistant austenitic structure.
1. Recommended Hot Working Temperature Range
Typical hot-working / forging temperature range is about 950–1,200°C (1,740–2,190°F).
Deformation is usually started towards the upper end of this range for best plasticity.
Work should generally be finished above roughly 900°C (1,650°F) to avoid cracking as ductility falls at lower temperatures.
Exact temperatures and soaking times should follow the relevant mill or product specification.
2. Heating and Forging Practice
Heat the material slowly and uniformly through the section before applying heavy deformation.
Use firm, substantial reductions per pass rather than light tapping to promote good grain refinement.
For large forgings or complex shapes, reheat the workpiece as soon as its temperature drops near the lower working limit.
Avoid excessive soaking at the highest temperatures to limit grain growth and surface scaling.
3. Cooling After Hot Working
After forging or hot forming, parts are usually cooled in air, then solution annealed to restore a fully austenitic structure.
Solution annealing is normally followed by rapid cooling (often water quench) to maximise corrosion resistance and prevent sensitization.
Very slow cooling through the sensitization range (about 425–870°C / 800–1600°F) should be avoided if possible, especially for parts that will see corrosive service.
4. Surface Scale, Decarburisation and Cleanup
At hot-working temperatures, 347 will form oxide scale and may experience some surface roughening.
Allow sufficient machining or grinding allowance to remove scale and any decarburised or damaged surface layer.
After hot working and solution annealing, use pickling, blasting or mechanical cleaning to achieve a clean, metallic surface suitable for service or further fabrication.
Good surface preparation is important for both corrosion resistance and fatigue performance.
5. Effect on Microstructure and Mechanical Properties
Correctly performed hot working produces a fine, uniform austenitic grain structure.
A refined grain size improves toughness, ductility and formability for later cold work or fabrication steps.
Subsequent solution annealing “resets” the microstructure, ensuring good resistance to intergranular corrosion and stable behaviour in high-temperature service.
Poor control of temperature or reduction (overheating, too little deformation) can leave a coarse or uneven grain structure, which may reduce mechanical performance.
6. Distortion, Cracking Control and Design Considerations
Preforms and forgings should be designed with smooth section transitions and generous radii to reduce stress concentrations.
Avoid sharp corners, abrupt thickness changes and heavy localised reductions that can encourage cracking during forging or cooling.
For long shafts, tubes or complex shapes, careful support and handling during hot working and cooling help minimise bending and distortion.
Inspection for laps, folds and surface cracks before final machining and solution annealing reduces scrap risk and rework.
Summary
347 stainless steel can be hot worked effectively over a typical range of about 950–1,200°C, using uniform heating, substantial reductions and controlled cooling, followed by solution annealing and rapid cooling; careful attention to temperature, deformation, surface cleanup and section design ensures a fine, stable austenitic microstructure with good toughness and corrosion resistance for high-temperature and welded service.
Heat Resistance
Heat Resistance of 347 Stainless Steel
347 stainless steel (AISI 347 / UNS S34700 / 0Cr18Ni11Nb / SUS347) is specifically designed for welded and high-temperature service, especially in the sensitization range where standard 304-type grades can suffer intergranular corrosion. Its niobium stabilization and austenitic structure give it good stability and oxidation resistance at elevated temperatures.
1. Service Temperature Range
347 is commonly used in continuous service up to about 870°C (1600°F) in oxidizing or mildly corrosive high-temperature environments.
It is particularly suitable for equipment operating in the 425–870°C (800–1600°F) band, where unstabilized 304/316 are prone to sensitization.
For long-term high-temperature service, design stresses are reduced to account for creep and relaxation, as with any stainless steel.
2. Resistance to Sensitization and Intergranular Attack
Niobium additions stabilise carbon as Nb carbides instead of chromium carbides.
This stabilization greatly improves resistance to intergranular corrosion after exposure in the sensitization range (425–870°C).
