EVALUATION OF THE MECHANICAL PROPERTIES OF DIFFUSION LAYER IN THE PROCESS OF MICROARC STEEL VANADATION

Traditional processes of thermochemical treatment of steel have a longer duration, so there are proposed the new methods of intensification of diffusion saturation with high-energy impacts on the material surface. In the process of micro-arc alloying the steel product is immersed in a container filled with powder of coal, and is heated by passing electric current. In a powder environment, microdischarges are formed, which are concentrated around the product and create an area of gas discharge with the formation of a carbonaceous gas environment, which enables carburizing of steel. The application of coating containing diffusant allows forming coating of a carbide type due to simultaneous carbon diffusion into alloying elements. The influence of micro-arc surface alloying of steel with vanadium on mechanical properties of diffusion coatings is studied, and the primary mechanism of steel hardening at microarc alloying is revealed. Cylindrical samples of 20 steel were used; the source diffusant was a powder of ferrovanadium. Current density on the sample surface was 0.3 A/cm2, total duration of the process was 3 min. The mechanical properties of coatings were evaluated by means of indentation using pyramidal indenter, at loads of 2.5 mN, 20 mN and 100mN. The diffusion layer with thickness of 170 – 180 μm consists of a base with hardness of 8 – 9 GPa, containing mild etching inclusions of up to 5 μm with microhardness of 21 – 25 GPa. The base of the layer represents an α-solid solution of vanadium in iron, and inclusions are carbides of VC0.863 type. By atomic force microscopy it was established, that the surface relief is defined by single, relatively large carbide particles with a size of up to 3 μm, and by plural nano-sized carbide particles, which act as  the strengthening phase, providing high microhardness of the coating. By method of indentation of the hardened layer cross section using different loads hardening effect of the carbide particles is proven. Estimation of possible mechanisms of hardening have shown that the greatest contribution to diffusion layer hardening is made by dispersion component significantly increasing the yield stress of α-solid solution of iron in comparison with the initial state, which is 38 times greater than the contribution of solid-solution hardening.

1. Voroshnin L.G., Mendeleeva O.L., Smetkin V.A. Teoriya i tekhnologiya khimiko-termicheskoi obrabotki: ucheb. posobie [Theory and technology of chemical-thermal treatment: Manual]. Moscow; Minsk: Novoe znanie, 2010, 304 p. (In Russ.).

2. Berlin E.V., Koval’ N.N., Seidman L.A. Plazmennaya khimikotermicheskaya obrabotka poverkhnosti stal’nykh detalei [Plasma chemical-thermal treatment of the surface of steel parts]. Moscow: Tekhnosfera, 2012, 464 c. (In Russ.).

3. Suminov I.V., Belkin P.N., Epel’fel’d A.V.‚ Lyudin V.B., Krit B.L., Borisov A.M. Plazmenno-elektroliticheskoe modifitsirovanie poverkhnosti metallov i splavov. V 2-kh t. T. 1 [Plasma-electrolytic 630 Известия высших учебных заведений. Черная металлургия. 2018. Том 61. № 8 modification of the surface of metals and alloys. In 2 vols. Vol. 1]. Suminov I.V. ed. Moscow: Tekhnosfera, 2011, 464 p. (In Russ.).

4. Tyurin Yu.N., Zhadkevich M.L. Plazmennye uprochnyayushchie tekhnologii [Plasma strengthening technologies]. Kiev: Naukova dumka, 2008, 216 p. (In Russ.).

5. Sosnin N.A., Ermakov S.A., Topolyanskii P.A. Plazmennye tekhnologii. Svarka, nanesenie pokrytii, uprochnenie [Plasma technologies. Welding, coating, hardening]. Moscow: Mashinostroenie, 2008, 406 p. (In Russ.).

6. Kidin I.N., Andryushechkin V.I., Volkov V.A., Kholin A.S. Elektrokhimiko- termicheskaya obrabotka metallov i splavov [Electrochemical and thermal treatment of metals and alloys]. Moscow: Metallurgiya, 1978, 320 p. (In Russ.).

