Oxidation and morphology of iron and iron alloy droplets.

The oxidation characteristics of iron droplets and a range of binary iron alloy droplets have been investigated at 1600°C. The droplets were generated by a novel technique of melting the iron or iron-alloy, in the form of wire, in a levitation coil. The droplets were held for several minutes in hydrogen prior to releasing them into a mass transfer column separated from the levitation chamber by a thin plastic film. The oxidised droplets were quenched. The oxidising or reducing characteristics of various quenching media were investigated and silicone oil was selected as the quenchant.Pure iron and a range of iron-manganese, iron-chromium and iron-silicon alloys were investigated. Whereas the iron-manganese system showed increasing oxygen pick-up with increasing manganese content, the iron-chromium and iron-silicon systems showed an inversion in oxidation behaviour after an initial increase in oxidation rate. The oxidation characteristics of these systems have been related to the surface morphology of the droplets as revealed by Scanning Electron Microscopy and to SIMS analysis of the thin surface oxide films, and mechanisms are proposed for each system.A mathematical model has been developed which simulates the mass transfer of oxygen by forced convection to an accelerating molten iron droplet. The model generates an almost linear oxidation rate for iron, for the period of flight of the droplet and is in good agreement with the experimental work when the model is compensated for circulation within the droplet. The model predicts that temperature has a minimal effect on oxidation rate whereas droplet size is of major importance. Both of these effects were confirmed by the experimental studies. The roles of the alloying elements in relation to the characteristics of oxide films reduced in hydrogen at 1150°C have also been considered. Reduction mechanisms are postulated to account for the morphology of the reduced droplets and the final oxygen levels in the droplets.