Determination of the PLE/NPLE transition boundary in the Fe-C-Si system: Application to ferrite growing in bull-eye structure in spheroidal graphite cast irons under continuous cooling conditions

Abstract

The ferrite growth in SG cast irons is expected to take place at temperatures lower than T_α, when C_α/γ−C_α/G turns to positive values. Nevertheless, once the alloy reaches T_αo it could grow driven by C_γ/α−C_γ. In spite of the presence of C_γ/α−C_γ at temperatures lower than T_αo, no ferrite growth is evidenced for intermediate temperatures between T_αo and T_α under continuous cooling conditions. Carbon flux driven by C_γ/α−C_γ recalls the one that rules proeutectoid ferrite in steels. Nevertheless, unlike steels, in case of SG cast irons, silicon and alloying elements (Mn and Cu are the more common ones) develop a segregation profile that is inherited by austenite; later on these profiles are acquired by the final microstructure since no diffusion of alloying elements is expected to take place during solid-state transformations [1]. Considering all this, growth in SG cast irons was studied at austenite in contact with graphite, as it is accepted to start right there [1,2]. To set the composition at this point for the different samples, measured microsegregation profiles from a previous work [3] were employed. On the other hand, Thermo-Calc software (TC) was used for calculating carbon at the beginning of the solid-state transformation. Later, isopleth FeCSi isotherm sections were built using TC for determining the transition temperatures from – slow– partitioning local equilibrium (PLE) to –fast– negligible partitioning local equilibrium (NPLE) [4]. These constructions were necessary since no data was available neither for the temperature range of interest in SG cast irons and nor for the high silicon contents registered at austenite in contact with graphite. In the calculation of these transition temperatures is the answer to the absence of ferrite growth during the referenced temperature gap for continuous cooling conditions. This should be taken into account when developing solid-state growth models to get a more accurate description of ferrite growth during continuous cooling in SG cast irons. In addition, an explanation for the pearlite promoting effect of alloying elements is proposed.