Studies on decarburisation and desiliconisation of levitated Fe-C-Si alloy droplets

摘要

The rate of decarburization and desiliconisation has been studied in several Fe-C-Si alloys containing 4 percent C and 0.37-0.71 percent Si using electromagnetically levitated droplets at 1 400 and 1 660 deg C in oxygen and helium atmospheres containing 10 and 20 percent oxygen. It has been found that the decarburization proceeds till a carbon content of about 0.5 percent is reached at a constant rate (the rates are 0.11 and 0.17 percent s respectively for 10 and 20 percent oxygen) that is independent of temperature and silicon content. Beyond this carbon level, not only the decarburization rate has decreased but also the silicon content has started falling down. Optical microscopy studies showed typical white iron structures in the initial stages of decarburization (due to the small size of the droplet as well as rapid cooling) while low carbon acicular ferrite structure when percent O2 was 20 percent and medium carbon structure when percent O_2 was 10 percent. This corresponded to the higher carbon content (-0.38 percent) of the latter. SEM studies did not reveal any oxide layer on the surface but EDS showed that the oxygen concentration was higher (0.02 to 0.24 percent) that extended to about 0.5 to 1 mu m depth suggesting that oxygen gradients do occur even in the initial stages of decarburization due to the high level of turbulence within the droplet due to the electromagnetic effects in levitation. Calculations based on Thermocalc gave logarithmic decrease for decarburisation while it was found to be linear experimentally. This could be due to operating parameters other than diffusion (like convection) taking place or nonequilibrium conditions (created due to turbulence) and shorter times used for levitation.