ASTM A276 UNS S31803: Composition, Properties and Equivalent Standards

What ASTM A276 UNS S31803 Is

UNS S31803 is a duplex stainless steel. The name duplex describes the microstructure: roughly half ferrite and half austenite, produced by balancing chromium, nickel, molybdenum and nitrogen and then controlling the heat treatment. That mixed structure delivers a combination single-phase steels cannot match, namely high mechanical strength together with good resistance to chloride pitting and stress corrosion cracking.

ASTM A276 is the specification for wrought stainless steel bars, so S31803 supplied to A276 covers hot-finished and cold-finished bars in round, square, hexagonal and flat sections for general engineering, machining and structural use. Typical applications include offshore and marine hardware, pump and valve components, shafts, fasteners, and heat exchanger parts where pressure design rules do not govern the material choice.

Chemical Composition of S31803

The balance between the ferrite-forming elements, chromium and molybdenum, and the austenite-forming elements, nickel and nitrogen, is what keeps the structure near 50/50 and prevents harmful third phases such as sigma phase after processing.

Element	Content, %
Chromium (Cr)	21.0 - 23.0
Nickel (Ni)	4.5 - 6.5
Molybdenum (Mo)	2.5 - 3.5
Nitrogen (N)	0.08 - 0.20
Carbon (C)	0.030 max.
Manganese (Mn)	2.00 max.
Silicon (Si)	1.00 max.
Phosphorus (P)	0.030 max.
Sulfur (S)	0.020 max.
Iron (Fe)	Balance

Chromium and molybdenum provide the passive film and the resistance to chloride attack, nitrogen raises strength and delays the formation of intermetallic phases, and the low carbon limit keeps weldability and intergranular corrosion resistance under control.

Mechanical Properties of S31803 Bar

Property	Value
Tensile strength, min.	620 MPa
Yield strength, 0.2% offset, min.	450 MPa
Elongation in 50 mm, min.	25%
Hardness, max.	290 HBW

Mechanical performance is the main commercial reason engineers choose duplex over austenitic grades. Compared with 304L or 316L, whose minimum yield strength is around 170 MPa, S31803 offers roughly two and a half times the yield strength and about 20% more tensile strength. In practical terms that allows thinner sections, lighter components and fewer supports for the same design load, which is often where the cost case for duplex is made. The trade-off is that duplex is less ductile than austenitic steel and needs more careful forming and welding.

S31803, S32205 and the Name 2205

UNS S32205 is a later refinement of S31803 with tighter limits, notably chromium from 22.0 to 23.0%, molybdenum from 3.0 to 3.5% and nitrogen from 0.14 to 0.20%. Those tighter ranges raise the pitting resistance equivalent and make corrosion performance more consistent from heat to heat. Because most modern duplex producers already work inside the S32205 windows, the two grades are often treated as interchangeable in the market, and both are widely sold under the descriptive name 2205.

For a project that specifies one UNS number, buy to that number. If a certificate reports S32205 against an S31803 order the material is normally acceptable, because S32205 chemistry satisfies the wider S31803 ranges, but the reverse is not true: an S31803 heat sitting at the low end of nitrogen and molybdenum will not satisfy S32205.

Choosing the Right Specification and Checking the Certificate

A276 is a bar specification for general use and does not by itself qualify material for pressure-retaining service. The correct choice depends on the product form and the design code:

General machined or structural bar: ASTM A276.

Bar for pressure vessels and heat exchangers: ASTM A479, which adds stricter heat treatment and testing requirements.

Forged flanges, valves and fittings: ASTM A182, where duplex grades appear as F51 for S31803 and F60 for S32205.

Plate for pressure vessels: ASTM A240; seamless or welded pipe: ASTM A790; butt-welding fittings: ASTM A815.

Using an A276 bar where a pressure design code applies is a common selection error, because the certification, heat treatment verification and testing regime of A479 or A182 are what the code relies on. When ordering duplex bar, state the UNS number rather than only the trade name 2205, require a mill test certificate showing actual chemical analysis, mechanical results, hardness and the heat treatment record for each heat, ask for evidence of a ferrite content of roughly 40 to 60% by volume, and confirm in advance that the welding procedure uses a matching duplex filler metal.

Frequently Asked Questions

Q: Is UNS S31803 the same as 2205?
2205 is a commercial trade name for the duplex family that includes both UNS S31803 and UNS S32205. The two UNS numbers sit in the same family but have different composition ranges, so a specification should always quote the UNS number.

Q: Can S31803 replace 316L for a machined component?
In many cases yes, because S31803 provides higher strength and better chloride resistance. The substitution still needs review for ductility, welding, forming and code compliance, since duplex behaves differently from austenitic steel.

Q: What is the difference between S31803 and S32205?
S32205 is a tightened version of S31803, with chromium 22.0 to 23.0%, molybdenum 3.0 to 3.5% and nitrogen 0.14 to 0.20%. It gives more consistent corrosion performance, and most duplex production today meets the S32205 ranges.

Q: Is ASTM A276 suitable for pressure applications?
No. A276 covers stainless steel bars for general structural and mechanical use. Pressure vessels and heat exchangers require ASTM A479 bar, or ASTM A182 forgings, which add stricter heat treatment and testing requirements.

Q: Why is S31803 stronger than 304L or 316L?
Because of its duplex structure and its nitrogen content. Nitrogen strengthens the austenite, and the ferrite-austenite phase mixture resists deformation more effectively, giving a minimum yield strength around 450 MPa against about 170 MPa for the austenitic grades.

Q: What should be checked on the mill certificate for duplex bar?
The chemical analysis against the specified UNS number, the tensile, yield and elongation results, the hardness, the heat treatment record and, where agreed, the ferrite-austenite phase balance of roughly 40 to 60% ferrite by volume.