Stainless Steel, Duplex
ASTM A815 (S31803/S32205)
A duplex stainless steel with a microstructure of 50:50 austenite and ferrite.
ASTM A815 / UNS S31803 refers to duplex stainless steel pipe fittings made from grade S31803 (2205) – a ferritic–austenitic (duplex) stainless steel with about 22% Cr, 5–6% Ni, 3% Mo and nitrogen, offering high strength and excellent resistance to chloride stress-corrosion cracking, pitting and crevice corrosion.
Duplex stainless steels are extremely corrosion resistant, work hardenable alloys. Their microstructures consist of a mixture of austenite and ferrite phases. As a result, duplex stainless steels display properties characteristic of both austenitic and ferritic stainless steels. This combination of properties can mean some compromise when compared with pure austenitic and pure ferritic grades.
Duplex stainless steels are in most cases, tougher than ferritic stainless steels. Strengths of duplex stainless steels can in some cases be double that for austenitic stainless steels.
Whilst duplex stainless steels are considered resistant to stress corrosion cracking, they are not as resistant to this form of attack as ferritic stainless steels. However, the corrosion resistance of the least resistant duplex stainless steels is greater than that for the most commonly used grades of stainless steels, i.e. 304 and 316.
Duplex steels are also magnetic, a property that can be used to easily differentiate them from common austenitic grades of stainless.
Property data given in this document is typical for flat rolled products covered by ASTM A240/A240M. ASTM, EN or other standards may cover products sold. It is reasonable to expect specifications in these standards to be similar but not necessarily identical to those given in this datasheet.
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S31803/S32205 Stainless Steel Related Specifications
| System / Standard | Country / Region | Grade / Designation |
| UNS | International | S31803 |
| Common Name | International | 2205 duplex |
| EN / W.Nr. | Europe | 1.4462 |
| EN Name | Europe | X2CrNiMoN22-5-3 |
| ASTM A182 | USA | F51 (forgings, flanges) |
| ASTM A240 | USA | 2205 (plate, sheet, strip) |
| ASTM A276 / A479 | USA | 2205 / S31803 (bars, shapes) |
| ASTM A789 / A790 | USA | S31803 (duplex tubes / pipes) |
| ASTM A815 | USA | WPS S31803 duplex pipe fittings |
| GB / China | China | 022Cr22Ni5Mo3N (duplex 2205 equivalent) |
| JIS | Japan | SUS329J3L (2205 / S31803 family) |
Properties
Chemical Composition
| Chemical Element | % Present |
| Carbon (C) | 0.00 - 0.03 |
| Chromium (Cr) | 21.00 - 23.00 |
| Phosphorous (P) | 0.00 - 0.03 |
| Sulphur (S) | 0.00 - 0.02 |
| Nickel (Ni) | 4.50 - 6.50 |
| Nitrogen (N) | 0.08 - 0.20 |
| Molybdenum (Mo) | 2.50 - 3.50 |
| Iron (Fe) | Balance |
| Manganese (Mn) | Balance |
| Silicon (Si) | Balance |
Mechanical Properties
| Mechanical Property | Value |
| Proof Stress | 450 Min MPa |
| Tensile Strength | 620 Min MPa |
| Elongation A50 mm | 25 Min % |
| Hardness Brinell | 290 Max HB |
General Physical Properties
| Physical Property | Value |
| Density | 7.805 g/cm³ |
| Thermal Expansion | 13.7 x10^-6 /K |
| Modulus of Elasticity | 200 GPa |
| Thermal Conductivity | 19.0 W/m.K |
| Electrical Resistivity | 0.85 x10^-6 Ω .m |
Applications of ASTM A815 / S31803 (2205 Duplex) Stainless Steel
ASTM A815 / S31803 covers duplex stainless steel butt-weld fittings (such as elbows, tees, reducers, caps and stub-ends) manufactured in 2205 duplex. These fittings combine high strength (≈2× 304/316) with excellent resistance to pitting, crevice corrosion and chloride stress-corrosion cracking, making them ideal for demanding piping systems.
