Duplex stainless steel pipe

With the continuous deepening of industrialization, the performance requirements for pipe materials are getting higher and higher: they not only need to withstand high pressure and high temperature, but also maintain long-term stability in corrosive environments containing chlorine, strong acids, etc. Stainless steel pipes are widely used due to their high anti-corrosion performance. Meanwhile, there are many different types of stainless steel pipes, each with its own distinct characteristics and properties. In recent years, Duplex stainless steel pipes have become one of the preferred materials in the petrochemical, Marine, chemical and papermaking industries due to their unique austenitic-ferrite duplex structure.

What is duplex stainless steel?

Duplex stainless steel is a type of stainless steel in which approximately 50% austenite and 50% ferrite phases coexist in the metal microstructure. It is also known as austenitic-ferrite stainless steel. Austenite provides excellent plasticity and toughness, while ferrite endows it with high strength and resistance to stress corrosion cracking. By reasonably adding elements such as chromium (Cr), nickel (Ni), molybdenum (Mo) and nitrogen (N) to steel, the phase ratio and performance indicators can be precisely controlled, forming an ideal combination that has both high strength and corrosion resistance.

In addition to these characteristics, duplex stainless steel also has high toughness at low temperatures. It can maintain its mechanical properties in acidic and alkaline environments up to 700°C, making it an ideal material for industrial applications such as heat exchangers, storage tanks, and pipes in the chemical processing industry.

Outstanding mechanical properties: Yield strength ≥450 MPa, tensile strength ≥620 MPa, elongation ≥25%, hardness ≤280 HB.

Outstanding corrosion resistance:

Pitting and crevice corrosion: In a 3.5% NaCl solution, the critical potential for pitting resistance of duplex 2205 is significantly higher than that of 316L, and its corrosion resistance is 2 to 3 times that of ordinary austenitic stainless steel.

Stress corrosion cracking (SCC) : By combining the crack-resistant advantage of the ferritic phase with the high nitrogen content of the austenitic phase, duplex stainless steel has excellent resistance to SCC in a chloride environment.

Acidic media: Due to the presence of molybdenum and nitrogen, it also has good resistance to medium-concentration sulfuric acid, hydrochloric acid and other acidic media.

Key points of welding and processing: Duplex stainless steel pipes can be welded by conventional processes such as TIG, MIG, and submerged arc welding. To prevent the precipitation of brittle σ phase in the heat-affected zone of welding, it is necessary to strictly control the cooling rate and carry out solution treatment after welding (holding at about 1050 ℃ for 15-30 minutes and air cooling) to restore the two-phase structure. Due to the high work hardening rate of duplex stainless steel, greater cutting force and reasonable rotational speed should be combined during cutting, and wear-resistant tools should be selected to ensure processing quality.

Types of duplex stainless steel

Duplex stainless steel has many types, each with different properties and characteristics. The most commonly used duplex steel types in pipelines and other equipment include SAF 2205, SAF 2507, SAF 2304 and Zeron 100, etc. 2507 is also known as super duplex steel, and its strength and corrosion resistance are even higher than those of 2205. It is very suitable for use in harsh environments. Zeron 100 is a kind of highly corrosion-resistant duplex steel and is often used in the chemical and petrochemical industries. Because 2205 has excellent comprehensive performance and a relatively moderate cost, it is the most widely used.

The application of duplex stainless steel pipes

Duplex stainless steel pipes are widely used in various industries. They are typically employed in the oil and gas extraction, chemical, petrochemical, and food processing sectors.

Marine engineering: Submarine pipelines, platform heat exchangers, seawater pump casings.
Petrochemical and chemical industry: Pipelines and reactors for transporting concentrated sulfuric acid and chloride media.
Pulp and paper: Chemical transportation and circulation systems.
Food and pharmaceutical: High hygiene requirements for pipelines and equipment.
Construction engineering: Bridges, curtain walls and decorative pipe fittings.

In extreme corrosive and high-stress environments, duplex stainless steel pipes, with their high strength, high corrosion resistance and economy, are constantly replacing traditional stainless steel and titanium alloys, becoming the preferred material for upgrading and replacing pipeline systems.

Read more: Schedule 40 stainless steel pipe  or  Schedule 80 stainless steel pipe