On the Formation of non-metallic inclusions in fe-50 pct Ni type alloys by deoxidation with Mn,Si, and Al

This investigation deals with deoxidation experiments in 30 g lab melts of Fe-50 pct Ni alloys. After deoxidation with different amounts of Mn, Si and Al and their combinations the samples were quenched into water at different times. Metallographic studies comprising light microscopy, scanning electron microscopy, electron microprobe and image analysis were performed. Classical nucleation theory was used for computation of the different supersaturation with oxygen or the deoxidant necessary for homogeneous nucleation. The different deoxidation reactions and the transformation of inclusions due to diffusion of oxygen, or the deoxidant, from or into the inclusions was treated for the different cases of deoxidation. Most deoxidation reactions take place within some seconds. The experimental results were to be used to estimate the pertinent interfacial tensions between the oxides and the melt and the values obtained for the different oxides seemed to be reasonable. The diffusional computations were successfully used for predicting the different transformations taking place. For example, in deoxidation with 0.03 pct Si the oxygen solubility is controlled by the equilibrium with liquid FeO ⋅ SiO₂. The time taken to reach equilibrium is determined by the number of inclusions and the particle size. In deoxidation with 0.1 pct Si or more, the equilibrium is controlled by SiO₂ inclusions and the time taken to reach equilibrium, less than 1 s, is much shorter compared to the samples with 0.03 pct Si. The deoxidation reactions with aluminum were treated in the same way, and it was shown that the number of particles determined the time elapsing before equilibrium with respect to the formation of FeOAl₂O₃ or A1₂O₃. It was further shown that transformation of primary liquid FeOAl₂O₃ with high contents of FeO into solid FeOAl₂O₃ was expected to occur within one second. However, the experiments showed that it took somewhat longer, due to formation of solid FeOAl₂O₃ around the liquid FeOAl₂O₃ inclusions, thereby preventing the diffusion of aluminum into the particles.