[Back]

E. Povoden-Karadeniz, W. Mayer, E. Kozeschnik:
"Thermodynamic modeling of carbon clusters in martensite";
Poster: CALPHADXLIV, Loano; 2015-05-31 - 2015-06-05.

Metastable carbon-rich cluster stages, dissolving about 12 at.% C, are observed in martensite at low temperatures. These features are either interpreted as decomposition product of the martensitic structure by ordering [1] or clustering [2]. This cluster phase is of particular interest since its transformation to transition carbide phases during re-heating influences the further microstructural evolution of martensitic steel. To understand these interrelations, a thermodynamic model description for the C-cluster phase is developed, which reproduces observed composition and (meta)stability ranges in various steel grades. The model considers the C-cluster phase as particular compartments of the martensite, described by (Me)(C,Va)3. Its description (Me)1(C,Va)1 essentially assumes that trapped C preferentially collects ("self-trapping") in 1/3rd of the interstitial sites of martensite (Me)1(Va)3. The full cluster-description adopts all metallic end-members and interactions from the martensite "parent" phase. Me-C energies and interactions among C and vacancies determine the stability of the cluster phase. Typically, cluster formation and C-segregation to sinks such as dislocations compete during thermal treatment of martensitic steel. We combine the thermodynamic modeling of both mechanisms in thermokinetic simulations of low to medium carbon steels and predict the constrained formation of transition carbides and cementite during tempering.
[1] C. Zener, Trans. AIME 167 (1946) 550.
[2] J.-M.R. Genin, P.A. Flinn, Trans. AIME (1968) 1419.