7. Spiridonov N.V., Kobyakov O.S., Kupriyanov I.L. Plazmennye i lazernye metody uprochneniya detalei mashin [Plasma and laser methods for machine parts hardening]. Minsk: Vysheishaya shkola, 1988, 155 p. (In Russ.).

8. Mazanko V.F., Pokoev A.V., Mironov V.M. etc. Diffuzionnye protsessy v metallakh pod deistviem magnitnykh polei i impul’snykh deformatsii. T. 2 [Diffusion processes in metals under the influence of magnetic fields and impulse deformations. Vol. 2]. Moscow: Mashinostroenie, 2006, 323 p. (In Russ.).

9. Stepanov M.S., Dombrovskii Yu.M., Pustovoit V.N. Diffusion saturation of carbon steel under microarc heating. Metal Science and Heat Treatment. 2017, vol. 59, no. 1-2, pp. 55–59.

10. Stepanov M.S., Dombrovskii Yu.M., Pustovoit V.N. Micro-Arc Diffusion impregnation of steel with carbon and carbide-forming elements. Metal Science and Heat Treatment. 2017, vol. 59, no. 5-6, pp. 308–312.

11. Dombrovskii Yu.M., Stepanov M.S. Formation of carbide type coating in the process of microarc steel vanadation. Izvestiya VUZov. Chernaya metallurgiya = Izvestiya. Ferrous Metallurgy. 2017, vol. 60, no. 4, pp. 262–267. (In Russ.).

12. Stepanov M.S., Dombrovskii Yu.M. Thermodynamic analysis of carbide layer formation in steel with microarc saturation by molybdenum. Steel in Translation. 2016, vol. 46, no. 2, pp. 79–82.

13. Oliver W.C., Pharr G.M.. Measurement of hardness and elastic modulus by instrumented indentation: Advances in understanding and refinements to methodology. J. Mater. Res. 2004, vol. 19, no. 1, pp. 3–20.

14. Schuh С.А. Nanoindentation studies of materials. Materials Today. 2006, vol. 9, no. 5, pp. 32–40.

15. Fuqian Yang, James C.M.L. Micro and nano mechanical testing of materials and devices. New York: Springer, 2008, 387 р.

16. GOST R 8.748-2011. Metally i splavy. Izmerenie tverdosti i drugikh kharakteristik materialov pri instrumental’nom indentirovanii. Chast’ 1. Metod ispytanii. Utv. i vveden v deistvie prikazom federal’nogo agentstva po tekhnicheskomu regulirovaniyu i metrologii ot 13 dekabrya 2011 g. № 1071 [GOST R 8.748-2011. Metals and alloys. Measurement of hardness and other characteristics of

17. materials in tool indenting. Part 1. Test method. Approved and put into effect by the order of the Federal Agency for Technical Regulation and Metrology of December 13, 2011, no. 1071]. (In Russ.).

18. Gol’dshtein M.I., Farber V.M. Dispersionnoe uprochnenie stali [Dispersion hardening of steel]. Moscow: Metallurgiya, 1979, 208 p. (In Russ.).

19. Gol’dshtein M.I., Litvinov V.S., Bronfin B.M. Metallofizika vysokoprochnykh splavov [Metallophysics of high-strength alloys]. Moscow: Metallurgiya, 1986, 312 p. (In Russ.).

20. Gol’dshtein M.I., Grachev S.V., Veksler Yu.G. Spetsial’nye stali: Uchebnik dlya vuzov [Special steels: University manual]. Moscow: MISIS, 1999, 408 p. (In Russ.).

21. Arzamasov B.N., Brostrem V.A., Bushe N.A. etc. Konstruktsionnye materialy: Spravochnik [Structural materials: Reference book]. Arzamasov B.N. ed. Moscow: Mashinostroenie, 1990, 688 p. (In Russ.).