STUDY OF THE OXIDATION DEGREE OF CHROMIUM IN OXIDE-FLUORIDE SLAGS FOR ESR

Billets from high-chromium steels possessing the necessary complex of mechanical and corrosion properties are widely used in the manufacture of critical products of heavy and power engineering. One of the effective technologies widely used in the manufacture of such work-pieces is electroslag remelting (ESR). ESR, included in the production process chain, for the management of refining and solidification processes allows to ensure a high homogeneity of metallurgical characteristics (chemical composition, structure, non-metallic inclusions, etc.) of the metal and, as a result, the complex of mechanical properties of the product. The choice of slag, maintaining its optimum oxidation-reduction potential at an optimum level, is a prerequisite for the effective refining of high-chromium steels at the ESR, since chromium and other elements present in the slag in various oxidation degree participate in the transport of oxygen from the gas phase to the liquid metal. From standpoint of the theory of electronic structure of slag systems, the effect of slag oxidation (equilibrium partial pressure of oxygen Р_O2 ) on the oxidation degree of chromium has been studied for widely used slags in Russia such as ANF-1, ANF-6, and ANF-29. Dependence of the ratio of Cr⁺³/Cr⁺² concentrations on the temperature, oxidation level and optical slag basicity is established. A thermodynamic model of changing the oxidation state of chromium in slag depending on its oxidation is presented. The calculated results are compared with the experimental data for slag systems at a temperature of 1873 K. It is shown that the average oxidation degree of chromium decreases with increasing temperature, decreasing of the oxygen partial pressure and the optical slag basicity. The presence of fluorine in the slag affects the varia- tion ratio Cr⁺³/Cr⁺². It is shown that with decrease in the oxygen partial pressure from 10^-4 to 10^-12Pa at a temperature of 1873 K, the average value of chromium oxidation degree in fluoride-oxide slags decreases from +3 to +2. A correlation is proposed, which makes it possible to estimate the Cr⁺³/Cr⁺² ratio in fluoride-oxide slags, taking into account the temperature and oxidation of slag.

oxidation degree, partial oxygen pressure, electroslag remelt- ing, optical slag basicity, electronegativity

1. Kern T.-U., Scarlin B., Zeiler G. etc. The European COST536 project for the development of new high temperature rotor materials. 16. Proc. 17th IFM. 2008, 1081 p. 2.

2. Utkina K.N., Balikoev A.G., Levkov L.Ya., Dub V.S. etc. Principal technology of production of a new nanostructured corrosion-resis17. tant duplex steel. In.: Sb. dokladov: 19­ya Konferentsiya molodykh spetsialistov po yadernym energeticheskim ustanovkam, 13 aprelya 2017g., Podol’sk [19th Conference of Young Specialists on Nuclear 18. Power Plants on April, 13, 2017, Podolsk]. AO OKB “GIDROPRESS”, Podol’sk, 2017, pp.. 351–359. (In Russ.).

3. Mitchell A., Reyes-Carmona F., Wei C.-H. Deoxidation in the electroslag process. Proc. of 39th Electric Furnace Conference. 1981, pp. 103–107.

4. Reitz J., Maurischat M., Friedrich B. Optimized control of slag chemistry for the electroslag remelting of large size ingots. Proc. of IFM. 2008, pp. 28–36.

5. Hernandez-Morales B., Mitchell A. Review of mathematical models of fluid flow, heat transfer and mass transfer in electroslag remelting process. Ironmaking and Steelmaking. 1999, vol. 26, no. 6, pp. 423–438.

6. Ponomarenko A.G., Inozemtseva E.N. On the valence of metals in oxide and salt melts. In.: Nauchnye soobshcheniya 4­i Vsesoyuz. konf. po stroeniyu i svoistvam metallicheskikh i shlakovykh rasplavov [Scientific Reports of the 4th All-Union. Conf. on the structure and properties of metallic and slag melts]. Part. 3. 1980, pp. 67–70. (In Russ.).

7. Pavlov A.V. Fiziko­khimicheskie svoistva polivalentnykh elementov v rasplavakh i razrabotka energoresursosberegayushchikh metallurgicheskikh tekhnologii: dis. ... dokt. tekhn. nauk [Physicochemical properties of polyvalent elements in melts and development of energy-resource-saving metallurgical technologies]. Moscow, 2002, 351 p. (In Russ.).

8. Bartie N.J. The effects of temperature, slag chemistry and oxygen partial pressure on the behaviour of chromium oxide in melter slags. University of Stellenbosch, 2004, 137 p.

9. Jahanshahi S., Sun S., Zhang L. Recent developments in physicochemical characterisation and modelling of ferroalloy slag systems. In: 10th Int. Ferroalloys Congress INFACON. South Africa. Februa­ ry 2004, pp. 316–332.

10. Mikelsons J. Degree of oxidation of iron in СaO – SiО2 – FeОn slag melts as a Function of the oxygen partial pressure of the gas phase // Archiv für das Eisenhüttenwesen. 1982. Vol. 53. No. 6. P. 251 – 265.

11. Khrapko S.A. Termodinamicheskaya model’ sistemy metall­shlak dlya ASU i mashinnykh eksperimentov po optimizatsii tekhnologii staleplavil’nogo protsessa: dis. ... kand. tekhn. nauk [Thermodynamic model of the metal-slag system for automatic control systems and machine experiments on optimization of the technology of the steelmaking process]. Donetsk: DPI, 1990, 81 p. (In Russ.).

12. Mitchell A., Et’en M. Titanium losses in the process of electroslag remelting. In: Elektroshlakovyi pereplav [Electroslag remelting]. Issue. 2. Moscow: Metallurgiya, 1971, pp. 161–169. (In Russ.).

