Steel Times International Digital July August 2018

Page 16



Precise material verification Optical emission spectrometry means easy and precise identification of duplex steels via nitrogen content THE development of duplex steels began in the 1970s and their use has steadily increased. This is especially the case in the oil and gas industry, for chemical tankers, the paper industry and for water systems, where the content of much more expensive nickel can be reduced through the 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 safety critical, their quality must be strictly controlled. Optical emission spectrometry (OES) technology provides precise material verification and positive material identification (PMI) in order to prevent the fatal consequences of material mix-ups. Duplex, super-duplex and hyperduplex 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. (see Table 1) The important role of OES A characteristic of duplex steels is

Digital Edition - July/August 2018


C %

Cr %

Ni %

Mo %







0.08 – 0.22






0.15 – 0.4






0.3 – 0.6

Table 1. Average composition of typical duplex, super-duplex and hyper-duplex steels

their 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 their nitrogen content. On site, this is only possible with OES technology. While in the laboratory, the advantages of OES compared to the combustion method include considerably easier specimen preparation, full analysis and much lower operating costs. On-site high performance analysis In cases where analysis in the laboratory is not possible (for example, unwieldy items and 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 analyser, like the Foundry Master Optimum 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 a chloride containing environment. Although it’s also a ferrite former, its concentration is usually limited at 7.5% maximum.

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