The development of duplex steels began in the 1970s and their use has steadily increased. Especially in the oil and gas industry, for chemical tankers, in the paper industry and for water systems, where the content of much more expensive nickel can be reduced through addition of nitrogen and as a result, production costs can be lowered. This has meant that the production of duplex steels has more than doubled in the last decade. However, due to the use of duplex steels in applications that are critical for safety, their quality must be strictly controlled. Optical emission spectrometry (OES) technology provides precise material verification and positive material identification (PMI) in order to prevent fatal consequences of material mix-ups.
Duplex, super-duplex and hyper-duplex steels consist of an austenitic and ferritic structure, hence the designation “duplex”, and are classified as stainless steels. They set themselves apart with high structural stability, outstanding weldability characteristics and excellent corrosion resistance.
Average composition of typical duplex, super-duplex and hyper-duplex steels.
The important role of OES
A characteristic of duplex steels is the elevated nitrogen content, which considerably increases their corrosion resistance and solid solution strengthening due to pronounced austenite formation. Duplex steels are therefore easy to identify by means of the nitrogen content. On site, this is only possible with OES technology. Whilst in the laboratory, the advantages of OES compared to the combustion method include considerably easier specimen preparation, full analysis and much lower operating costs.
In cases where analysis in the laboratory is not possible (e.g. unwieldy items, petrochemical plants), the PMI-MASTER Smart portable spark spectrometer in combination with a UVTouch smart probe from Hitachi High-Tech Analytical Science for example can be used to conduct a high-performance analysis on site, including measuring nitrogen. Analysis at close to laboratory level is possible thanks to the patented optics in lightweight construction, dynamic spectrum evaluation and optimised excitation.
In a laboratory, the smallest stationary OES analyzer, like the FOUNDRY-MASTER by Hitachi High-Tech Analytical Science, has a working range of 0.05 – 1.2% and a precision level otherwise seen only in far larger spark spectrometers.
The basics of duplex steel composition
Four alloying elements are responsible for the key properties: Chromium (Cr), Molybdenum (Mo),
Nitrogen (N) and Nickel (Ni). Cr is a ferrite former and provides the necessary corrosion resistance even at elevated temperatures. Mo supports the Cr effects and increases drastically the resistance against pitting and crevice corrosion in chloride containing environment. Although it’s also a ferrite former, its concentration is usually limited at 7.5% max.
Although Nitrogen is a cheap alloying element, it has a major influence on the properties. N supports the corrosion resistance and increases substantially strength and toughness of the Duplex Steel. It has another important role during fabrication by delaying intermetallic phase forming and therefore conserving the dual microstructure during the production steps. As a strong austenitic former it could replace some expensive Nickel in the alloy.
Lastly, Ni is the austenitic stabilizer which promotes the change of the crystal structure from body centred cubic (ferritic) to face-centred cubic (austenitic). A typical chemical composition of duplex steel (1.4462, S32205): C 0.03% Cr 22.5% Ni 5.0% Mo 3.25% N 0.18%.
Guido Odenthal, from Odenthal’s Engineering Office, which is an accredited test laboratory, commented: “Due to their better corrosion resistance and higher strength, duplex and super-duplex steels are being used more and more often in apparatus and pipeline construction over the last few years, either as base materials or also for the entire welded structures. Since duplex steel is easy to distinguish based on the nitrogen content, we use a spark spectrometer from Hitachi High-Tech for testing.”
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Author: Willy Sanders, Product Manager OES