Duplex (Semi-Austenitic) Stainless Steels

General Information

Overview
Duplex stainless steels consist of both austenitic and ferritic phases, allowing them to combine the characteristics and advantages of these two stainless steel types.

Strengths

They offer corrosion resistance comparable to austenitic stainless steels while providing superior resistance to stress-corrosion cracking and pitting corrosion compared to both ferritic and austenitic grades. Duplex stainless steels also have higher tensile and yield strength, along with good ductility and toughness, although their toughness is somewhat lower than that of austenitic stainless steels.

Limitations

Compared with austenitic stainless steels, duplex grades have lower elongation.

Typical Applications

Duplex stainless steel is commonly used for shell-and-tube heat exchangers, cooling coils, wastewater treatment systems, oil and gas production equipment, sour gas welds, chemical processing facilities, pulp and paper manufacturing, and food processing applications.

Composition Overview

Compositional Summary

Fe/20–28Cr/1.5–8Ni/0.3–4Mo, with varying additions of Cu, Co, Mn, Si, C, P, S, and N.

Property	Description
Material Family	Metal (ferrous)
Base Material	Fe (Iron)

Microstructure

The microstructure consists of both austenite (fcc) and ferrite (bcc) phases.

Effect of Composition

Certain duplex alloys contain added nitrogen (N) to reduce alloy segregation between the ferrite and austenite phases. This enhances the corrosion resistance of the steel in the as-welded condition.

Processing Properties

Available Forms

Duplex stainless steel is available in sheet, strip, bar, plate, foil, tube plates, and most other wrought product forms.

Forming

Cold working increases the strength of duplex stainless steel to levels that cannot be achieved with ferritic grades or most austenitic stainless steels.

Machining

Machining is more challenging due to the material’s high strength in the annealed condition. There are currently no enhanced-machinability grades available for duplex stainless steel alloys.