What Is The Numbering System for Duplex Stainless Steels?

Duplex stainless steel materials are known for their strength and corrosion resistance. However, choosing the right grade can be confusing, especially with different numbering systems used in different countries.

What Is Duplex Stainless Steel

The "duplex" in duplex stainless steel means that austenite and ferrite coexist in the one material. Duplex stainless steel is a versatile steel with excellent corrosion resistance, high strength and easy processing. Its physical properties are between austenitic and ferritic stainless steels, but closer to ferritic stainless steels and carbon steels.

The pitting and corrosion resistance of duplex steels is related to its chromium, molybdenum, tungsten and nitrogen content, which can be similar to 316 stainless steel or higher than stainless steel for seawater such as 6% Mo austenitic stainless steel. All duplex stainless steels have significantly stronger resistance to chloride stress corrosion cracking than 300 series austenitic stainless steels, and their strength is also much higher than austenitic stainless steels, while showing good plasticity and toughness.

Types of Duplex Steel

Duplex stainless steels are arranged according to their corrosion properties, which depend on their alloy composition. Modern duplex stainless steels can be divided into 5 types:

• Economic duplex steels such as 2304.

• Standard duplex stainless steels such as 2205 steel are the main grades, accounting for more than 80% of duplex steel usage.

• 25Cr duplex steels such as alloy 255, with a PREN value of less than 40.

• Super duplex stainless steels, with a PREN value of 40~45 containing 25%~26%Cr, and increased molybdenum and nitrogen content compared to 25%Cr duplex stainless steels, such as 2507.

Why Do We Need a Numbering System?

Duplex stainless steels are identified using several internationally recognized systems. The two most important are:

UNS (Unified Numbering System): This is the primary system used in North America and widely recognized globally. UNS numbers for duplex steels usually start with S32XXX or S33XXX. For example:

• 2205 duplex stainless steel is most commonly UNS S32205. You might also see the older UNS S31803, but S32205 has tighter controls on composition for better performance and is now the standard.

• 2507 super duplex stainless steel is UNS S32750. Another common super duplex grade is UNS S32760 (often called Zeron 100).

• The term "2250 duplex stainless steel" likely refers to UNS S32250, an older lean duplex grade. However, 2205 duplex stainless steel (S32205) is far more prevalent today.

• The UNS numbering system precisely defines the chemical composition ranges.

EN (European Norm) Standards: Widely used in Europe and increasingly globally, especially for material specifications. EN standards use a 1.XXXX format. Key examples include:

• 2205 duplex stainless steel: EN 1.4462.

• 2507 duplex stainless steel: EN 1.4410 for UNS S32750.

• Other duplex grades have their own EN numbers, like lean duplex EN 1.4162 for UNS S32101.

Comparison table of main duplex steel numbering systems

Grade/Country	China	America	Sweden	Germany	France	Japan
Low alloy	00Cr23Ni4N	UN 23	SS232	W. Nr. 1.4362	UR35N	DP11
Medium alloy	00Cr18Ni5Mo3Si2 00Cr22Ni5Mo3N	UNSS31500 UNSS31803	SS2376 SS2377	W.Nr. 1.4417 W.Nr. 1.4462	UR45N	DP1 DP8
High alloy	0Cr25Ni5Mo2 00Cr25Ni7Mo3WCuN	UNSS32900 UNSS31260	SS2324	W.Nr. 1.4460 W.Nr. 1.4501		329J1 329J2L
Super duplex steel	00Cr25Ni7Mo4N 00Cr25Ni6Mo3CuN	UNSS32750 UNSS32550	SS2328	W. Nr. 1.4410 W. Nr. 1.4507		UR47N UR52N

Common Names: The "2205" and "2507" You Know

Alongside the formal UNS and EN numbers, duplex stainless steels are very often referred to by common names based on their key alloying elements:

• 2205 Duplex Stainless Steel: This name comes from its typical composition: approximately 22% Chromium (Cr) and 5% Nickel (Ni). This is the most widely used duplex grade. You'll see it called duplex 2205 stainless steel, duplex stainless steel 2205, or simply duplex 2205 steel coil when referring to that product form. It offers an excellent balance of strength, corrosion resistance, and cost. Its UNS is S32205, EN is 1.4462.

• 2507 Duplex Stainless Steel: This name comes from ~25% Chromium (Cr) and 7% Nickel (Ni). This is a super duplex stainless steel grade (UNS S32750, EN 1.4410). It provides significantly higher corrosion resistance and strength than standard 2205. Products include alloy 2507 duplex stainless steel plate and alloy 2507 duplex stainless steel sheet.

