Origins in the 1940s
Duplex stainless steel was developed in the United States during the 1940s, when metallurgists looked for a material combining the strength of ferritic steel with the corrosion resistance and toughness of austenitic steel. The idea was to balance the microstructure so that ferrite and austenite were present in roughly equal proportions, letting each phase compensate for the weaknesses of the other.
The earliest commercial grades were rich in chromium and carbon and contained no deliberate nitrogen addition. They gave useful strength and chloride resistance, but they were difficult to weld and tended to form brittle intermetallic phases after prolonged exposure to elevated temperature, so the first generation was adopted mainly in cast form.
What Makes the Duplex Structure Different
In a conventional austenitic stainless steel the microstructure is essentially single phase. A duplex grade is deliberately two phase, with ferrite and austenite interleaved on a scale of a few micrometres. The ferrite raises strength and improves resistance to chloride stress corrosion cracking, while the austenite provides toughness, weldability and general corrosion resistance.
Yield strength of 400–550 MPa, roughly twice that of ordinary austenitic stainless steel, allowing thinner sections and lower equipment weight
Marked improvement in resistance to pitting, crevice corrosion, stress corrosion cracking and corrosion fatigue, particularly in seawater and high-chloride media
Lower thermal expansion and higher thermal conductivity than austenitic grades, which reduces thermal stress in heat-exchange service
Corrosion performance comparable with high-alloy austenitic stainless steels, and often better where chloride stress corrosion is the controlling risk
Three Generations of Development
The family is conventionally divided into three generations, each defined by better control of composition and of the ferrite-austenite balance.
| Generation | Period | Defining features |
|---|---|---|
| First generation | 1940s–1960s | High chromium and carbon content, no deliberate nitrogen addition. Limited weldability and a tendency to form intermetallic phases, so the grades were used mainly as castings. |
| Second generation | 1970s–1980s | Deliberate nitrogen alloying and improved melting practice. The familiar 22% chromium, 5% nickel family became established in process plant, chemical tankers, pulp and paper equipment and offshore structures. |
| Third generation | late 1980s onward | Super duplex and hyper duplex grades with about 25% chromium, higher molybdenum and nitrogen up to roughly 0.3%. Pitting resistance equivalent numbers exceed 40, and the grades serve seawater systems, desalination plant and offshore flowlines. |
Corrosion Performance in Chloride Media
The reputation of the family rests largely on behaviour where chlorides are present. Under those conditions ordinary austenitic grades can suffer pitting, crevice attack and, above about 60 °C, chloride stress corrosion cracking. The ferrite phase interrupts the path along which such a crack would otherwise propagate through austenitic grain boundaries, which is why duplex components often survive where a standard grade would fail.
Alloying level is normally compared using the pitting resistance equivalent number, calculated as chromium content plus 3.3 times molybdenum content plus 16 times nitrogen content. Values above about 40 indicate suitability for seawater and other aggressive chloride environments, while lower values point to less demanding duty.
Where Duplex Grades Are Used Today
Chemical tankers, storage vessels and process piping handling chlorides and organic acids
Seawater cooling systems, desalination plant and firewater lines
Offshore oil and gas production equipment, flowlines and risers
Pulp and paper digesters, bleaching equipment and heat exchangers
Structural and architectural members exposed to salt-laden atmospheres and de-icing salts
Frequently Asked Questions
Q: When was duplex stainless steel first made?
The first duplex stainless steels were produced in the United States in the 1940s. They used high chromium and carbon contents without deliberate nitrogen, and industrial use was initially limited to castings.
Q: Why is the material called duplex?
Because its microstructure contains two phases, ferrite and austenite, in roughly equal proportions. The two-phase structure provides the combination of high strength, toughness and chloride resistance.
Q: Is it stronger than a standard austenitic grade?
Yes in terms of yield strength. A yield strength of 400–550 MPa is about double that of ordinary austenitic stainless steel, so thinner walls can carry the same load and material weight and cost can be reduced.
Q: What limits the use of duplex grades at high temperature?
Prolonged exposure above roughly 300 °C allows unwanted intermetallic phases to form, which embrittles the material and lowers corrosion resistance. The family is therefore selected for moderate-temperature service.
Q: Are all duplex grades weldable?
Second and third generation grades are welded routinely, with care taken to control heat input, interpass temperature and shielding. First generation grades were much less weldable, which is one reason their use was restricted to cast products.