1. Chemical and Petrochemical Process Piping
ASTM A815 S31803 fittings are widely used in aggressive chemical services where 304/316 are at their limit:
Process and transfer lines handling chloride-bearing or mildly acidic media
Header, manifold and branch connections on reactors, towers and scrubbers
Pump suction/discharge spools, by-pass lines and instrument connections
Places where pitting/crevice corrosion and SCC are concerns, but nickel alloys would be too costly
2. Oil & Gas, Offshore and Subsea Systems
2205 duplex fittings to ASTM A815 are a standard choice for high-load, corrosive oil & gas environments:
Topside and subsea flowlines, manifolds and water-injection systems
Produced-water, seawater lift and firewater lines exposed to warm chlorides
High-pressure piping spools, riser tie-ins and gathering systems
Valve, pump and separator connections where strength + SCC resistance are critical
3. Desalination, Water and Wastewater Treatment
In water treatment and desalination plants, S31803 duplex fittings offer long-life performance:
High-pressure RO and thermal desalination piping systems (feed, brine and product lines)
Seawater intake/discharge lines and chlorinated cooling-water systems
Clarifier, filter and contact-tank headers, distribution and collection lines
Wastewater treatment plant piping in saline or industrial effluent environments
4. Pulp, Paper and Bleach Plant Piping
The combination of strength, chloride resistance and SCC resistance makes 2205 fittings attractive in pulp and paper service:
White, green and black liquor headers and branch connections
Bleach plant piping where chloride + oxidizing conditions would damage 316L
Evaporator, washer and filtrate lines exposed to hot, corrosive liquors
Areas subject to corrosion–fatigue from flow, vibration and pressure cycling
5. Marine, Coastal and Structural Pipework
ASTM A815 S31803 fittings are used in marine and structural systems exposed to seawater and spray:
Shipboard seawater cooling, ballast and firewater piping systems
Offshore platform utility and service lines in splash and tidal zones
Coastal and port facility piping where de-icing salts or seawater are present
Structural or handrail piping where higher strength and corrosion resistance than 316L are required
6. Power Generation and General Industrial Service
In power and general industry, 2205 duplex fittings provide a robust upgrade over 304/316 and carbon steel:
Cooling-water and service-water systems in fossil, nuclear and renewable power plants
FGD (flue gas desulfurization) absorber, recycle and slurry lines where chlorides are present
High-strength process and utility piping in mining, fertilizer and general industrial plants
Any situation where designers want thinner walls / lighter fittings without sacrificing pressure capability or corrosion resistance
Summary
ASTM A815 / S31803 (2205 duplex) stainless steel fittings are widely used in chemical and petrochemical plants, oil & gas (onshore/offshore), desalination and water treatment, pulp and paper, marine and coastal installations, power generation and general industrial piping wherever systems require high-strength, weldable stainless fittings with much better pitting, crevice and chloride SCC resistance than 304/316, while still avoiding the cost and complexity of nickel-based alloys.
Characteristics of UNS S31803 (2205) / ASTM A815 S31803
UNS S31803 (2205) is a duplex (austenite + ferrite) stainless steel used for butt-weld fittings under ASTM A815 (elbows, tees, reducers, caps, etc.). It combines high strength, excellent localized corrosion resistance and good weldability, making it a workhorse material for demanding piping systems.
1. Duplex Microstructure and Standardization
Approximately 50% ferrite / 50% austenite in the solution-annealed condition.
Duplex structure provides a balance of:
High yield strength
Good toughness
Strong resistance to stress-corrosion cracking (SCC).
ASTM A815 S31803 fittings are produced and tested to pressure-piping and corrosion-resistant service standards, ensuring good consistency in microstructure and properties.
2. High Yield Strength and Wall-Thickness Efficiency
Yield strength is typically about twice that of 304L / 316L.
Enables:
Thinner fitting walls for the same pressure rating (subject to code rules).
Lighter piping systems and reduced support loads.
Especially valuable in high-pressure or large-diameter systems where weight and space are critical.
3. Corrosion Resistance in Chloride Media
Better resistance than 304L / 316L to:
Pitting corrosion
Crevice corrosion
Chloride stress-corrosion cracking
Well suited to:
Seawater, brackish water and chlorinated cooling water
Many chloride-bearing chemical and process streams
Correct pickling/passivation and good surface finish on fittings are important to fully realize this corrosion performance.
4. Weldability and Fabrication of Fittings
2205 duplex is readily weldable, provided duplex-specific procedures are used:
Controlled heat input and interpass temperature
Duplex or over-alloyed austenitic filler metals
ASTM A815 S31803 fittings are designed for:
Butt welding to matching pipe and components
Good weld soundness and HAZ toughness when proper procedures are followed
Correct welding keeps the ferrite–austenite balance and preserves corrosion resistance at the welds.
5. Toughness, Fatigue and Service Reliability
Toughness is lower than fully austenitic grades but adequate for most pressure and structural duties when microstructure is correct.
Duplex structure gives good fatigue and corrosion-fatigue performance, important in:
Pump, compressor and rotating-equipment piping
Systems subject to vibration, pressure cycling and flow-induced loading
When combined with good design and support, fittings offer long, reliable service life in demanding systems.
