This investigation studies the partitioning of Mn and Cu – and Si– between ferrite and cementite in the pearlite of two alloys containing similar Mn and Si concentrations, but different Cu contents, keeping the Cu concentration within the usual industrial limits. The results confirm the partitioning of all the elements between ferrite and cementite during pearlite growth. Furthermore, the presence of nanometric Cu-rich precipitates was detected for an alloy with typical Cu contents for achieving pearlitic structures in spheroidal graphite cast irons. This might be linked to the microsegregation of Cu developed during the solidification step. These precipitates have not been reported before and cannot be ignored as they can affect the transformation kinetics and the mechanical properties of the alloy.

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.

Tempered AISI 4140 steel bars were processed by cold drawing. An effective deformation of ≈0.9 was achieved. The quench and tempered, and the subsequently deformed microstructures were analysed by optical and transmission electron microscopy. Intralath, interlath and spherically shaped carbides were identified in both cases. The deformed specimens revealed a mixed structure of thinned laths and dislocations cell-blocks composed of cell-block and cell boundaries. After the deformation process, alignment and fragmentation of the carbides occurred, mainly of those located in lath boundaries what would explain the partial preservation of the lath boundaries after drawing as they would arrest the destruction of lath martensite boundaries. These microstructural characteristics shall play an important role to enhance grain refinement in subsequent heat treatments.