13. Biele H., Pateisky G., Fleischer H.J. The reactions of titanium and silicon with Al2O3-CaO-CaF2 slags in the ESR process. Journal of Vacuum Science and Technology. 1972, vol. 9, no. 6, pp. 1318–1321.

14. Levkov L.Ya. Teoreticheskie predposylki i prakticheskie metody upravleniya fiziko­khimicheskimi i teplofizicheskimi protsessami pri elektroshlakovom pereplave, opredelyayushchie kachestvo otvetstvennykh izdelii: dis. ... dokt. tekhn. nauk [Theoretical prerequisites and practical methods of controlling physical-chemical and thermophysical processes in electroslag remelting, which determine the quality of critical products]. Moscow, 2016, 339 p. (In Russ.).

15. Okoukoni P.I. Razrabotka elementov SAPR tekhnologii plavki stali: dis.... kand. tekhn. nauk [Development of CAD elements for steel melting technology]. Donetsk: DPI, 1993, 168 p. (In Russ.).

16. Xiao Y., Holappa L. Slags in ferroalloys production-review of present knowledge. The Journal of The South African Institute of Mining and Metallurgy. 2004, August, pp. 437–439.

17. Schwerdtfeger K., Mirzayousef-Jadid A. Redox Equilibria of Chromium in Calcium Silicate Base Melts. In: Proc. of the Belton Symposium. 2000. AIME ISS, pp. 108–119.

18. Mohanty A., Kay A. Activity of chromic oxide in the CaF2-CaOCr2O3 and CaF2-Al2O3-Cr2O3 systems. Metal. Trans. B. 1975, vol. 6, pp. 159–166.

19. Lijun Wang, Seshadri Seetharaman. Experimental Studies on the Sulphide Capacities of CaO-SiO2-CrOx Slags. Metallurgical and Materials Transactions B. 2010, vol. 41, Issue 2, pp. 367–373.

20. Yan B., Zhang J., Song Q. Thermodynamic behaviour of transition metal (Cr, Ti, Nb, V) oxides in molten slags. In: Proc. of “MOLTEN – 2009”, Chile. 2009. Chapter 1, pp. 309-317.

21. Morita K., Sano N. Activity of chromium oxide in CaO-SiO2 based slags at 1873 K. In: VII Int. Conf. on Molten Slags Fluxes and Salts (South Africa), 2004, pp. 113–117.

22. Morita K., Inoue A., Takayama N., Sano N. The solubility of MgO∙Cr2O3 in MgO-Al2O3-SiO2-CaO slag at 1600 °C under reducing conditions. Tetsu­to­Hagane. 1988, vol. 74, no. 6, pp. 999–1005.

23. Pauling L. The Nature of the Chemical Bond. Cornell University Press , USA, 1967. (Russ. ed.: Pauling L. Priroda khimicheskoi svyazi. Moscow – Leningrad: Goskhimizdat, 1947, 440 p.).

24. Mendeleev 0.4.3 documentation. Electronic resource. Available at URL: http://mendeleev.readthedocs.io/en/stable/data.html#electronegativity (Accessed 12.03.2018.)

25. Keyan, Xue Dongfeng. Estimation of electronegativity values of ele ments in different valence states. J. of Physical Chemistry A. 2006, vol. 110, Issue 39, pp. 11332–11337.

26. Cherkasov A.R., Galkin V.I., Zueva E.M., Cherkasov R.A. The concept of electronegativity. The current state of the problem. Russian Chemical Reviews. 1998, vol. 67, no. 5, pp. 375–392.

27. Wegman Dwight D. Investigation into critical parameters which determine the oxygen refining capability of the slag during electroslag remelting of alloy 718: theses and dissertations: Dr. Tech. Sci. Diss. 1993, 180 p.

28. Lakomskii V.V., Grigorenko G.M. Approach to evaluation of the basicity of slag melt in gas-slag-metal system. Sovremennaya elektrometallurgiya. 2009, no. 2, pp. 48–49. (In Russ.).

29. Povolotskii D.Ya. Fiziko­khimicheskie osnovy protsessov proizvodstva stali: Uchebnoe posobie dlya vuzov [Physicochemical foundations of steel production processes: A manual for universities]. Chelyabinsk: Izd-vo YuUrGU, 2006, 183 p. (In Russ.).

30. Rudnenko T.B., Ponomarenko A.G., Inozemtseva E.N. etc. Thermodynamic evaluation of the distribution of elements between slag and metal phases in the process of ESR. Problemy spetsial’noi elektrometallurgii. 1987, no. 4, pp. 15–21. (In Russ.).

31. Klechkovskii V.M. Raspredelenie atomnykh elektronov i pravilo posledovatel’nogo zapolneniya (n + l)­grupp [Distribution of atomic electrons and the rule of successive filling of (n + l)-groups]. Moscow: Atomizdat, 1968, 432 p. (In Russ.).

32. Konovalov Yu.V. Statisticheskoe modelirovanie s ispol’zovaniem regressionnogo analiza: Metodicheskie ukazaniya k vypolneniyu kursovoi raboty po distsipline “Komp’yuternoe i statisticheskoe modelirovanie” [Statistical modeling using regression analysis: Methodical instructions for the execution of the course work on the discipline “Computer and Statistical Modeling”]. Moscow: Izd-vo MGTU im. N.E. Baumana, 2013, 73 p. (In Russ.).

33. Pei W., Wijk O.Experimental study on the activity of chromium oxide in the CaO-SiO2-Al2O3-MgOsat.-CrOx slag. Scand. J. Metallurgy. 1994, vol. 23, pp. 228–235.

34. Morita K., Mori M., Sano N. etc. Activity of chromium oxide and phase relations for the CaO-SiO2-CrOx system at 1873 K under moderately reducing conditions. Steel Research. 1999, no. 8-9, pp. 319–324.