6. Dimensional Accuracy, Pressure Integrity and Standards Compliance
ASTM A815 sets requirements for:
Chemical composition and mechanical properties
Heat treatment (solution annealed)
Nondestructive examination and pressure-related quality controls (where specified by grade/class)
Fittings are manufactured with controlled dimensions, wall thickness and end preparation, making them:
Suitable for code piping systems (e.g. ASME/EN pressure design)
Compatible with standard duplex pipes, flanges and valves.
7. Physical Properties and Magnetism
Higher elastic modulus and lower thermal expansion than 304/316:
Improved dimensional stability under load and temperature changes
Magnetic in all conditions due to ferrite phase (unlike annealed 300-series).
Thermal conductivity slightly higher than austenitic grades, beneficial for some heat-transfer and thermal-cycling applications.
Summary
UNS S31803 (2205) / ASTM A815 S31803 duplex stainless steel fittings combine a duplex microstructure, high yield strength, excellent resistance to pitting/crevice attack and chloride SCC, good weldability with duplex-appropriate procedures, and robust dimensional/pressure integrity, making them a preferred choice for high-reliability piping systems in chemical, petrochemical, oil & gas, desalination, water treatment, pulp & paper, marine and other chloride-exposed industrial applications.
Additional Information
Weldability
Weldability of UNS S31803 (2205) / ASTM A815 S31803
UNS S31803 (2205) duplex stainless steel fittings to ASTM A815 are readily weldable butt-weld components, but—like all duplex steels—they require duplex-specific welding procedures to maintain correct ferrite–austenite balance, toughness and high corrosion resistance in the weld and heat-affected zone.
1. General Weldability Characteristics
Designed for butt welding to matching duplex pipe and components.
Weldable with all common stainless processes, but with a narrower process window than 304/316.
Key control targets:
Maintain a balanced duplex structure in weld metal and HAZ.
Avoid formation of intermetallic phases (e.g. sigma) from excessive heat input.
Preserve pitting, crevice and SCC resistance at and near welds.
2. Suitable Welding Processes for ASTM A815 Fittings
Common processes:
GTAW (TIG) – ideal for root passes on pipe–fitting joints and for thinner wall sections.
GMAW (MIG/MAG) – widely used for production welding of duplex piping systems.
SMAW (manual stick) – often used on site and for repairs with duplex electrodes.
FCAW – used in shop fabrication with duplex flux-cored wires.
Autogenous welding (no filler) should be avoided except for very thin sections; filler composition is important to match corrosion resistance and phase balance.
3. Filler Metal Selection and Matching
Use duplex filler metals designed for 2205-type alloys, giving:
Pitting resistance at least equal to, preferably higher than, the S31803 base metal.
A suitable ferrite–austenite ratio in the weld metal after cooling.
For dissimilar joints (e.g. duplex fitting to austenitic pipe):
Over-alloyed austenitic fillers may be used, but weld metal must still meet strength and corrosion requirements of the system.
4. Heat Input, Interpass Temperature and Cooling
Moderate heat input is critical:
Too low → high ferrite content, reduced toughness and corrosion resistance.
Too high / too slow cooling → increased risk of sigma and other intermetallic phases.
Typical practice:
Follow a qualified heat-input window for duplex steels.
Limit interpass temperature and avoid overheating thick multi-pass joints.
Allow welds to cool in still air; no quench required.
5. Joint Design, Root Quality and Shielding
Butt-weld fittings (elbows, tees, reducers, caps):
Prepared with standard pipe bevels for full-penetration welds.
Require correct root gap, alignment and fit-up to avoid lack of fusion or excessive penetration.
Root quality and shielding:
Use inert-gas back purging on pipe–fitting joints to prevent heavy oxidation on the internal root.
Good shielding on the face side prevents loss of chromium/nitrogen and protects pitting resistance.
6. Weld Properties and Corrosion Behaviour
With correct procedures, welds on S31803 fittings provide:
High strength, typically comparable to or above austenitic stainless welds.
Adequate toughness for pressure and structural service.
Pitting, crevice and SCC resistance close to that of the base metal in chloride environments.
Poor welding (wrong filler, excessive heat input, poor shielding) can lead to:
Ferrite-rich or intermetallic-containing HAZ/weld metal with low toughness.
Noticeably worse pitting/crevice resistance at and near welds—critical in seawater and chemical service.
7. Post-Weld Heat Treatment and Surface Finishing
Normally, no post-weld heat treatment (PWHT) is applied to 2205 duplex fittings; properties are controlled through welding parameters.
Standard carbon-steel stress-relief cycles are not suitable and may damage duplex properties.
After welding:
Remove slag, spatter, heat tint and oxide scale by grinding or mechanical cleaning.
