Thermodynamic prediction of the eutectoid transformation temperatures of low-alloy steels

The experimental eutectoid transformation temperatures (A₁) of low-alloy steels, as reported in the USS Atlas of I-T diagrams, have been compared to the thermodynamic predictions of a model proposed by Kirkaldy and Venugopalan. The analysis is consistent with the model prediction that Cr atoms are almost fully partitioned, while Ni and Mo atoms are scarcely partitioned, during the eutectoid transformation. This study also shows that Mn atoms are partitioned fully or partly in C-Mn, Cr-Mn, and Mo-Mn steels, while they are scarcely partitioned in Ni-Mn steels. The difference (ΔT) between the orthoequilibrium (OE) eutectoid temperature (A_e1) and the paraequilibrium (PE) eutectoid temperature (A_p1) has been investigated as a function of the content of each substitutional alloying element. The slope of ΔT increases with substitutions of Mo, Ni, Mn, Si, and Cr, with Mo having the least effect, Ni the next-greatest effect, and so on. Considering both Mn partitioning and the slope of ΔT, the equation for the prediction of A₁ temperatures of low-alloy steels proposed by Kirkaldy and Venugopalan is modified. This new equation is in better agreement with the experimental A₁ temperatures.