Phase and structural transformations when forming a welded joint from rail steel. Report 1. Thermokinetic diagram of decomposition of supercooled austenite of R350LHT rail steel

A thermokinetic diagram of decomposition of supercooled austenite of R350LHT steel was constructed based on the results of its dilatometric, metallographic and hardness analysis during continuous cooling and in isothermal conditions. It was found that cooling at a rate of 0.1 and 1 °C/s causes the austenite decomposition in R350LHT steel by the pearlite mechanism. After cooling at a lower rate, the pearlite structure is coarser and has lower hardness (289 HV). This is due to the higher temperature range of transformation, in which diffusion processes associated with the transformation of austenite into pearlite occur more actively. In the range of rates from 5 to 10 °C/s, the austenite decomposition occurs according to the pearlite and martensitic mechanism, which leads to the formation of a pearlite-martensite structure. When the austenite of the steel under study is cooled at a rate of 30 and 100 °C/s, the austenite transforms according to the martensitic mechanism, and a martensitic structure with high hardness is formed. With an increase in the cooling rate of R350LHT steel, an increase in hardness is observed from 289 (at 0.1 °C/s) to 864 – 0 896 HV (at 100 and 30 °C/s, respectively). The conducted studies allow the boundaries of the search for optimal parameters of welding and heat treatment modes of the investigated rail steel to be narrowed. To obtain the required structures and physical and mechanical properties (austenite of R350LHT steel undergoes decomposition by the pearlite mechanism), cooling should be carried out at a rate of no more than 1 °С/s.

1. Polevoi E.V., Volkov K.V., Kuznetsov E.P., Golovatenko A.V., Atkonova O.P., Yunusov A.M. Development of technology for the production of differentially heat-strengthened rails at JSC “EVRAZ ZSMK”. In: Improvement of the Quality and Operating Conditions of Rails and Rail Fasteners. Collection of Scientific Papers. Yekaterinburg: OAO “UIM”, 2014, pp. 93–101. (In Russ.).

2. Myers J., Geiger G.H., Poirier D.R. Structure and properties of thermite welds in rails // Welding Journal. 1982. Vol. 258. No. 8. P. 258–268.

3. Tikhomirova L.B., Il’inykh A.S., Galai M.S., Sidorov E.S. Investigation of structure and mechanical properties aluminothermic welded joints of rails. Bulletin of South Ural State University. Series “Metallurgy”. 2016, vol. 16, no. 3, pp. 90–95. (In Russ.). http://doi.org/10.14529/met160313

4. Konovalov A.V., Kurkin A.S., Makarov E.V., Nerovnyi V.M., Yakushin B.F. Theory of Welding Processes. Nerovnyi V.M. ed. Moscow: Bauman St. Tech. Univ., 2007, 559 p. (In Russ.).

5. Weingrill L., Krutzler J., Enzinger N. Temperature field evolution during flash-butt welding of railway rails // Materials Science Forum. 2016. Vol. 879. P. 2088–2093. http://doi.org/10.4028/www.scientific.net/MSF.879.2088

6. Ryuichi Y., Yuichi K., Yasuto F. Experimental examination for understanding of transition behaviour of oxide inclusions on gaspressure weld interface: Joining phenomena of gas pressure welding // Welding International. 2014. Vol. 28. No. 7. P. 510–520. http://doi.org/10.1080/09507116.2012.753237

7. Fujii M., Nakanowatari H., Nariai K. Rail flash-butt welding technology // JFE Technical Report. 2015. No. 20. P. 159–163.

8. Saita K., Karimine K., Ueda M., Iwano K., Yamamoto T., Hiroguchi K. Trends in rail welding technologies and our future approach // Nippon Steel and Sumitomo Metal Technical Report. 2013. No. 105. P. 84–92.

9. Dahl B., Mogard B., Gretoft B., Ulander B. Repair of rails on-site by welding // Svetsaren. 1995. Vol. 50. No. 2. P. 10–14.

10. Takimoto T. Latest welding technology for long rail and its reliability // Tetsu-to-Hagane. 1984. Vol. 70. No. 10. P. 40–45. http://doi.org/10.2355/tetsutohagane1955.70.10_1348

11. Tachikawa H., Uneta T., Nishimoto H., Sasaki Y., Yanal J. Steel welding technologies for civil construction applications // Nippon Steel Technical Report. 2000. Vol. 82. No. 7. P. 35–41.

12. Okumura M., Karimine K., Uchino K., Yurioka N. Development of field fusion welding technology for rail-road rails // Nippon Steel Technical Report. 1995. Vol. 65. No. 4. P. 41–49.

13. Kozyrev N.A., Shevchenko R.A., Usol’tsev A.A., Prudnikov A.N., Bashchenko L.P. Welding of differentially heat-strengthened rails. Industrial testing. Izvestiya. Ferrous Metallurgy. 2020, vol. 63, no. 5, pp. 305–312. (In Russ.). http://doi.org/10.17073/0368-0797-2020-5-305-312

14. Shevchenko R.A., Kozyrev N.A., Kratko S.N., Kryukov R.E., Mikhno A.R. Development of technology for manufacturing rail strings for railway access roads to mines // IOP Conference Series: Earth and Environmental. 2019. Vol. 377. Article 0121021. http://doi.org/10.1088/1755-1315/377/1/012021

15. Kalashnikov E.A., Korolev Yu.A. Rail welding technologies: Trends in Russia and abroad. Put’ i putevoe khozyaistvo. 2015, no. 8, pp. 2–6. (In Russ.).

16. Shur E.A. Influence of structure on rails service life. In: Influence of Metal Matrix Properties on Rails Service Life. Collection of Papers. Yekaterinburg: UIM, 2006, pp. 37–63. (In Russ.).

17. Shur E.A., Rezanov V.A. Complex method of rails resistance welding. Vestnik VNIIZhT. 2012, no. 3, pp. 20–22. (In Russ.).

18. Blanter M.E. Heat Treatment Theory. Moscow: Metallurgiya, 1984, 328 p. (In Russ.).

19. Popov A.A., Popova L.E. Heat-Treater Reference Book. Isothermal and Thermokinetic Diagrams of Supercooled Austenite Decomposition. Moscow; Sverdlovsk: Mashgiz, 1961, 430 p. (In Russ.).

20. Gol’dshtein M.I., Grachev S.V., Veksler Yu.G. Special Steels. Moscow: Metallurgiya, 1985, 408 p. (In Russ.).