Use appropriate pickling and/or passivation to restore a clean, chromium-rich passive film.
Smooth internal and external weld profiles are particularly important in flowing, chloride-containing service.
Summary
UNS S31803 (2205) / ASTM A815 S31803 duplex stainless steel fittings have good weldability for butt-weld piping systems, provided you use duplex-appropriate filler metals, control heat input and interpass temperature, ensure high-quality root shielding and joint preparation, and thoroughly clean and passivate welds; with these measures, welds maintain the high strength, toughness and excellent chloride-corrosion resistance that make 2205 duplex fittings the preferred choice for demanding process and seawater piping.
Fabrication
Fabrication of UNS S31803 (2205) / ASTM A815 S31803
ASTM A815 S31803 fittings (2205 duplex elbows, tees, reducers, caps, etc.) are supplied in the solution-annealed and pickled condition. Fabrication mainly involves fit-up, machining of ends, welding into pipe spools, and surface finishing, all done with duplex-appropriate practices.
1. General Fabrication Approach
Fittings are delivered with:
Correct wall thickness and bevels for butt welding.
Solution-annealed duplex microstructure and cleaned surfaces.
In normal shop/field work, you will:
Align and fit fittings to pipe and other components.
Make any necessary minor machining/facing of weld ends.
Weld using duplex welding procedures and then clean/passivate the weld area.
No further bulk heat treatment is usually required after supply.
2. Forming and Cold Working of Fittings
ASTM A815 fittings are typically factory-formed; additional heavy forming on site is uncommon.
If any further cold work is applied (small adjustments, slight reshaping):
Remember 2205 has higher yield strength and lower ductility than 304/316.
Use moderate deformation only, with generous radii to avoid cracking.
Heavy re-forming of finished fittings is not recommended; if unavoidable, it may require re-qualification or fresh solution annealing by the manufacturer.
3. Machining, End Preparation and Fit-Up
Machining operations are usually limited to:
Facing ends to length.
Adjusting bevel angle / land to match piping specs.
Light boring or profiling for alignment or flow profile.
Duplex machinability is more demanding than 304/316:
Use carbide tools, rigid setups and moderate speeds with adequate feed.
Provide plenty of coolant and chip breaking for tough duplex chips.
Fit-up:
Ensure proper root gap, alignment and high-low control to avoid lack of fusion or excessive mismatch.
Use clamps/fixtures strong enough for the higher strength of 2205.
4. Welding Within Piping Spools
Fittings are designed for butt welding into duplex or compatible piping systems.
Key points (in addition to the Weldability section):
Use qualified duplex WPS/PQR for each joint type and thickness.
Control heat input and interpass temperature within duplex limits.
Use duplex or over-alloyed austenitic filler metals suitable for 2205.
Practical spool build sequence:
Tack and check alignment → weld in controlled passes → inspect (NDE as required) → then finalize supports and external attachments.
5. Surface Cleaning, Pickling and Passivation
To achieve full 2205 corrosion resistance, all fabricated areas must be properly cleaned:
Remove slag, spatter and heat tint from welds and HAZ by grinding or brushing.
Where appropriate, perform pickling (or high-quality mechanical cleaning) and passivation to rebuild a clean chromium-rich passive film.
Smooth internal surfaces and weld profiles are especially important in:
Seawater, brine and chemical service.
High-velocity or crevice-sensitive areas (reducers, tees, elbows).
6. Distortion, Handling and Dimensional Control
2205 has:
Higher strength and stiffness than 304/316 → good resistance to deformation, but
Still subject to weld shrinkage and distortion if poorly sequenced.
Good practice:
Use balanced welding sequences and adequate fixturing to keep fittings aligned.
Check angle, face-to-face distance and squareness after welding, especially for elbows and reducers.
Avoid excessive local cold straightening that could over-strain small regions.
Summary
UNS S31803 (2205) / ASTM A815 S31803 duplex fittings are fabricated mainly by accurate fit-up, limited machining of weld ends, duplex-controlled welding and thorough post-weld cleaning/passivation. When you respect the higher strength and duplex-specific welding requirements, these fittings deliver reliable geometry, weld integrity and the high corrosion performance expected in demanding process and seawater piping systems.
Hot Working
Hot Working of UNS S31803 (2205) / ASTM A815 S31803
UNS S31803 (2205) duplex stainless steel used in ASTM A815 fittings can be hot worked (forging, hot forming of elbows/tees/reducers), but temperature control, reductions and cooling are critical to preserve a good duplex (ferrite + austenite) microstructure and full corrosion resistance.
1. Recommended Hot Working Temperature Range
Typical hot-working / forging range: about 950–1,150°C.
Start deformation towards the upper end of this range for best plasticity.
