Fig. 1: Duplex microstructure in cross-section under the optical microscope (light: ferrite, dark: austenite)
Duplex steels combine very good corrosion resistance with excellent mechanical properties. They are used in sectors including the chemical industry, construction, and offshore and marine applications. In this case, the W.S. MaterialtestCenter examined the microstructure of a forged component from a plastics processing plant.
The characteristic duplex microstructure
Compared with conventional stainless steels such as 1.4301 or 1.4401, duplex steels have a higher chromium content of more than 20%. Chromium acts as a ferrite-forming element. The content of austenite-forming elements – particularly nickel, nitrogen and manganese – and the heat treatment are carefully controlled to produce a two-phase microstructure at room temperature. This consists of a ferritic matrix containing islands of austenite.
Good mechanical properties even at low temperatures
This distinctive microstructure is visible in the optical micrographs taken by our colleagues at the W.S. MaterialtestCenter. The combination of the two phases gives duplex steels greater resistance to acid attack and improved resistance to stress corrosion cracking compared with conventional CrNiMo steels.
The phase boundary between ferrite and austenite plays a particularly important role in the latter: cracks are often deflected or their growth slowed at these interfaces. This mechanism helps duplex steels maintain good mechanical properties even at low temperatures down to approximately −100 °C.
Conclusion: Duplex steels
The combination of high strength, corrosion resistance and temperature resistance makes duplex steels attractive materials for demanding applications.
