Prediction of alloy hardenability from thermodynamic and kinetic data

Abstract

A simple algorithm for the calculation of the hardenability of low alloy eutectoid steels is presented. This involves the semiempirical prediction of concentration-dependent CCT start and pearlite velocity curves using fundamental thermodynamic and kinetic data and current theories of nucleation and growth. The combination of analytic cooling curves with these predictions and a pearlite growth model based on site saturation at grain corners leads to a good prediction of the hardenability of steel 4068, extensively examined by Jominy, and a grain-size dependence which is qualitatively correct. It is also demonstrated via Taylor and binomial expansions of the undercooling within the various kinetic expressions that the hardenability at low alloy concentrations is correctly represented by linear or quadratic addition formulas. At the same time, it is unequivocally demonstrated that Grossman type multiplication formulas are theoretically incorrect. The quadratic terms in the addition formula quantify the so-called “synergistic” effects. The most important positive terms involve an interaction between austenite and ferrite stabilizers. Austenite stabilizers, by depressing the effective temperature of nucleation and diffusional growth processes involving partition of ferrite stabilizers in pearlite, make the latter processes more sluggish. In agreement with long-standing experience, it is concluded that the strongest and most economic hardenability effects can be obtained via mixtures which include both austenite and ferrite stabilizers.