Avoid working below roughly 900°C, where ductility falls and cracking risk increases.
Exact temperatures and soaking times should follow the specific mill data for the product (plate, pipe, forging stock for fittings).
2. Heating and Forming Practice for Fittings
Heat stock slowly and uniformly through the section before heavy deformation (elbows, tees, reducers, caps).
Use substantial reductions per pass rather than light tapping to refine the grain structure.
Reheat when temperature falls near the lower working limit; do not continue working “too cold”.
Avoid very long soaking at the top end of the range to limit grain growth, scaling and unnecessary surface damage.
3. Cooling After Hot Working and Solution Annealing
After hot forming, allow parts to cool in still air to a safe handling temperature.
For duplex 2205 fittings, a full solution anneal + rapid cooling after hot working is normally required to:
Restore a balanced ferrite–austenite microstructure
Recover toughness and pitting/crevice/SCC resistance
Typical route:
Solution anneal in the specified high-temperature range for 2205
Hold for full through-heating
Rapidly cool (usually water quench, or very fast air for thin sections)
4. Surface Scale, Machining Allowance and Cleaning
At hot-working temperatures, 2205 develops oxide scale and possible surface roughening.
Leave enough machining/grinding allowance on forgings or pre-forms to remove:
Scale
Any decarburised or mechanically damaged surface layers
After solution annealing, surfaces should be:
Mechanically cleaned (grinding, brushing, blasting) and/or
Pickled and passivated
to restore a clean, metallic, chromium-rich surface for best corrosion resistance.
5. Effect on Microstructure and Properties
Correct hot working followed by solution annealing gives:
A fine, balanced duplex (ferrite + austenite) structure
High yield strength with good toughness
Excellent pitting, crevice and SCC resistance typical of 2205 duplex
Poor practice (working too cold, overheating, slow cooling through intermediate temperatures) can:
Coarsen grains and reduce impact toughness
Promote intermetallic phase formation (e.g. sigma), severely degrading toughness and localized corrosion resistance
6. Dimensional Control, Distortion and Cracking
Preform shapes for elbows, tees and reducers should have:
Smooth transitions and generous radii
Avoidance of sharp corners and abrupt thickness changes
During hot working and cooling:
Support components properly to limit bending and distortion
Inspect for laps, folds, surface cracks before committing to solution anneal and finish machining
Final dimensions (angles, face-to-face, squareness) are usually brought into tolerance after heat treatment by machining or grinding.
Summary
Hot working of UNS S31803 (2205) / ASTM A815 S31803 duplex fittings should be done in a controlled ≈950–1,150°C range with uniform heating, substantial reductions and timely reheating, followed by solution annealing and rapid cooling plus thorough surface cleaning; this preserves a fine duplex microstructure, high strength, good toughness and the excellent chloride-corrosion resistance required for demanding piping systems.
Heat Resistance
Heat Resistance of UNS S31803 (2205) / ASTM A815 S31803
UNS S31803 (2205) duplex stainless steel fittings to ASTM A815 are designed mainly for ambient to moderately elevated temperatures in corrosive, chloride-bearing piping systems, not for very high-temperature or creep-critical service.
1. Recommended Service Temperature Range
Typically used from sub-zero temperatures up to about 250–300°C in continuous service.
Within this range, fittings retain:
High yield strength (well above 304L/316L)
Adequate toughness for pressure and structural piping duties
Excellent pitting, crevice and SCC resistance in chlorides when surfaces and welds are properly finished
Long-term service significantly above this range is generally not recommended for critical corrosive applications.
2. Strength and Toughness at Elevated Temperatures
As temperature increases:
Yield and tensile strength decrease, but remain higher than austenitic 304L/316L at the same temperature.
Fatigue strength and impact toughness are reduced, as with all steels.
For typical process and utility piping temperatures, 2205 fittings provide:
A favourable strength-to-weight ratio
Reliable impact performance when produced and welded to specification
3. Behaviour in Hot Chloride and Process Environments
Duplex 2205 is widely used where hot seawater, brackish water or chlorinated cooling water is present:
Much better chloride stress-corrosion cracking resistance than 304/316 at elevated temperatures.
Strong resistance to localized corrosion (pitting/crevice) in many hot process waters and brines.
This makes ASTM A815 S31803 fittings suitable for:
Hot cooling-water circuits
Warm seawater systems
Moderately elevated-temperature chemical piping where 316L is marginal
4. Microstructural Stability and Intermetallic Phases
Like other duplex steels, 2205 is sensitive to intermetallic phase formation (e.g. sigma phase) and 475°C-type embrittlement if held too long in certain intermediate temperature ranges.
