Effects of Melting Conditions on the Viscosity of Molten Cast Iron

Abstract

  This paper is a report on the results of viscosity measurement of molten cast iron by a rotation viscometer with an internal rotor.
  Experimental procedures were as follows : An alumina crucible was set in a siliconit furnace and heated to a fixed temperature after which a molten iron was poured in the crucible. The furnace was then thrust up and the internal rotor was immersed in the molten iron coaxially with the center line of the crucible. Then the internal rotor was rotated. The internal rotor was made of electrode graphite and heated preliminarily with a heating element. Viscosity torque arising from the rotation of the internal rotor was converted into an electric quantity from which the coefficient of viscosity was obtained.
  The diameter of the internal rotor was 30 mm and the length of its immersed part was 60 mm. The internal diameter of the alumina crucible was 45 mm. The internal diameter of the alumina crucible was 45 mm. The speed of rotation of the internal rotor was 15.7rpm.
  Since a laminar flow was necessary for viscosity measurement with the rotation viscometer, the rotor had to be rotated slowly. Therefore, the viscosity torque was very weak and hard to detect. This viscosity torque was detected as the increase in the load current of the DC shunt motor which rotated the internal rotor. A feedback compensator of a field control system which made the revolution rate uniform was used to raise the sensitivity of measurement. The above revolution rate of 15.7 rpm was the limit, i.e., it was the minimum revolution rate.
  Even with such a high sensitivity measurement, the flow was a little out of the laminar state in low viscosity areas. Hence viscosities in these areas seemed to be measured as a little lower than true values. It was believed, however, that the laminar flow was kept in high viscosity areas and reliable values could be obtained.
  Chemical composition of cast iron used for the viscosity measurement were: 3.26%C, 1.36%Si, 0.46%Mn, 0.072%P, 0.085%S. This iron was melted in a graphite crucible using a kryptol furnace under the following three melting conditions, and effects that these melting conditions had upon viscosity were investigated:
  1) Normal melting: Iron was melted at 1,480°C.
  2) Oxidized melting: 1%Fe₃O₄ was added after normal melting.
  3) Inoculation: 0.5%Ca-Si was added after normal melting.
  It was clarified from results obtained that the viscosity of molten cast iron changed remarkably with melting conditions as summarized in the follwing :
  1) The viscosity of oxidized iron was the highest. It rose rapidly especially at low temperatures and reached, for instance, 25 cp at 1,225°C.
  2) The viscosity of inoculated iron was 3.0-4.3 cp at 1,300-1,250°C which was the same as in normally melted iron and its rising gradient at low temperatures was the most gentle, being only 7.5 cp at 1,225°C.
  3) The viscosity of normally melted iron was 9.4-11.0 cp at 1,225-1,220°C.