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Introduction to duplex stainless steel

Introduction to duplex stainless steel

Duplex stainless steel (DSS) is a type of stainless steel where the microstructure consists of approximately equal amounts of ferrite and austenite, typically with at least 30% of the lesser phase. This unique composition grants DSS a range of advantageous properties, making it suitable for various demanding applications. The chromium content in DSS ranges from 18% to 28%, and the nickel content ranges from 3% to 10%, with some grades containing molybdenum (Mo), copper (Cu), niobium (Nb), titanium (Ti), and nitrogen (N).

Haihao Group

Haihao Group

Why stainless steel does not rust?

Stainless steel’s exceptional corrosion resistance is due to its chromium content. When the chromium content in steel reaches or exceeds 12%, a thin, dense oxide layer forms on the surface, known as a passive layer. This layer prevents further oxidation and corrosion, giving stainless steel its renowned durability and longevity.

Composition of duplex stainless steel

Duplex Stainless Steel typically consists of 50% ferrite and 50% austenite, with a minimum of 30% for the lesser phase. In addition to chromium (18%-28%) and nickel (3%-10%), DSS may contain alloying elements such as molybdenum, copper, niobium, titanium, and nitrogen, enhancing its mechanical and corrosion-resistant properties.

Classification of duplex stainless steel

Low-Alloy Type:

Example: UNS S32304 (23Cr-4Ni-0.1N)

Molybdenum-free, with a PREN (Pitting Resistance Equivalent Number) of 24-25.

Suitable for replacing AISI 304 or 316 in stress corrosion applications.

Medium-Alloy Type:

Example: UNS S31803 (22Cr-5Ni-3Mo-0.15N)

PREN of 32-33, offering corrosion resistance between AISI 316L and high Mo+N austenitic stainless steels.

High-Alloy Type:

Example: UNS S32550 (25Cr-6Ni-3Mo-2Cu-0.2N)

PREN of 38-39, providing superior corrosion resistance compared to 22% Cr DSS.

Stainless steel pipe fittings

Stainless steel pipe fittings

Super duplex stainless steel:

Example: UNS S32750 (25Cr-7Ni-3.7Mo-0.3N)

PREN above 40, suitable for harsh environments, offering excellent corrosion resistance and mechanical properties comparable to super austenitic stainless steels.

Duplex Stainless Steel vs. Austenitic & Ferritic Stainless Steel

Advantages of DSS over austenitic stainless steel:

Higher Yield Strength: DSS has over twice the yield strength of common austenitic stainless steels, allowing for thinner walls in tanks and pressure vessels, reducing material costs.

Stress Corrosion Cracking Resistance: DSS, even in its lowest alloyed form, offers superior resistance to stress corrosion cracking, particularly in chloride environments.

Corrosion Resistance: DSS like 2205 has better corrosion resistance than 316L, and super duplex grades can replace high-alloy austenitic steels and corrosion-resistant alloys in specific media.

Localized Corrosion Resistance: DSS outperforms austenitic steels in terms of wear and fatigue corrosion resistance.

Low Thermal Expansion: DSS has a lower thermal expansion coefficient, making it more compatible with carbon steel in composite or lined structures.

Energy Absorption: DSS has higher energy absorption under dynamic or static loads, beneficial in crash or explosion scenarios.

Disadvantages of DSS compared to austenitic stainless steel:

Temperature Limitations: DSS is less versatile than austenitic steel, with usage temperatures needing to be kept below 250°C.

Lower Ductility and Formability: DSS has lower plasticity and formability, making it less suitable for certain manufacturing processes.

Brittle Zone: DSS is sensitive to the brittle zone, requiring careful control of heat treatment and welding processes to avoid detrimental phases.

Advantages of DSS over ferritic stainless steel:

Superior Mechanical Properties: DSS offers better overall mechanical properties, particularly in terms of plasticity and toughness.

Corrosion Resistance: DSS has better local corrosion resistance compared to ferritic steels.

Cold Working: DSS is more amenable to cold working and forming processes.

Welding: DSS has better welding properties, often not requiring preheating or post-weld heat treatment.

Broader Application Range: DSS has a wider application range than ferritic stainless steel.

Disadvantages of DSS compared to ferritic stainless steel:

Higher Alloy Content and Cost: DSS is more expensive due to its higher alloy content, particularly nickel, which is generally absent in ferritic steels.

Duplex stainless steel offers a balanced combination of properties from both ferritic and austenitic stainless steels, making it a versatile and high-performing material for a wide range of industrial applications. Its enhanced strength, excellent corrosion resistance, and adaptability to different environments make it a preferred choice for demanding conditions.