The influence of the Si and Mn concentrations on the kinetics of the bainite transformation in Fe-C-Si-Mn alloys

Five Fe-C-Si-Mn alloys were investigated by dilatometry, optical microscopy, and transmission electron microscopy (TEM) analysis to determine the effects of Si and Mn content and austenitizing temperatures on the kinetics of bainite transformation. Segregation of Mn at prior austenite grain boundaries at a temperature below B_s was detected by the scanning transmission electron microscopy + energy dispersive spectroscopy (STEM + EDS) and secondary ion mass spectroscopy (SIMS) techniques. The fraction of intragranular ferrite increases with Mn content. The time needed for the initiation of the bainite transformation tends to decrease as the austenitizing temperature decreases. The results obtained were compared with those from Fe-0.38C-1.73Si and Fe-0.38C-3.11Mn alloys. Equilibrium and nonequilibrium segregation of Mn at prior austenite grain boundaries and the effect of the interaction between Si and Mn on the segregation of Mn are used to explain the results obtained and the large difference between the kinetics of the two ternary alloys and those of the Fe-C-Si-Mn alloys. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Metals Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.