Determination of PLE/NPLE trasition boundaries for ferrite growth in FeCSi system: application to decomposition of austenite in spheroidal graphite cast irons

GARCÍA, LAURA N.; CARAZO, FERNANDO D.

Juriquilla

Conferencia; CALPHAD XLVII Conference; 2018

Texas A&M University

Ferrite growth in spheroidal graphite (SG) cast irons is expected to take place down the lower limit of the three-phase field (Tα), when the difference in carbon concentration between ferrite/austenite and ferrite/graphite interfaces (C_C^(α/γ)-C_C^(α/G)) turns to positive values [1]. Nevertheless, ones the alloy reaches the upper limit of the three-phase field (Tαº) it could grow driven by the difference in carbon content between austenite/ferrite interface and the corresponding to the carbon content in austenite far away from it (C_C^(γ/α)-C_C^γ) [2]. In spite of the presence of C_C^(γ/α)-C_C^γ at temperatures lower than Tαº (Figure 1), no ferrite growth is evidenced for intermediate temperatures between Tαº and Tα under continuous cooling conditions [1].Carbon flux driven by C_C^(γ/α)-C_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.