347 maintains better grain-boundary chromium levels than 304/316 under repeated heating/welding cycles, making it well suited for welded high-temperature piping and structures that cannot be solution annealed after fabrication.
3. High-Temperature Strength and Creep Behaviour
347 retains useful tensile and creep-rupture strength at temperatures where standard 304 grades begin to lose structural reliability.
It has better creep resistance than unstabilized 304 in comparable service, particularly in boiler tubes, heater coils and high-temperature manifolds.
For very long-term or extremely high-stress high-temperature service, however, dedicated creep-resistant alloys (special austenitic or nickel-base) may still be preferred.
4. Oxidation and Scaling Resistance
With about 18% Cr and ~10–11% Ni, 347 has good oxidation resistance in air and combustion gases at elevated temperatures.
It forms a stable chromium-rich oxide film that protects against rapid scaling in furnace, flue gas and exhaust applications.
Proper surface condition (clean, scale-free and smooth) helps maintain oxidation resistance and extend service life under cyclic heating.
5. Comparison with 304, 316 and 321 at High Temperature
Compared with 304, 347 offers:
Better resistance to sensitization and intergranular attack after high-temperature exposure or heavy welding
Improved structural stability in the 425–870°C range
Compared with 316, 347:
Has similar general high-temperature oxidation resistance
But 316 may offer better pitting resistance in chloride-containing aqueous environments at lower temperatures
Compared with 321 (Ti-stabilized):
Both are stabilized 18-8 steels; 347 often shows slightly better creep strength and stabilization at the highest temperatures
Choice between 321 and 347 often depends on specific code, availability and detailed property requirements
Summary
347 stainless steel provides reliable heat resistance and structural stability up to about 870°C (1600°F), with niobium stabilization giving superior resistance to sensitization and intergranular corrosion compared with 304/316 in the 425–870°C range, making it a preferred choice for welded high-temperature piping, boiler, furnace and exhaust components operating under cyclic or long-term elevated temperature service.
Machinability
Machinability of 347 Stainless Steel
347 stainless steel (AISI 347 / UNS S34700 / 0Cr18Ni11Nb / SUS347) is an austenitic, niobium-stabilized stainless steel with machinability similar to 304 / 321. It is not a free-cutting grade and tends to work harden, so correct tooling, cutting data and coolant are important for good productivity and surface finish.
1. General Machining Behaviour
347 machines much like other 18-8 austenitic stainless steels.
It work hardens under the tool, especially if feeds are too light or tools are dull.
Cutting forces are moderate, but heat generation can be high if speeds are excessive.
Compared with carbon steel, tools wear faster and require more frequent sharpening or insert changes.
2. Tooling and Cutting Parameters
Use carbide tooling for production turning, milling and drilling; high-quality HSS may be used for light or intermittent work.
Select tool grades and geometries designed for stainless steels (positive rake, good toughness).
Apply moderate cutting speeds with relatively high feed to cut beneath the work-hardened layer.
Avoid rubbing cuts and very shallow passes that only polish the surface and increase hardening.
3. Coolant and Chip Control
Use plenty of cutting fluid or coolant to control temperature, reduce built-up edge and improve tool life.
For milling and drilling, ensure coolant reaches the cutting zone, especially in deep holes.
347 can produce long, tough chips; use chip-breaker inserts and suitable feed/depth of cut to promote chip breaking.
Good chip control improves surface finish, reduces tool chipping and helps automatic machines run reliably.
4. Drilling, Tapping and Threading
For drilling, use cobalt HSS or carbide drills with steady feed and avoid dwelling at the bottom of the hole.
Use peck drilling for deep holes to clear chips and maintain cooling.
For tapping, choose strong, high-quality taps and generous lubrication to minimise torque and galling.
Thread milling is often preferred for critical or large threads to reduce risk of tap breakage and to give better control of fit.
5. Surface Finish and Work-Hardening Management
Because 347 work hardens, finish cuts should be definite and continuous, not a series of light skims.