Extended exposure at elevated temperatures can:
Reduce impact toughness
Lower pitting and crevice-corrosion resistance
For this reason, 2205 is not chosen for continuous high-temperature or creep-controlled service, and fabrication heat cycles (hot working, welding, any PWHT) must respect duplex temperature limits.
5. Design Considerations for Elevated-Temperature Piping
Treat ASTM A815 S31803 fittings as high-strength, corrosion-resistant components for moderate temperatures, not as primary high-temperature alloys.
In design you should:
Use temperature-dependent allowable stresses from the relevant piping/pressure code.
Keep continuous operating temperatures within the qualified range for 2205 in the given code/service.
Avoid designs that create local heat traps or long-term exposure in intermetallic-forming bands.
Combine correct material selection with good weld procedures and post-weld cleaning to maintain corrosion performance at higher service temperatures.
Summary
UNS S31803 (2205) / ASTM A815 S31803 duplex stainless steel fittings offer reliable strength and excellent corrosion resistance in chloride-bearing piping systems at low to moderately elevated temperatures (up to ~250–300°C), with far better chloride SCC and localized-corrosion performance than 304/316; however, they are not intended for long-term, high-temperature or creep-critical service, where the risk of intermetallic phase formation and property degradation becomes unacceptable and more heat-resistant alloys are required.
Machinability
Machinability of UNS S31803 (2205) / ASTM A815 S31803
UNS S31803 (2205) duplex stainless steel used in ASTM A815 fittings has moderate but demanding machinability: it is clearly more difficult to machine than 304/316 due to higher strength and work hardening, but performs well with the right tooling, parameters and setups.
1. General Machining Behaviour
Duplex (ferrite + austenite) structure and high yield strength →
higher cutting forces and tool loads than 304/316.Tends to work harden if tools rub instead of cut (too light feed, dull tools).
Produces tough, sometimes continuous chips, especially in turning and boring.
Overall machinability is similar to other duplex grades and more demanding than standard austenitics.
2. Preferred Condition for Machining
ASTM A815 fittings are supplied solution-annealed and pickled – this is the best condition for machining.
Typical operations on fittings include:
Facing and squaring weld ends
Adjusting bevel angles/lands
Light boring or profiling for alignment and flow control
Avoid heavy cold work on areas that will be finish machined, as this increases surface hardness and tool wear.
3. Tooling and Cutting Parameters
Use carbide tooling designed for stainless / duplex steels.
Setup recommendations:
Rigid machine, fixturing and toolholders to minimise chatter.
Positive or mildly positive rake geometries to reduce cutting forces.
Cutting practice (conceptually):
Moderate speeds, lower than for 304/316.
Adequate feed and depth of cut to cut beneath any work-hardened skin.
Avoid “skimming” passes that only rub and harden the surface.
4. Coolant and Chip Control
Duplex steels generate significant heat in cutting:
Use plenty of coolant/emulsion directed into the cutting zone.
For internal machining/boring, ensure coolant reaches the tool tip and chips are flushed.
Chips can be long and tough:
Choose inserts with effective chip breakers.
Adjust feed and depth of cut to encourage chip breaking and prevent bird-nesting.
Good chip control improves tool life, surface finish and CNC reliability.
5. Drilling, Boring and Facing of Fitting Ends
Drilling / Boring
Use carbide or high-quality cobalt HSS tools.
Apply steady feed; avoid dwelling at the bottom of holes or bores.
For deeper bores, use peck cycles for chip evacuation.
Facing / Bevel Adjustment
Use rigid tooling and controlled feed to maintain square, flat faces and accurate bevel angles.
For weld prep, maintain consistent land thickness and surface finish to support good weld quality.
6. Surface Finish and Dimensional Control
With appropriate parameters, 2205 fittings can achieve smooth, weld-ready faces and bores.
For dimensional control:
Check angle, face-to-face length and squareness after machining, especially on elbows and reducers.
Avoid local overheating in heavy cuts or grinding, which can introduce residual stress and slight distortion.
Use multiple controlled passes rather than one very heavy cut on thin-walled fittings.
Summary
UNS S31803 (2205) / ASTM A815 S31803 duplex fittings have demanding but manageable machinability: treat them as a high-strength duplex alloy—machine in the solution-annealed condition using rigid setups, duplex-rated carbide tools, moderate speeds with solid feeds, abundant coolant and good chip-breaking strategies—to obtain accurate dimensions, clean weld-prep surfaces and reliable tool life in production of high-performance piping systems.
Corrosion Resistance
Corrosion Resistance of UNS S31803 (2205) / ASTM A815 S31803
UNS S31803 (2205) duplex stainless steel fittings to ASTM A815 offer significantly better corrosion resistance than 304L/316L, especially in chloride-bearing environments, while also resisting chloride stress-corrosion cracking far better than austenitic grades.
