ASTM A815 UNS S31803
ASTM A815 UNS S31803 Duplex Stainless Steel Pipe Fittings
ASTM A815 UNS S31803 is a standard specification for wrought duplex stainless steel pipe fittings used in pressure piping and pressure vessel service. UNS S31803, also known as Duplex 2205, offers high resistance to pitting, crevice corrosion, and stress corrosion cracking, making it ideal for harsh environments like marine, chemical processing, and oil & gas industries.
Key Features
Duplex microstructure (ferritic-austenitic)
High strength compared to austenitic stainless steels (like 304/316)
Excellent chloride stress corrosion resistance
Superior erosion and fatigue resistance
Excellent weldability and formability
ASTM A815 UNS S31803 Chemical Composition
| Element | Content (%) |
|---|---|
| Chromium (Cr) | 21.0–23.0 |
| Nickel (Ni) | 4.5–6.5 |
| Molybdenum (Mo) | 2.5–3.5 |
| Manganese (Mn) | ≤ 2.0 |
| Nitrogen (N) | 0.08–0.20 |
| Carbon (C) | ≤ 0.030 |
| Silicon (Si) | ≤ 1.0 |
| Phosphorus (P) | ≤ 0.030 |
| Sulfur (S) | ≤ 0.020 |
| Iron (Fe) | Balance |
ASTM A815 UNS S31803 Mechanical Properties
| Property | Value |
|---|---|
| Tensile Strength | ≥ 620 MPa |
| Yield Strength (0.2% offset) | ≥ 450 MPa |
| Elongation | ≥ 25% |
| Hardness | ≤ 290 HBW |
Fabrication
UNS S31803 duplex stainless steel fittings are fabricated using conventional metalworking techniques. However, due to the high strength of duplex alloys, higher forming forces are required compared to austenitic stainless steels. Cold forming is possible but should be done with caution to avoid work hardening. Post-fabrication annealing may be necessary to restore the desired microstructure.
Weldability
Duplex 2205 (UNS S31803) offers good weldability. Welding methods such as TIG, MIG, and SMAW can be used, but interpass temperatures should be kept low (typically below 150°C) to avoid undesirable phases. No preheat is generally needed, but post-weld heat treatment is not typically required unless used in highly corrosive environments.
Key Weldability Notes:
Use matching duplex filler metals (e.g., ER2209).
Avoid heat input that causes sigma phase formation.
Ensure proper shielding to avoid porosity and contamination.
Heat Treatment
Duplex S31803 cannot be hardened by heat treatment. The primary heat treatment is solution annealing between 1020–1100°C, followed by rapid quenching. This restores the ideal 50/50 austenite-ferrite microstructure and improves corrosion resistance and mechanical properties.
Heat Resistance
UNS S31803 can withstand oxidation up to ~925°C in intermittent service and ~870°C in continuous service. However, prolonged exposure at elevated temperatures may cause embrittlement due to the precipitation of intermetallic phases such as sigma.
Corrosion Resistance
UNS S31803 has excellent corrosion resistance:
Pitting and crevice corrosion resistance due to high Cr, Mo, and N content.
Stress corrosion cracking (SCC) resistance superior to 300-series stainless steels, especially in chloride environments.
General corrosion resistance comparable or superior to SS 316L.
It is widely used in seawater, brine, acidic, and chloride-rich environments.
Machinability
Machining of duplex S31803 is more challenging than austenitic grades due to higher strength and work-hardening rate. Carbide tools, slower speeds, and rigid setups are recommended. Chip control may require high feed rates and constant cutting.
Tips for Machining:
Use powerful machine tools and sharp tooling.
Employ generous coolant flow.
Avoid vibration and tool chatter.
Applications
Heat exchangers and pressure vessels
Marine & offshore platforms
Oil & gas piping systems
Chemical processing equipment
Desalination plants