ORALS

Now-a-days, high Si steel with bainitic structure has wide applications in railway and automobile industries due to its properties of high strength, high ductility and high toughness. These properties of the bainitic steels can be varied by the variation of austempering temperature and time [1, 2]. The present work represents the crevice corrosion behavior of a set of newly developed high strength and highly ductile multiphase steels consisting of various fractions of bainite, retained austenite (RA), intercritical ferrite (IF) and pearlite. The steels were made by various combinations of continuous cooling in the ferrite-pearlite zone followed by isothermal heat treatment in the bainite zone. Crevice corrosion tests of the multiphase steel specimens were carried out in 3.5 wt.% NaCl solution for 9 months. Steel rusts after crevice tests were analysed with the help of scanning electron microscopy (SEM), Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR). Ultrasonically cleaned multiphase steel specimens were characterized with the help of SEM and optical profilometry. Multiphase interaction and rust constituents uniquely control the crevice corrosion behaviour of the steels. Considering the effect of various fractions of bainite, RA, IF and pearlite present in the multiphase steels and their tendency to form the galvanic cell corrosion, it has been observed that the multiphase steel isothermally heat treated at 300℃ after 0 s of continuous cooling time from austenitization temperature shows less corrosion. Though, all the multiphase steel specimens have same identical chemical composition, they exhibit different modes and various degrees of corrosion attack due to the difference in the microstructures, morphological distribution and fraction of different constituent phases and their subsequent formation of micro galvanic couples. The combination of high strength and ductility in association with excellent crevice corrosion resistance of the multiphase steels implies its great potential in rail applications.

References:
[1] A. Varshney, S. Sangal, S. Kundu, K. Mondal, Super strong and highly ductile low alloy multiphase steels consisting of bainite, ferrite and retained austenite, Mater. Des. 95 (2016) 75–88.\n[2] A.P. Moon, S. Sangal, S. Layek, S. Giribaskar, K. Mondal, Corrosion behavior of high-strength bainitic rail steels, Metall. Mater. Trans. A 46 (2015) 1500–1518.