The use of duplex steels has steadily increased from the time their development started in the 1970s. The content of comparatively more expensive nickel can be reduced by adding nitrogen, specifically for chemical tankers, for water systems, in the oil and gas industry, and in the paper industry. Consequently, production costs are reduced. This indicates that the manufacture of duplex steels has more than doubled in the past 10 years. However, since duplex steels are used in applications that are critical for safety, strict control of their quality is mandatory. Optical emission spectrometry (OES) technology enables precise verification of materials and positive material identification (PMI) to avoid fatal outcomes of material mix-ups.
The structure of duplex, super-duplex, and hyper-duplex steels is austenitic and ferritic, hence the term “duplex.” They are classified as stainless steels. Their superior corrosion resistance, excellent weldability properties, and high structural stability set them apart.
Average composition of typical duplex, super-duplex, and hyper-duplex steels
||0.08 – 0.22
||0.15 – 0.4
||0.3 – 0.6
The Important Role of OES
One of the properties of duplex steels is the higher nitrogen content, which largely enhances their corrosion resistance and solid solution strengthening owing to the evident austenite formation. Hence, duplex steels can be easily identified by analyzing their nitrogen content. However, on site, this can be achieved only with the help of OES technology. While in the laboratory, the benefits of using OES rather than the combustion method include complete analysis, much easier specimen preparation, and considerably lower operating costs.
If it is not feasible to perform the analysis in the laboratory (such as in the case of petrochemical plants and when unwieldy items are used), for instance, the PMI-MASTER Smart portable spark spectrometer can be combined with a Hitachi High-Tech Analytical Science UVTouch smart probe to carry out a high-performance analysis on site, including the measurement of nitrogen. The dynamic spectrum evaluation, patented optics in lightweight construction, and optimized excitation enable the analysis to be performed close to laboratory levels.
In laboratory settings, the smallest stationary OES analyzer, such as the Hitachi High-Tech Analytical Science FOUNDRY-MASTER, has a working range of 0.05%–1.2% as well as a precision level that is otherwise observed only in much more huge spark spectrometers.
The Basics of Duplex Steel Composition
The important properties are the result of four alloying elements: Nitrogen (N), Chromium (Cr), Nickel (Ni), and Molybdenum (Mo). Cr is a ferrite former and offers the required corrosion resistance even at higher temperatures. Mo supports the effects of Cr and considerably increases the resistance against crevice corrosion and pitting in a chloride-containing environment. Despite being a ferrite former, the concentration of Mo is generally limited to a maximum of 7.5%.
Nitrogen is an inexpensive alloying element. However, it has a major impact on the properties. It considerably increases the toughness and strength of the duplex steel and supports the corrosion resistance. Another significant role played by N at the time of fabrication is that it delays intermetallic phase forming, thereby conserving the dual microstructure in the production steps. Since it is a robust austenitic former, it can be an alternative to some amount of the high-cost Nickel in the alloy.
Finally, Ni functions as the austenitic stabilizer, promoting the transformation of the crystal structure from body-centered cubic, or ferritic, to face-centered cubic, or austenitic. A typical chemical make-up of duplex steel (1.4462, S32205) contains 22.5% Cr, 3.25% Mo, 5.0% Ni, 0.18% N, and 0.03% C.
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, FOUNDRY-MASTER Optimum, for testing.
Guido Odenthal, Engineering Office, Odenthal
This information has been sourced, reviewed and adapted from materials provided by Hitachi High-Tech Analytical Science.
For more information on this source, please visit Hitachi High-Tech Analytical Science.