Identification of the intermetallic sigma phase in superduplex steels using metallographic analysis methods

The class of duplex and super duplex steels, with its unique combination of high corrosion resistance and strength, is increasingly used in the chemical, oil and gas and shipbuilding industries The process of formation of intermetallic σ-phase in duplex and super duplex steels is described. The presence of this phase in the structure reduces the physical and mechanical properties of this class of steel. The majority of metallographic research methods allow the most complete identification of the main structural components of duplex steels - α- and γ-phase; however it is rather difficult to determine the presence of intermetallic σ-phase in the structure. In order to reveal this structural component the microstructure of super duplex steel samples after etching by various methods – electrolytic etching in 10% aqueous oxalic acid solution and in 20% aqueous NaOH solution, chemical etching in Beraha, Kalling and Piranha reagents were investigated. Samples for metallographic studies were selected from blanks of different profile sizes, as well as with different heat treatments. Electrolytic etching in both 10% oxalic acid and 20% NaOH solution has a number of drawbacks: deep rastrations (oxalic acid), similar colouring of ferrite and σ-phase (alkali) which can make structure identification difficult when using low magnifications. It has been established that for full visual identification of the structural components of duplex and super duplex steels by means of optical microscopy, the most reliable method is the use of chemical etching techniques with Beraha and Kalling reagents. Both reagents produce a similar colouring pattern for the phases: the austenitic grains are light grey, the ferrite phase is dark grey and the σ-phase remains white.The study was carried out with financial support from the Ministry of Science and Higher Education of the Russian Federation (state assignment for fundamental scientific research No. FENU-2023-0013 (2023013ГЗ)).