Duplex, no more a Perplex

Stainless steels are preferred for their superior corrosion resistance. Its derivatives are formed by altering composition of the various alloying elements. They are broadly divided into- austenitic, ferritic, martensitic, duplex and precipitation hardened stainless steels. This article will focus on the brief history, classification, properties and applications of Duplex Stainless steel.

Duplex stainless steels have a two-phase micro structure of austenitic and ferritic steels. Mechanical properties of duplex stainless steel depend on the relative amounts of ferrite and austenite forming elements present in the composition. Main alloying elements (other than iron) for the most popular 2205 grade are Chromium (21 -23%), Nickel (4.5 – 6.5%) and Molybdenum (2.5- 3.5%).

Duplex stainless steel was first conceptualized in 1930s; during this period maintaining chemistry was difficult due to manufacturing constraints. Nonetheless, it was used for some applications. With advancement in technology, particularly with introduction of decarburization in 1960s, manufacturing of duplex stainless steel became practically feasible. Duplex 2205, which is considered the workhorse of duplex grades, was developed during this period. 1980s saw the birth of superior Super & Hyper duplex grades. These new grades can survive the most corrosive of environments. Duplex stainless steel was introduced to the oil and gas industry with the development of these superior Super and Hyper duplex grades.

Classification of Duplex Stainless steel 

Below are a few of the popular duplex stainless steel grades.

Properties of Duplex Stainless Steel  

Corrosion Resistance

Duplex Stainless Steel grades have corrosion resistance properties equivalent or even superior to that of austenitic grades. Higher Chromium content paired with Molybdenum impart it higher resistance to pitting and crevice corrosion. Pitting Resistance Equivalent Number (PREN) is used to rank the grades based on their resistance to pitting. Higher the PREN number, higher will be its resistance to pitting corrosion.

Stress Corrosion Cracking (SCC) Resistance

Austenitic grades are susceptible to SCC whereas ferritic grades have higher resistance to it. Duplex Stainless steels have better SCC resistance compared to austenitic grades but inferior to ferritic stainless steel grades. In duplex stainless steel grades SCC initiates in austenitic region but is arrested at ferritic crystal structure present at the boundary, thus preventing propagation of crack and increasing uptime.

Mechanical Strength

Duplex Stainless steels have higher mechanical strength than austenitic grades and ferritic grades. It permits use of thinner sections and helps in weight reduction. Machining of duplex stainless steel is difficult and slower compared to austenite, owing to its higher mechanical strength.

Welding

Welding is difficult compared to austenite. Close control on heat input and cooling rate is required in order to avoid formation of undesired intermetallic phases.

Advantage of Duplex stainless steel over its austenitic counter parts- 

• Higher strength permits use of thinner gauges which is useful when lighter fabrications are desired
• Stainless steel prices are based mainly on one particular commodity i.e. Nickel. Duplex stainless steel have relatively low Nickel content, thus making it immune to commodity price fluctuations.

Both the above factors make Duplex stainless steel relatively cheaper with superior properties for specific applications.

Applications and Uses of Duplex Stainless Steels

• Chemical processing, transport and storage – pressure vessels, tanks, piping, and heat exchangers
• Oil and gas exploration and processing equipment – piping, tubing, and heat exchangers
• Marine and other high chloride environments
• Pulp and paper industry – digesters, bleaching equipment, and stock-handling systems
• Cargo tanks for ships and trucks
• Food processing equipment
• Biofuels plants

Available Grades

About Us

Ambica Steels Limited is one of the leading stainless steel long products manufacturers from India. We have over 5 decades of experience is stainless steel manufacturing. Our product range of duplex stainless steel covers the following grades-

For any further information on any of the Stainless steel grades, feel free to get in touch with us at info@ambicasteels.com or https://ambicasteels.com/contact-us/

About Author

Aniruddha Bauskar is responsible for New Business Development and Sales at Ambica Steels Limited. He likes reading and writing articles on various industries including Steel, Automobile and FMCG. You can find him on LinkedIn.”