Control of Microstructure during the Eutectoid Transformation in the As‐cast Spheroidal Graphite Cast Iron with »in‐situ« Dilatation Analysis and Quenching Experiments

Abstract

Transformation of austenite can proceed into ferrite and graphite (γFe→αFe+ G) and/or in pearlite (γFe→αFe+Fe3C). Investigation of the eutectoid transformation has been undertaken by evaluation of the “in‐situ” dilatation curves in connection with metallographic examinations, DTA and dilatation analyses of the solid state. On a basis of numerous quantitative relations, such as the relation between ferrite and pearlite fractions in as‐cast spheroidal graphite cast iron (SGI), the ratio between ferrite and pearlite in the microstructure is determined in a very short time. Thus the melt composition was corrected using the “in situ” dilatation analyses by addition of Cu and/or Mn or Si respectively. From the kinetics of austenite transformation and temperature dependence of the ferrite or pearlite growth, the following characteristic temperatures of the eutectoid transformation have been established: the ferrite nucleation T0α, the beginning of the ferrite growth Tα, and pearlite growth Tp, respectively. Both temperatures T0α and Tp are determined from the differential inflection point of the dilatometric curves. For mainly ferrite and pearlite SGI, the kinetic curves, which show the fraction of the single microstructure constituents in the microstructure in dependence of the transformation time, are well represented by the physical sigmoidal Boltzmann model. In case of the kinetics temperature dependence, however, pearlite growth does not follow the sigmoidal transformation course, which results from the recalescence of the pearlite reaction. The time of transformation is a more suitable criterion for the description of the transformation.