Sharp tools and appropriate feeds help maintain a clean, bright surface with minimal tearing.
If the surface has been heavily work-hardened (e.g. by forming or prior machining), consider removing the hardened layer with a slightly deeper first cut.
Grinding and polishing can produce excellent finishes for sealing surfaces, flanges, shafts and hygienic process equipment.
6. Distortion and Residual Stress Considerations
As an austenitic alloy, 347 has relatively high thermal expansion and can distort under uneven heating.
Heavy machining on one side of a section can introduce residual stresses and minor distortion.
Rough machining followed by stress-relief or solution anneal (if the process route allows) and then finish machining is good practice for tight-tolerance or highly stressed components.
Balanced machining from both sides of plates and rings helps minimise distortion.
7. Process Planning and Tool Life Optimisation
Plan machining so that heavy stock removal is done before final heat exposure or surface finishing.
Use rigid fixturing and minimise vibration to extend tool life and maintain dimensional accuracy.
Monitor tool wear closely; running tools too long in a worn state increases heat, hardening and surface damage.
For series production, optimising cutting data and tool grades specifically for 347 can significantly improve productivity compared with generic “stainless” settings.
Summary
347 stainless steel has machinability comparable to 304 / 321: it is workable but not free-cutting, and it tends to work harden, so good results require sharp tools, moderate speeds with adequate feed, effective coolant, proper chip control and sensible process planning to achieve accurate dimensions, stable tool life and high-quality surfaces on welded and high-temperature service components.
Corrosion Resistance
Corrosion Resistance of 347 Stainless Steel
347 stainless steel (AISI 347 / UNS S34700 / 0Cr18Ni11Nb / SUS347) is a niobium-stabilized austenitic stainless steel with corrosion resistance broadly similar to 304, but with much better resistance to intergranular corrosion in welded and high-temperature service.
1. General Corrosion Behaviour
Good overall corrosion resistance in many atmospheric, fresh-water and mildly corrosive industrial environments
Performance broadly comparable to 304 in most general aqueous conditions
Niobium stabilization is aimed more at weld/high-temperature stability than at increasing basic pitting resistance
2. Atmospheric and Aqueous Environments
Good resistance to rusting and staining in rural, urban and light industrial atmospheres
Suitable for potable water, cooling water and many process waters with moderate chloride content
Used for tanks, piping and structural fabrications that see condensation, splash and wash-down
3. Intergranular Corrosion and Sensitization
Key advantage of 347 is its resistance to intergranular corrosion after exposure in the sensitization range (≈425–870°C / 800–1600°F)
Niobium ties up carbon as Nb carbides, reducing chromium carbide precipitation at grain boundaries
This greatly lowers the risk of “weld decay” in the heat-affected zone of heavily welded structures that cannot be post-weld solution annealed
4. High-Temperature and Flue-Gas Environments
Good resistance to oxidation and hot corrosion in combustion gases and flue streams up to about 870°C (1600°F)
Common for boiler tubes, heater coils, flue gas ducting and exhaust components
Maintains grain-boundary chromium levels better than 304 during long high-temperature exposures, helping preserve corrosion resistance in service
5. Chloride and Localized Corrosion
Pitting and crevice corrosion resistance in chlorides is similar to 304 and lower than 316
Not ideal for hot, concentrated or stagnant chloride solutions, especially where crevices exist
For severe marine immersion, brine or chloride-rich process media, 316 or Mo-alloyed / duplex grades are usually preferred
6. Stress Corrosion Cracking and Fabrication Effects
Like other austenitic grades, 347 can suffer chloride stress-corrosion cracking at elevated temperatures under tensile stress
Good welding practice (347-type filler, controlled heat input, proper cleaning) helps maintain corrosion resistance in the weld and HAZ
Surface finish strongly influences performance: smooth, pickled / passivated surfaces resist pitting and crevice attack better than rough or heat-tinted surfaces
Summary
347 stainless steel offers general corrosion resistance similar to 304, but with significantly improved resistance to intergranular corrosion and sensitization in welded and high-temperature service, together with good oxidation performance—making it a preferred choice for high-temperature piping, boiler, furnace and exhaust applications where 304/316 may lose grain-boundary corrosion resistance after repeated heating and welding.
