Abstract
The aim of this research was to confirm the Bhadeshia hypothesis about the tetragonal lattice of the α'-phase, i.e. ferrite in the carbide-free bainite, as well as to compare the results of the thermodynamic calculation of the carbon concentration in the α'-phase and in the residual austenite with the results of the experiments. The objective of the study was to understand how carbon forms and what structure, formation kinetics and behavior it has at the moment when carbide-free bainite is forming in the steel of the following composition: 0.98 С, 1.52 Si, 1.91 Mn, 1.44 Cr, 0.11% V. The originality of this research is in establishing a number of regularities (including the weak tetragonality of the α'-phase) and a marked difference between the calculated (for tetragonal ferrite) and observed carbon concentrations. The authors suggest there may be thin Fe16C2 crystals inside the α'-phase crystals that formed during the spinodal decomposition of the α-phase, then lost a coherent connection with low-carbon formations and eventually became "invisible" in the X-ray diffraction analysis due to their small thicknesses, as it results in a diffuse diffraction maximum which basically blends into the background. During the bainite transformation (which starts as a diffusionless transformation) the carbon content in the α'-phase decreases, apparently as a result of the diffusion transition of carbon from the α'-phase to the γ-phase. After 10 hours of decomposition at 300 °C, when the α'-phase accounts for 41%, the carbon content in the α'-phase drops to 0.26%. After 20-hour long soaking the carbon content in the ferrite drops to 0.16% whereas the amount of α'-ferrite rises to 54%. In the long run, following 30 hours of soaking the carbon content drops to 0.1% with the α'-phase increasing to 83%. The practical relevance of this study lies in obtaining new theoretical and experimental data necessary for understanding the processes that take place in carbide-free bainitic steels, which is important for creating new steels of this class and defining the applicable heat treatment modes.
Keywords
Manganese-silicon high-carbon steel, low-temperature bainite transformation, structure, carbide-free bainite, tetragonal lattice.
1. Shchastlivtsev V.M., Mirzaev D.A., Yakovleva I.L. Struktura termicheski obrabotannoy stali [The structure of heat-treated steel]. Moscow: Metallurgiya, 1994, 228 p.
2. Bhadeshia H.K.D.H., 2001, Bainite in steels. London: The Institute of Materials, UK. 2001. 460.
3. Garcia-Mateo C., Sourmail T., Caballero F.G., et al., Nanostructured steel industrialisation: plausible reality, Materials Science and Technology. 30, (9). 2014. 1071-1078.
4. Agren J.A., 1979, Thermodynamic Analysis of the Fe-C and Fe-N Phase Diagrams. Metal. Trans. 10A, 1979. 1847-1852.
5. Mirzaev D.A., Mirzoev D.A., Buldashev I.V et al. Termodinamicheskiy analiz vozniknoveniya tetragonalnogo beinita v stalyakh [Thermodynamic analysis of the formation of tetragonal bainite in steels]. Physics of Metals and Metallography. 2017, vol. 118, no. 6, pp. 547-553.
6. Mirzaev D.A., Mirzoev A.A., Buldashev I.V. et al. Metastable equilibrium of tetragonal bainitic ferrite and austenite in steels with carbide-free bainite. Vestnik Magnitogorskogo gosudarstvennogo tekhnicheskogo universiteta im.G.I. Nosova [Vestnik of Nosov Magnitogorsk State Technical University]. 2017, vol. 15, no. 1, pp. 27-36.
7. Gustafson P., 1985, Thermodynamic evaluation of the Fe-C system. Scandinavian Journal of Metallurgy. 14(5), 1985, 259-267.
8. Mogutnov B.M., Tomilin I.A., Shvartsman L.A. Termodinamika zhelezo-uglerodistykh splavov [Thermodynamics of iron-carbon alloys]. Moscow: Metallurgiya, 1972, 328 p.
9. Chipman J., 1972, Thermodynamics and phase diagram of the Fe-C system. Metall. Trans. 1972, vol. 3, no. 1, pp. 55–64.
10. Gorelik S.S., Skakov Yu.A., Rastorguev L.N. Rentgenograficheskiy i elektronno-mikroskopicheskiy analiz [X-ray and electron-microscopic analysis]. Moscow: MISiS, 1994, 328 p.
11. Smirnov M.A., Shchastlivtsev V.M., Zhuravlev L.G. Osnovy termicheskoy obrabotki stali [Basics of heat treatment of steels]. Moscow: Nauka i tekhnologii, 2002, 519 p.
12. Shtremel M.A. Razrushenie. V 2 kn. Kn. 2. Razrushenie struktur [Fracture. In 2 books. Book 2. Structural fracture]. Moscow: Publishing House of MISiS, 2015, 976 p.
13. Kurdyumov G.V., Utevskiy L.M., Entin R.I. Prevrashcheniya v zheleze i stali [Transformations in iron and steel]. Moscow: Nauka, 1977, 236 p.
14. Okishev K.Yu., Mirzaev D.A. Spetsialnye stali [Special steels]. Chelyabinsk, Publishing House of SUSU, 2013, p. 7.
15. Kurdyumov G.V., Perkas M.D. On the mechanism of austenite decomposition in the intermediate temperature range.Sb. trudov TsNIICher Met “Problemy metallovedeniya i fiziki metallov” [Problems of Metallurgy and Metal Physics: Proceedings of the Bardin Central Research Institute of Ferrous Metallurgy]. Moscow, 1951, pp. 169-175.
16. Bernstein M.L., Kaputkina L.M., Prokoshkin S.D. Otpusk stali [Tempering]. Moscow: MISiS, 1997, 335 p.
17. Sinclair C.W., Perez M., Veiga G.A., et al. Molecular dynamics study of the ordering of carbon in highly supersaturated α-Fe, Physical Review B, 81, (22), 2010, 224204.
18. Eduard Donazil, Tömas Podravcky, Jiri Svejca, 1982, Untersuchung der Bainitumwandlung in Siliciumstahl. Arch. Eisenhüttewesen, B52, ( 7), 1982, 289-293.
19. Caballero F.G., Miller M.K., Garsia-Mateo C., et al Temperature dependence of carbon supersaturation of ferrite in bainitic steels. Scripta Materialia, 67, 2012. 846-849.