  
  

Understanding Super Duplex Stainless Steel

So, what is super duplex stainless steel? It's a sub-category of duplex stainless steels for even more extreme environments. Key differences:

• Higher Alloy Content: Significantly more Chromium (Cr), Molybdenum (Mo), and often Nitrogen (N) and sometimes Tungsten (W) compared to standard duplex grades like 2205.

• Higher PREN (Pitting Resistance Equivalent Number): PREN is a formula (PREN = %Cr + 3.3x%Mo + 16x%N) used to predict resistance to pitting corrosion. Super duplex stainless steels have a PREN greater than 40. Standard duplex (like 2205) typically has a PREN around 34-38.

• Higher Strength: Even greater yield and tensile strength than standard duplex.

• Higher Cost: Due to the higher alloy content.

Duplex and super duplex steel together cover a wide range of applications, from moderately aggressive conditions to highly aggressive seawater, chemical, and oil & gas environments. Alloy 2205 duplex stainless steel sheet is a workhorse, while alloy 2507 duplex stainless steel plate tackles the toughest jobs. Together, they form the essential categories of duplex and super duplex stainless steel.

Jinie Duplex Stainless Steel Grades

Let's look closer at the two most important grades:

2205 Duplex Stainless Steel (UNS S32205 / EN 1.4462):

• Composition: ~22% Cr, ~5-6% Ni, ~3% Mo, ~0.15-0.20% N.

• PREN: ~34-38.

• Properties: Excellent strength (about 2x 316 stainless steel), very good resistance to stress corrosion cracking, good pitting and crevice corrosion resistance, good weldability.

• Applications: Chemical tankers, pressure vessels, heat exchangers, piping systems, structural components in marine environments, pulp and paper digesters.

JN provides duplex stainless steel pipe, duplex stainless steel plate, duplex 2205 steel coil, and alloy 2205 duplex stainless steel sheet. Contact me for the latest news and prices！

2507 Super Duplex Stainless Steel (UNS S32750 / EN 1.4410):

• Composition: ~25% Cr, ~7% Ni, ~4% Mo, ~0.25-0.30% N.

• PREN: > 40 (typically ~42-43).

• Properties: Exceptional pitting and crevice corrosion resistance, very high strength (even higher than 2205), excellent resistance to chloride stress corrosion cracking.

• Applications: Offshore oil & gas platforms (seawater systems, umbilicals), desalination plants (high-pressure RO tubes), flue gas desulfurization, highly aggressive chemical processing. Supplied as alloy 2507 duplex stainless steel plate, alloy 2507 duplex stainless steel sheet, pipe, and fittings. Stainless steel duplex of this grade is for the most demanding tasks.

Product Forms: Pipe, Plate, Sheet, Coil

1. Duplex Stainless Steel Pipe: Used extensively for transporting corrosive fluids under pressure in chemical plants, offshore, and desalination. Both seamless and welded pipes are common.

2. Duplex Stainless Steel Plate: Used for pressure vessels, tanks, structural components requiring high strength and corrosion resistance. Duplex stainless steel plate is common for 2205 and 2507.

3. Duplex Stainless Steel Sheet and Coil: Used for fabrication of tanks, containers, linings, and parts. Thinner gauges are supplied as alloy 2205 duplex stainless steel sheet or alloy 2507 duplex stainless steel sheet. Duplex 2205 steel coil is the raw material fed into machines to produce tubes or formed parts.

Duplex Stainless Steel vs 316

A common question is duplex stainless steel vs 316. Let's compare standard 316L austenitic stainless steel (UNS S31603) to 2205 duplex:

1. Strength: Duplex stainless steel 2205 has roughly double the yield strength of 316L. This means you can often use thinner sheets for the same pressure rating, saving weight and potentially material cost.

2. Corrosion Resistance: 2205 duplex offers significantly better resistance to pitting, crevice corrosion, and stress corrosion cracking in chloride-containing environments than 316L. 316L can suffer SCC in warm chloride solutions; 2205 is highly resistant.

3. Cost: While the price per pound of duplex steel is higher than alloy 316L, the higher strength often using thinner material. The total component cost can be comparable or even lower for duplex, plus you get better corrosion resistance.

4. Toughness and Fabrication: Alloy 316L generally has slightly better toughness at very low temperatures. Both are readily weldable, though duplex requires more specific procedures to maintain the correct phase balance. Machining duplex can be harder than 316L.

Conclusion

Understanding the numbering system for duplex stainless steels – the UNS, EN, and common names – is crucial for specifying, sourcing, and using these high-performance materials correctly. Whether you need duplex stainless steel pipe, duplex stainless steel plate, duplex 2205 steel coil, alloy 2205 duplex stainless steel sheet, or alloy 2507 duplex stainless steel plate, knowing the precise grade through its numbering ensures you get the performance you require. If you have any questions, you can contact me at any time and I will help you get the most professional advice.