Heat Treatment
Heat Treatment of 347 Stainless Steel
347 stainless steel (AISI 347 / UNS S34700 / 0Cr18Ni11Nb / SUS347) is a niobium-stabilized austenitic stainless steel. It cannot be hardened by quenching; heat treatment is mainly used to restore ductility, relieve stress and optimise resistance to intergranular corrosion and high-temperature service.
1. General Heat-Treating Behaviour
347 is a fully austenitic stainless steel in the annealed condition.
It is not hardenable by heat treatment (no martensitic transformation), so strength is adjusted mainly by cold work.
The key heat treatments are:
Solution annealing (softening / full anneal)
Stabilizing (stabilization) treatment for severe high-temperature service
Stress relief in special cases
2. Solution Annealing (Softening / Full Anneal)
Purpose:
Restore a fully austenitic, soft and ductile structure
Dissolve chromium carbides and improve resistance to intergranular corrosion
Typical practice (exact range depends on spec and product):
Heat to about 1010–1120°C (≈1850–2050°F)
Hold long enough for through-heating
Rapid cooling, usually water quench or fast air cool, to avoid sensitization and carbide re-precipitation
After proper solution annealing and rapid cooling, 347 has:
Good toughness and ductility
Maximum general corrosion resistance
A good starting condition for further fabrication or service
3. Stabilizing (Stabilization) Heat Treatment
For components expected to see long-term temperatures up to about 870–900°C (≈1600–1650°F), a stabilizing treatment can be used to lock up carbon as niobium carbides and further improve intergranular-corrosion resistance.
Typical stabilization practice:
After solution annealing, heat to about 840–900°C (≈1550–1650°F)
Hold for a time appropriate to section thickness (for example, around 1 hour per 25 mm is a common guideline in similar stabilized grades)
Cool in air
Effect:
Promotes precipitation of Nb(C,N) rather than chromium carbides at grain boundaries
Provides maximum resistance to intergranular corrosion in the critical 425–870°C service range
4. Stress-Relief Heat Treatment
Austenitic stainless steels in general do not require stress relieving for most applications, but it may be used where residual stresses are a concern (e.g. heavy machining, forming or welding).
Key points:
Low-temperature stress relief (around 650–750°C) can reduce residual stresses, but time and temperature must be controlled to avoid sensitization.
For corrosion-critical service, it is often preferable to use a full solution anneal instead of a sub-critical stress-relief treatment, to avoid any risk of carbide precipitation in the sensitization range.
5. Post-Weld Heat Treatment (PWHT)
One of the main advantages of 347 is that it often does not require post-weld solution annealing for corrosion resistance, thanks to niobium stabilization.
In practice:
Welds are typically left as-welded for many applications, provided correct welding procedures and 347-type fillers are used.
For very severe high-temperature or critical service, some specifications may call for solution annealing with or without a subsequent stabilizing treatment after welding, to ensure full stabilization and to remove any sensitized regions.
6. Hardening, Cold Work and Heat Treatment
347 cannot be hardened by quenching; it is hardened only by cold working (cold rolling, drawing, forming).
Typical route for high-strength products:
Cold work to raise strength
Optional stress relief or solution anneal if corrosion resistance or ductility needs to be restored
For high-temperature / welded duty, a solution anneal (and possibly stabilization) is usually the final heat treatment before service
7. Effect of Heat Treatment on Corrosion Resistance
Correct heat treatment is essential to get the corrosion performance expected of 347:
Proper solution anneal + rapid cooling gives maximum resistance to general and intergranular corrosion.
Stabilizing treatment provides the best protection against intergranular attack during long-term exposure in the 425–870°C range, especially for welded components.