1. General Corrosion Behaviour
Very good resistance to uniform (general) corrosion in many industrial, marine and process environments.
Clearly superior to 304L/316L in most chloride-containing waters and process streams.
Well suited for long-life service in seawater, brackish water, chlorinated cooling water and many chemical plant environments.
2. Pitting and Crevice Corrosion in Chloride Media
Higher Cr, Mo and N contents give 2205 a much higher pitting resistance than 304L/316L.
Provides strong resistance to:
Pitting corrosion in seawater and brine
Crevice corrosion under gaskets, deposits and lap joints (assuming sensible design)
Commonly selected where 316L is at or beyond its limit for localized attack in hot or high-chloride service.
3. Chloride Stress-Corrosion Cracking (SCC)
Duplex structure (ferrite + austenite) gives excellent resistance to chloride SCC, far better than austenitic 304/316.
In hot, aerated chlorides (warm seawater, cooling water, process brines):
304/316 can crack under tensile stress.
2205 fittings typically remain sound within their normal temperature and stress limits.
This SCC resistance is a key reason 2205 fittings are widely used in offshore, desalination and cooling-water systems.
4. Performance in Chemical and Process Environments
Good resistance in many mild to moderately aggressive acids, alkalis and salt solutions, especially where chlorides are present.
Frequently used in:
Pulp and paper liquors (white, green, black liquors)
Many refinery, petrochemical and chemical-process streams containing chlorides
For extremely strong acids or highly reducing environments, more highly alloyed stainless or nickel alloys may still be required; 2205 is optimised for chloride-dominated conditions.
5. Welds, Heat-Affected Zones and Fabrication Effects
With proper duplex welding procedures (controlled heat input, correct filler, limited interpass temperature), welds and HAZ can have corrosion resistance close to the base metal.
Poor practice (overheating, wrong filler, inadequate shielding) can:
Increase ferrite or form intermetallic phases (e.g. sigma) in welds/HAZ.
Significantly reduce pitting, crevice and SCC resistance around welds.
For critical seawater or chemical service, qualified WPS/PQR and good QC are essential to maintain full corrosion performance.
6. Surface Finish, Cleaning and Design Considerations
As with all stainless steels, the actual corrosion resistance depends strongly on surface condition:
Remove weld scale, heat tint, slag and contamination by grinding or blasting.
Use appropriate pickling and/or passivation to restore a clean, chromium-rich passive film.
Smooth, well-finished internal surfaces reduce pitting, fouling and crevice attack.
Good design further improves performance:
Minimise crevices, dead legs and dirt traps in piping layouts.
Ensure proper drainage and clean-out possibilities.
Avoid unfavourable galvanic couples in wet service where possible.
Summary
UNS S31803 (2205) / ASTM A815 S31803 duplex stainless steel fittings provide excellent pitting and crevice corrosion resistance and markedly superior chloride SCC resistance compared with 304L/316L, especially in seawater, brines and chloride-bearing process streams; when fittings and welds are fabricated with duplex-appropriate procedures and properly cleaned/passivated, they deliver long, reliable corrosion performance in demanding industrial and marine piping systems.
Heat Treatment
Heat Treatment of UNS S31803 (2205) / ASTM A815 S31803
UNS S31803 (2205) duplex stainless steel fittings to ASTM A815 are not hardenable by quenching like martensitic steels. Heat treatment is mainly used to obtain or restore the correct duplex (ferrite + austenite) microstructure and to protect toughness and corrosion resistance by avoiding harmful intermetallic phases.
1. General Heat-Treatment Behaviour
Strength comes from composition + duplex structure, not from martensite formation.
Main practical treatments:
Solution annealing (the key treatment)
Very limited, carefully controlled stress relief (rarely required)
Prolonged exposure in certain mid-temperature ranges can form sigma and other intermetallic phases, causing embrittlement and loss of corrosion resistance.
2. Solution Annealing (Primary Heat Treatment)
Purpose
Restore a balanced ferrite–austenite microstructure after hot working or heavy cold work.
Dissolve unwanted precipitates and recover toughness + localized corrosion resistance.
Typical practice (conceptual)
Heat into a high-temperature solution-annealing range specified for 2205.
Hold long enough for full through-heating of the section.
Rapidly cool – typically water quench (or very fast air for thinner sections).
Correct solution annealing + rapid cooling gives:
Fine, balanced duplex structure.
High yield strength with good impact toughness.
Maximum pitting, crevice and SCC resistance for 2205.
