DOI: 10.52150/2522-9117-2026-40-002
O. I. Babachenko1, D. Sc. (Tech.), Corr. Member., Senior Researcher, ORCID 0000-0001-7501-4173
G. A. Kononenko1,*, D. Sc. (Tech.), Senior Researcher, ORCID 0000-0001-7446-4105
O. A. Podolska1, Researcher, PhD Student, ORCID 0000-0002-4032-4275
O. A. Shpak1, Junior Researcher, ORCID 0009-0005-2797-4790
O. L. Safronov1, Junior Researcher, ORCID 0009-0007-1308-5380
1 Iron and Steel Institute of Z. I. Nekrasov National Academy of Sciences of Ukraine
* Corresponding author: perlit@ua.fm
FRACTURE FEATURES DURING FRACTURE TOUGHNESS TESTING DEPENDING ON THE STRENGTH LEVEL OF STEEL FOR RAILWAY WHEELS
Abstract. Increasing railway operating speeds and axle loads impose more stringent requirements on the fracture resistance of wheel steels. Fracture toughness (KIC) is a critical parameter governing the reliability and operational safety of railway wheels. A fundamental strength-toughness trade-off exists; however, recent studies have demonstrated the possibility of mitigating this limitation through optimization of chemical composition and heat treatment. Purpose of the study. To investigate the effect of vanadium and molybdenum microalloying, as well as heat treatment conditions (single and double heat treatment with prior normalization), on the structural parameters, mechanical properties, and fracture toughness of R7 wheel steels, and to establish the relationship between microstructure, strength, and fracture behavior. Material and research methodology. Four industrial heats were investigated: a non-microalloyed steel (No. 4), vanadium-microalloyed steels (No. 1 and No. 2), and a vanadium–molybdenum microalloyed steel (No. 3). Static tensile tests, hardness measurements, impact toughness tests, and fracture toughness (KIC) measurements were performed. Microstructural analysis was carried out using optical microscopy, while fractographic analysis was performed using scanning electron microscopy. Main results. Vanadium microalloying resulted in grain refinement by 29–35% (from 0.034 mm to 0.022–0.024 mm) and doubled the ferrite content (from 8% to 15–16%). Double heat treatment was effective only for microalloyed steels, providing additional grain refinement of 9–22%. The increase in KIC due to microalloying reached 65–78% (from 60 to 100–108 MPa·m¹/²). The highest fracture toughness (118.5 MPa·m¹/²) was achieved for the V–Mo microalloyed steel after double heat treatment, indicating a synergistic effect. At the same time, the tensile strength of this specimen was 854 N/mm², which was the lowest among the investigated steels. A high KIC level corresponded to a ductile dimple fracture characterized by pronounced relief, a significant fraction of the fibrous component, well-developed tear ridges, and the absence of intergranular cracks. A low KIC level (60.2 MPa·m¹/²) corresponded to quasi-cleavage fracture with relatively flat fracture topography, a low fraction of the fibrous component, shallow facets, and the presence of intergranular cracks. Conclusions. The classical strength–toughness trade-off is not absolute. The decisive factor is not the strength level itself, but the microstructure responsible for achieving it. Vanadium microalloying, especially in combination with molybdenum addition and double heat treatment, enables the formation of a favorable microstructure (refined grain structure, dispersed carbides, and increased ferrite content), which promotes the transition from a quasi-brittle to a ductile fracture mechanism and increases KIC by more than 1.5 times while maintaining a high strength level. The obtained results are of considerable practical importance for the development of next-generation wheel steels with enhanced fracture resistance.
Keywords: fracture toughness (K1С), microalloying, heat treatment, average grain size, pearlite, microhardness, ferrite, crack resistance.