Prolonged holding or slow cooling in the sensitization range without adequate stabilization can still allow some carbide precipitation and reduce grain-boundary corrosion resistance.
Summary
Heat treatment of 347 stainless steel is based on solution annealing with rapid cooling, optional stabilizing treatment for severe high-temperature service, and carefully controlled stress-relief practices; the alloy cannot be hardened by quenching, so strength comes from cold work, while correctly chosen and executed heat-treatment cycles are used to restore ductility, control residual stress and, above all, maximise resistance to intergranular corrosion in welded and elevated-temperature applications.
Cold Working
Cold Working of 347 Stainless Steel
347 stainless steel (AISI 347 / UNS S34700 / 0Cr18Ni11Nb / SUS347) is a niobium-stabilized austenitic stainless steel with cold workability similar to 304, combined with improved resistance to intergranular corrosion. It can be extensively cold formed, but work-hardening and sensitization behaviour must be managed properly.
1. General Cold Workability
347 has good ductility and elongation, allowing substantial cold deformation.
Cold formability is broadly comparable to 304/321, so it is suitable for:
Deep drawing
Bending and roll forming
Spinning, expanding and flaring
Because it is austenitic, it does not harden by heat treatment, only by cold work.
2. Typical Cold Working Operations
Common cold working operations for 347 include:
Sheet and strip forming: deep-drawn vessels, bellows, convoluted piping
Bending and roll forming of pipes, tubes and sections
Cold rolling, drawing and swaging of bar, wire and tube to higher strength levels
Flanging, beading and edge forming on ducting and exhaust components
With appropriate tooling and lubrication, complex shapes and tight radii are achievable.
3. Work-Hardening Behaviour
Like other austenitic stainless steels, 347 work hardens significantly during cold deformation:
Yield strength and hardness increase as cold reduction increases
Forming loads rise as operations progress, so press capacity and tooling must be adequate
Excessive local strain can lead to orange-peel, surface cracking or loss of ductility if not controlled
Planning of forming sequences and, when needed, intermediate anneals is important for heavy reductions.
4. Effects on Mechanical Properties and Corrosion Behaviour
Cold work has several effects:
Raises strength and hardness—useful for springs, fasteners, drawn tube and high-strength strip
Reduces ductility and toughness in highly strained regions
Introduces residual stresses, which may influence distortion, fatigue and stress-corrosion behaviour
Niobium stabilization means 347 remains more resistant to intergranular corrosion than unstabilized 304 after cold work and subsequent heating, but very heavy cold work followed by exposure in the sensitization range may still justify a proper solution anneal for critical service.
5. Annealing and Stress Relief After Cold Work
To restore ductility and maximum corrosion resistance after significant cold work:
Use solution annealing (typical high-temperature anneal followed by rapid cooling) to:
Remove work hardening
Dissolve chromium carbides
Re-establish a clean, fully austenitic structure
Intermediate solution anneals are often used in multi-stage forming (e.g. deep drawing, heavy cold reduction of tube or bar).
Low-temperature stress relief alone is less common and must be carefully controlled to avoid sensitization; for corrosion-critical service, solution annealing is usually preferred.
6. Design and Process Recommendations
For successful cold working of 347:
Assume strong work-hardening and size presses and tooling accordingly
Use smooth tools, good lubrication and generous radii where possible to reduce galling and surface damage
For deep draws or severe forming, plan stepwise operations with intermediate anneals
For tight tolerances, adopt a route such as:
Heavy forming / cold work → solution anneal → final light forming or straightening → finish machining / polishing
This approach balances formability, dimensional accuracy and corrosion performance.
Summary
347 stainless steel has cold workability comparable to 304/321 and can be heavily cold formed into complex shapes, but it work hardens strongly, so forming loads, tooling, intermediate anneals and residual stresses must be carefully managed; after significant cold work, a proper solution anneal is recommended to restore ductility and maximise resistance to intergranular corrosion for high-temperature and welded-service applications.
