duplex

In metallurgy, duplex refers to a stainless steel or nickel alloy composed of roughly equal proportions of two distinct crystal phases: austenite and ferrite. This two-phase microstructure, typically achieved through controlled cooling and composition balancing, gives duplex stainless steels properties that neither phase possesses alone. Common duplex grades contain 22, 26% chromium, 5, 8% nickel, and 3, 5% molybdenum, with nitrogen added to stabilize austenite and strengthen the steel.

The duplex structure delivers a practical combination of strength and corrosion resistance. Tensile yield strength typically ranges from 450 to 550 MPa, roughly double that of standard 304 austenitic stainless steel. Pitting resistance equivalent number (PREN) values reach 35 or higher in lean duplex grades and exceed 40 in super-duplex formulations, making these alloys suitable for seawater environments and chloride-bearing industrial settings where single-phase austenitic steels fail.

Variants and applications

Lean duplex steels (22% chromium, ~3% molybdenum) balance cost and performance for pulp mills, water treatment plants, and heat exchangers. Super-duplex grades (25, 26% chromium, 4, 5% molybdenum) serve oil and gas subsea equipment, high-temperature corrosive piping, and desalination plants. Hyper-duplex variants exist for extreme environments but are rare in production.

The challenge in duplex metallurgy lies in welding and heat treatment. Rapid cooling favors ferrite over austenite, leaving brittle ferritic phases and reducing toughness. Intermetallic sigma phase can precipitate above 700°C, embrittling the metal. Controlled heat input, preheat, and post-weld annealing around 1080°C are essential. Cold work and solution annealing also require precision to maintain the intended phase balance.

Duplex stainless entered industrial use in the 1930s but remained niche until the 1970s, when advances in nitrogen alloying and welding practice made production reliable. The name reflects the microstructure directly: two distinct crystal structures visible under optical microscopy, each occupying roughly half the volume. This stands in contrast to austenitic stainless steels, which are single-phase, and ferritic stainless steels, which are also single-phase. Duplex alloys occupy a middle ground and have captured a growing share of the corrosion-resistant stainless market, especially in offshore and desalination sectors.