For citation: Babachenko, O. I., Kononenko, G. A., Podolska, O. A., Shpak, O. A., & Safronov, O. L. (2026). Fracture features during fracture toughness testing depending on the strength level of steel for railway wheels. Fundamental and applied problems of ferrous metallurgy, 40. 22-43. https://doi.org/10.52150/2522-9117-2026-40-002
References
1. Bi, N., Tang, H., Shi, Z., Wang, X., Han, F., & Liang, J. (2023). Effects of Vanadium Microalloying and Intercritical Annealing on Yield Strength–Ductility Trade-Offs of Medium-Manganese Steels. Materials. 2023. 16. https://doi.org/10.3390/ma16062220
2. Mudda, S., Hegde, A., Sharma, S. et al. (2026). A review of the latest developments regarding the heat treatment effects on hardness, impact toughness, and microstructure in AISI 1040, 4140, and 4340 medium carbon structural steels. Discover Materials, 6(1). https://doi.org/10.1007/s43939-025-00532-z
3. Zurnadzhy, V., Stavrovskaia, V., Chabak, Y., Petryshynets, I., Efremenko, B., Wu, K., Efremenko, V., & Brykov, M. (2024). Enhancing the Tensile Properties and Ductile-Brittle Transition Behavior of the EN S355 Grade Rolled Steel via Cost-Saving Processing Routes. Materials, 17(9), 1958. https://doi.org/10.3390/ma17091958
4. Brykov, M, Mierzwiński, D, Efremenko, V, Girzhon, V, Shalomeev, V, Shyrokov, O V, Petryshynets, I, Klymov, O, & Kapustyan, O. (2024). Increasing the Strength and Impact Toughness of Carbon Steel Using a Nanosized Eutectoid Resulting from Time-Controlled Quenching. Materials. 17(15). 3696. https://doi.org/10.3390/ma17153696
5. Zhou, S.-t., Li, Z.-d., Yang, C.-f., Xie, S.-k., Yong, Q.-l. (2019). Cleavage fracture and microstructural effects on the toughness of a medium carbon pearlitic steel for high-speed railway. Materials Science and Engineering: A, 761, 138036. https://doi.org/10.1016/j.msea.2019.138036
6. Strnadel B., Hausild P., Nohal S., Matocha K. Statistical scatter in the fracture toughness and Charpy impact energy of pearlitic steel (2007). Mater. Sci. Eng.: A. 486. 1-2. 208-214. https://doi.org/10.1016/j.msea.2007.08.081
7. Chattopadhyay, C., Sangal, S., Mondal, K., & Garg, A. (2012). Improved wear resistance of medium carbon microalloyed bainitic steels. Wear, 289, 168–179. https://doi.org/10.1016/j.wear.2012.03.005
8. Andrade, H., Akben, M. G., & Jonas, J. J. (1983). Effect of molybdenum, niobium, and vanadium on static recovery and recrystallization and on solute strengthening in microalloyed steels. Metallurgical Transactions A, 14, 1967-1977.
9. Zajac, S. (2002). Precipitation and grain refinement in vanadium-containing steels. Iron steel vanadium titanium. 23(1). 35-48.
10. Capdevila, C., Ferrer, J. P., García-Mateo, C., Caballero, F. G., López, V., & de Andrés, C. G. (2006). Influence of Deformation and Molybdenum Content on Acicular Ferrite Formation in Medium Carbon Steels. ISIJ Int., 46, 1093–1100. https://doi.org/10.2355/isijinternational.46.1093
11. Rezende, A. B., Fernandes, F. M., Fonseca, S. T., Farina, P. F. S., Goldenstein, H., & Mei, P. R. (2020). Effect of alloy elements in time temperature transformation diagrams of railway wheels. Defect and Diffusion Forum, 400, 11-20. https://doi.org/10.4028/www.scientific.net/DDF.400.11
12. Sakamoto, H., Toyama, K., & Hirakawa, K. (2000). Fracture toughness of medium-high carbon steel for railroad wheel. Mater. Sci. Eng.: A, 285(1-2), 288–292. https://doi.org/10.1016/s0921-5093(00)00648-1
13. Trivedi, R., Bhumika, Tandon, R., Mishra, G., Singh, R., Singh, J. K., Mahobia, G. S., Chauhan, A., Sarma, S., Ghosh, A., Karmakar, A., & Patra, S. (2024). Study of strength and toughness in pearlitic wheel steel via microstructural alteration. Materials Today Communications, 39, 109255. https://doi.org/10.1016/j.mtcomm.2024.109255
Рукопис надійшов до редакції / Received 01.04.2026
Рекомендовано до друку / Accepted 28.05.2026
Опубліковано / Published 30.05.2026