3. Stress Relief and PWHT for Fittings
For ASTM A815 S31803 fittings, post-weld heat treatment (PWHT) is normally not applied.
Residual stresses are usually managed by:
Proper weld sequencing and fixturing
Good fit-up and distortion control, rather than by heat treatment.
Conventional carbon-steel stress-relief cycles (long holds in intermediate temperature ranges) are not suitable; they can:
Promote intermetallic phase formation
Reduce toughness and localized corrosion resistance
If a code or client demands stress relief, it must follow duplex-specific guidance and be strictly time/temperature controlled.
4. Effect of Incorrect Heat Exposure
Overheating, slow cooling or long holds in the 600–1 000°C region can:
Form sigma and other intermetallic phases in base metal or HAZ
Sharply reduce impact toughness
Lower pitting and crevice-corrosion resistance, especially in seawater and chloride service
Multiple uncontrolled heat cycles do not increase strength and only risk property degradation.
5. Practical Heat-Treatment Route for Fittings
Mill/manufacturer:
Hot work / form elbows, tees, reducers, caps → solution anneal + rapid cool → pickle/clean → supply as solution-annealed fittings.
Fabricator / end user:
Perform machining, fit-up and welding using duplex-qualified procedures.
No further bulk heat treatment; just:
Proper welding parameters
Thorough post-weld cleaning, pickling and passivation to restore a clean passive surface.
Summary
For UNS S31803 (2205) / ASTM A815 S31803 duplex fittings, heat treatment is centred on high-temperature solution annealing followed by rapid cooling performed by the manufacturer to produce a fine, balanced duplex microstructure with high strength and excellent corrosion resistance; in service and fabrication, additional PWHT is normally avoided, and time at intermetallic-forming temperatures must be limited to prevent embrittlement and loss of pitting/crevice and SCC resistance.
Cold Working
Cold Working of UNS S31803 (2205) / ASTM A815 S31803
UNS S31803 (2205) duplex stainless steel has higher strength and lower ductility than 304/316, but still offers reasonable cold formability. For ASTM A815 fittings, cold work is usually limited to minor adjustments; heavy re-forming should be avoided unless the fitting is re-heat treated by the manufacturer.
1. General Cold Workability
Duplex (ferrite + austenite) structure →
Higher yield strength and forming loads than austenitic 304/316.Ductility is lower than 300-series, but adequate for moderate bends and shape adjustments.
Not suitable for very severe deep drawing or tight-radius bending on finished fittings without special precautions.
2. Typical Cold Work on ASTM A815 Fittings
In normal fabrication, cold work is usually limited to:
Small end corrections (slight facing, alignment tweaks).
Very minor angle adjustments on elbows or reducers.
Light straightening or fit-up corrections.
Fittings are already factory-formed and solution-annealed;
heavy re-forming on site is not recommended and can invalidate material certification.
3. Forming Practice and Limits
If any cold forming of fittings is unavoidable:
Use generous radii and smooth transitions; avoid sharp corners and severe local bends.
Apply deformation in gradual, controlled steps, not one heavy blow.
Use smooth, well-lubricated tooling to reduce friction and galling.
Monitor for signs of surface cracking, especially on the outside of bends and at welds.
4. Work Hardening and Residual Stresses
2205 work hardens under cold deformation:
Local strength and hardness increase.
Ductility decreases in heavily strained zones.
Heavy cold work introduces high residual stresses, which can:
Increase risk of distortion during welding.
Affect fatigue and corrosion-fatigue behaviour in service.
For critical service (seawater, hot chlorides), severe local strain in highly loaded regions should be avoided.
5. Interaction with Welding and Machining
Cold-worked regions:
Are more prone to distortion when welded because of stored strain energy.
May be harder to machine due to increased surface hardness.
Practical fabrication route for precise spools:
Fit and (if needed) lightly adjust → Weld using duplex procedures → Light straighten → Finish machine / dress weld ends.
6. When Solution Annealing May Be Needed
If a fitting has been subjected to substantial cold re-forming (beyond minor adjustments):
A full solution anneal + rapid cooling would be needed to restore:
Balanced duplex microstructure
Full toughness
Maximum pitting/crevice/SCC resistance
In practice, such heat treatment is normally done only by the manufacturer, not in the field; heavily re-formed fittings are often rejected and replaced instead.
Summary
Cold working of UNS S31803 (2205) / ASTM A815 S31803 fittings should be treated as limited, corrective forming on a high-strength duplex alloy: use only modest deformation with generous radii and good lubrication, avoid severe local strain in critical areas, recognise that heavy cold work raises hardness and residual stress, and rely on new or re-solution-annealed fittings rather than heavily re-forming standard solution-annealed products for demanding corrosive piping service.
