The activities of chromium in the liquid systems of iron-chromium and iron-chromium saturated with carbon were determined by a vaporization method using radioactive Cr51. The experiments were carried out at 1630°C up to about 40 at. pct chromium. It was found that iron-chromium system is a regular solution, showing negative deviation from the Raoult’s law. The interchange energy w was found to be about −10 kcal/mole. The value of w calculated from the phase diagram is also negative, but is not in good agreement with the above. It is theoretically inferred that the deviation from the Raoult’s law is larger in iron-chromium system than in iron-nickel system. The addition of carbon lowers the activities of chromium and the interaction parameter εCrC in carbon-saturated system is −12∼−16 over the composition range 0∼10 at. pct chromium.
The absorption phenomena of nitrogen by molten iron alloys (Fe-Cr, Fe-Mn and Fe-Ni binary alloys) were studied and the following results were obtained.With the increase of additional percentage of Cr or Mn to Fe, the saturation value of N2 increased from 0.039 Wt.%N2 in pure iron(at 1600°) up to 4 Wt.%N2 in pure Cr or to 1 Wt.%N2 in pure Mn (at about each melting point). In the case of Fe-Ni alloys, on the contrary, the saturation value decreased with the increase of Ni and the alloys containing over than 80%Ni hardly absorbed N2.The curves representing the relation between the saturation values and compositions are upward concave as shown in Figs. 2,4 and 6. These relations could be explained satisfactorily by the theoretical formula (5) or (6) derived from the statistical thermodynamics under the same conception formerly reported.One of the main conclusions obtaind from the comparison between experimental results and theory is as follows: From the standpoint of atomic arrangement, the metal atoms which have large affinity to N-atoms congregate more than the mean concentration of the alloy around absorbed N-atoms. That is,Cr or Mn atoms congregate around N-atoms in Fe-Cr or Fe-Mn alloys,and Fe-atoms congregate in Fe-Ni alloys.
A study on the origin of non-metallic inclusions was carried out by the use of radio-isotope tracers. The radio-isotope 45Ca or 95Zr was impregnated in the pouring refractories which were the fertile source of inclusions. Mild steel was melted and refined in a 150KVA. Héroult furnace, tapped into the tagged ladle, and an ingot of about 90kg was obtained. Non-metallic inclusions extracted from the ingot, ingot scum, slag and all other radio-active products were collected, weighed and counted.Thus, the behaviors of the eroded refractory materials carried along by the pouring stream were quantitatively traced with the following results:When nozzles, stoppers and fireclay mortar were tagged (pouring temperature: 1550°C) 70.071g of the tagged parts was eroded, 6.04% of which was once transported into the ingot case. 52.3% of the transported refractories was floated as the ingot scum, and the rest (47.7%) was retained in the ingot as non-metallic inclusions. The ratio of the non-metallic inclusions of the tagged refractory origin to the total inclusions in the ingot was 2.86%.In the case when all the refractories of the ladle were tagged (pouring temperature: 1550 °C), inclusions of refractory origin amounted to 8.74% of the total content of inclusions.The behaviors of the eroded refractory materials were found to depend largely on pouring conditions. For instance, when pouring temperature increased, the erosion became stronger, but a larger amount of the eroded materials was floated as ingot scum than in the case of lower pouring temperature, and a cleaner ingot was obtained.
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