Partitioning of silicon during pearlite growth in a eutectoid steel

Metastable Eutectoid Transformation in Spheroidal Graphite Cast Iron: Modeling and Validation

This paper presents a new microstructural model of the metastable eutectoid transformation in spheroidal graphite cast irons. The model takes into account the nucleation and growth of pearlite nodules. The nucleation is assumed to be continuous and dependent on the metastable undercooling associated with the upper limit of the three-phase field, while the growth rate is considered to be ruled by the silicon partitioning between ferrite and cementite at the pearlite/austenite front. The initial conditions for the metastable transformation are obtained from a microstructural simulation of solidification, graphite growth, and stable eutectoid transformation. These microstructural models are coupled with the thermal balance solved at a macroscopic level via the finite element method. The experimental validation of the metastable eutectoid model achieved by comparison with measured values of ferrite, graphite, and pearlite fractions at the end of the cooling process demonstrates the sound predictive capabilities of the proposed model.

Paraequilibrium and other Restricted Equilibria

After a review of the historic development of the concept of paraequilibrium in the field of alloyed steels, the basic principles of restricted thermodynamic equilibrium at a migrating interface during phase transformations are outlined. As a demonstration they are first applied to classical nucleation. In addition to the case of paraequilibrium they are then applied to adiabatic and to isochoric conditions. It is argued that a full equilibrium model, with a spike of the intensive property in front of the migrating interface may be more realistic. It is shown that the full equilibrium conditions require about twice as much undercooling or underpressure as the restricted equilibrium conditions because of the consumption of driving force in the spike. The concept of restricted equilibrium only applies to heterogeneous phase transformations. For homogeneous reactions, the same experimental conditions will give full equilibrium.