South Ural State University, Chelyabinsk, Russia

G. P. Vyatkin, Dr. Chem., Prof., Advisor to the Rector’s Office, e-mail: viatkingp@susu.ru
N. A. Shaburova, Cand. Eng., Associate Prof., Dept. of Materials Science and Physical Chemistry of Materials, e-mail: shaburovana@susu.ru

South Ural State University, branch in Zlatoust, Zlatoust, Russia

A. N. Anikeev, Cand. Eng., Associate Prof., Dept. of Engineering and Materials Production Technology, e-mail: anikeevan@susu.ru
V. V. Sedukhin, Head of the Educational Laboratory of the Dept. of Engineering and Materials Production Technology, e-mail: sedukhinvv@susu.ru

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)).

1. Nilsson J.-O. Super duplex stainless steels. Materials Science and Technology. 1992. Vol. 8, Iss. 8. pp. 685–700. DOI: 10.1179/mst.1992.8.8.685
2. Galtsov I. A., Korelskaya P. V. Application of duplex steel 1.4462 (Duplex) in modern shipbuilding. Resursosberegayushchie tekhnologii proizvodstva i obrabotki davleniem materialov v mashinostroenii. 2022. No. 2 (39). pp. 76–82.
3. Francis R., Byrne G. Duplex stainless steels – alloys for the 21^st century. Metals. 2021. Vol. 11, Iss. 5. 836. DOI: 10.3390/met11050836
4. Tumakova N. S., Tikhonov V. P., Smirnov A. S., Samokhvalov S. G., Bolshakov A. A., Nazarov V. G., Leushin I. O. Study of casting properties of superduplex and serial austenitic steel, development of technology and production of experimental castings for valve bodies. Trudy NGTU imeni R. Е. Alekseeva. 2015. No. 2 (109). pp. 242–250.
5. Kahar S. Duplex stainless steels – an overview. International Journal of Engineering Research and Applications. 2017. Vol. 7. pp. 27–36. DOI: 10.9790/9622-0704042736
6. Efimushkin A. S., Chumanov I. V., Anikeev A. N., Sedukhin V. V. Basic technological methods in the production of SUPERDUPLEX 25Cr steels and features of their production technology in the conditions of ZMZ. Metallurg. 2022. No. 4. pp. 31–36. DOI: 10.52351/00260827_2022_04_31
7. Chail G., Kangas P. Super and hyper duplex stainless steels: structures, properties and applications. Procedia Structural Integrity. 2016. Vol. 2. pp. 1755–1762. DOI: 10.1016/j.prostr.2016.06.221
8. Chumanov I. V., Shaburova N. A., Sedukhin V. V. Analysis of the variability of the composition of duplex steels in terms of nitrogen and carbon content. Elektrometallurgiya. 2022. No. 10. pp. 29–37. DOI: 10.31044/1684-5781-2022-0-10-29-37
9. Pohl M., Storz O., Glogowski T. Effect of intermetallic precipitations on the properties of duplex stainless steel. Materials Characterization. 2007. Vol. 58. pp. 65–71. DOI: 10.1016/j.matchar.2006.03.015
10. Martins M., Casteletti L. C. Sigma phase morphologies in cast and aged super duplex stainless steel. Materials Characterization. 2009. Vol. 60. pp. 792–795. DOI: 10.1016/J.MATCHAR.2009.01.005
11. Garzon C. M., Ramirez A. J. Growth kinetics of secondary austenite in the welding microstructure of a UNS S32304 duplex stainless steel. Acta Materialia. 2006. Vol. 54, Iss. 12. pp. 3321–3331. DOI: 10.1016/j.actamat.2006.03.018
12. Levina A. V. Formation of structure, phase composition and properties under thermal and deformation effects of austenitic-ferritic 03Kh14N10К5М2Yu2Т steel for elastic elements : Dissertation... of Candidate of Engineering Sciences. Ekaterinburg, 2015. 145 p.
13. Escriba D. M., Materna-Morris E., Plaut R. L., Padilha A. F. Intermetallic phase precipitation in duplex stainless steels during high temperature exposition. Materials Science Forum. 2010. Vol. 636–637. pp. 478–484.
14. Jacob A., Povoden-Karadeniz E. Predictive computations of intermetallic σ-phase evolution in duplex steel. II) Thermo-kinetic simulation in duplex and hyper duplex stainless steels. Calphad. 2020. Vol. 71. 101810. DOI: 10.1016/j.calphad.2020.101810
15. Hsieh C. C., Wu W. Overview of intermetallic sigma (σ) phase precipitation in stainless steels. ISRN Metallurgy. 2012. Vol. 2012. 732471. DOI: 10.5402/2012/732471
16. Rushchits S. V., Shaburova N. A., Sedukhin V. V., Akhmedyanov A. M., Samoilov S. P., Anikeev A. N., Chumanov I. V. Modeling of hot deformation of cast superduplex corrosion-resistant steel. Chernaya metallurgiya. Byulleten nauchno-tekhnicheskoy i ekonomicheskoy informatsii. 2022. Vol. 78. No. 11. pp. 967–977. DOI: 10.32339/0135-5910-2022-11-967-977
17. Kunitskaya I. N., Spektor Ya. I., Salnikov A. S., Orzhitskaya L. K. Features of the structure, properties and technological plasticity of metal products made from corrosion-resistant duplex 03Kh22N5АМ3 steel. Metallovedenie i termicheskaya obrabotka metallov. 2020. No. 6 (780). pp. 3–14.
18. Vander Voort G. F., Manilova E. P. Hints for imaging phases in steels. Advanced Materials and Processes. 2005. Vol. 163, Iss. 2. pp. 32–37.
19. Vander Voort G. F. Applied metallography: The metallography of stainless steels. Journal of the Minerals, Metals and Materials Society. 1989. Vol. 41. pp. 6–11.
20. Beckert M., Klemm H. Handbuch der metallographischen Aetzverfahren. Translated from Germany, 2^nd edition. Moscow : Metallurgiya, 1988. 400 p.
21. Specification Sheet: 2507 (UNS S32750). Available at: https://www.sandmeyersteel.com/images/2507-Spec-Sheet.pdf (accessed: 25.09.2023).
22. Abdolvand R., Atapour M., Shamanian M. Effects of cooling regimes on the microstructural and mechanical properties of the transient liquid phase joints of UNS S32750 super duplex stainless steel/BNi-2/AISI 304 stainless steel. Journal of Materials Science. 2022. Vol. 57. pp. 4383–4398. DOI: 10.1007/s10853-022-06928-z
23. Ferro P., Bonollo F., Timelli G. Sigma phase precipitation modeling in a UNS S32760 superduplex stainless steel. La Metallurgia Italiana. 2012. Vol. 104, Iss. 5. pp. 7–12.