[ Mining / Duplex ]
[ Mining / Duplex ]
Chemically, duplex grades are characterised by very low carbon content( typically less than 0.03 %), chromium levels ranging from 21.00 to 26.00 %, nickel from 3.50 to 8.00 %, and molybdenum up to 4.50 %. Their toughness and ductility fall between those of austenitic and ferritic stainless steels. Duplex stainless steels are generally classified into three categories based on corrosion resistance: lean duplex, standard duplex, and super duplex. Super duplex grades exhibit enhanced mechanical strength and superior resistance to pitting, crevice corrosion, and stress corrosion cracking compared to both standard duplex and austenitic grades. Duplex stainless steels are widely used in demanding applications such as marine structures, petrochemical and desalination plants, heat exchangers, and the pulp and paper industry. The duplex family is ideally suited to demanding deep-sea mining applications.
Characteristics of duplex Duplex products offer a unique combination of high strength and excellent corrosion resistance due to their dual-phase microstructure. Their approximately twofold strength advantage over austenitic and ferritic grades makes them ideal for weightreduction strategies while ensuring durability in harsh environments. Duplex steels demonstrate excellent resistance to localised corrosion forms, including pitting and crevice corrosion, as well as exceptional resistance to chlorideinduced stress corrosion cracking.
An artists representation of deepsea mining of polymetallic nodules on the seabed. Source: Wikimedia, Gringo from Zemgale https:// creativecommons. org / licenses / bysa / 4.0 / deed. en
They also maintain good mechanical performance at low temperatures( down to – 50 ° C) and up to moderate temperatures( generally below 300 ° C). With proper welding techniques, duplex stainless steels achieve reliable weldability while preserving their advantageous microstructure and mechanical properties. However, their high strength and work-hardening characteristics can pose challenges in machining, particularly in structural applications. The elevated yield strength enables the use of thinner wall sections, reducing overall material weight and contributing to cost efficiency in fabrication and installation.
Fabrication characteristics Corrosion resistance: Duplex stainless steels offer excellent corrosion resistance, particularly in chloride-rich environments like seawater, making them ideal for subsea and offshore applications. Fatigue resistance: High fatigue strength makes duplex steels well-suited for cyclic loading applications, including risers, umbilicals, and other dynamic structures. Weldability, cutting, and bending: Duplex grades combine strength with good formability and weldability. Bending requires greater force than austenitic grades but proceeds similarly beyond the yield point. Cutting demands appropriate tooling to avoid heat-affected zone( HAZ) issues, while welding necessitates strict heat input control to preserve microstructure and prevent cracking. Cost efficiency: The high strength of duplex stainless steels enables reduced wall thickness, lowering material consumption. Combined with good weldability, formability, and long-term corrosion resistance, duplex steels offer significant life-cycle cost savings, particularly in demanding marine and subsea environments.
A bed of manganese nodules from deep offshore of the Cook Islands; photo sourced from Hein et al., 2015, taken during a Japanese research cruise in the year 2000. Nodules range from about 2 to 10 centimeters across. Source: https:// www. usgs. gov / media / images / cook-islands-manganese-nodules
Categories of duplex Duplex stainless steelsare classified into lean, standard, and super duplex grades, each offering varying levels of strength and corrosion resistance to suit specific seabed mining applications.
• Lean Duplex( 21 – 23 % Cr, 1 – 3 % Ni, low Mo and N) provides good corrosion resistance at a lower cost, with easier weldability due to reduced alloying. Grades like 2304 are ideal where moderate corrosion
24 Stainless Steel World November 2025 www. stainless-steel-world. net