Etude enthalpimetrique des differentes etapes du revenu des aciers a ressort 50CV4 et 61SC7: Stabilite de l'austenite residuelle, transformations η → χ et χ → θ

Abstract

During annealing, the heat flux accompanying the matrix relaxation shows the following steps: formation of ϵ phase, then of η, both by carbon diffusion, transformation of η into χ then into θ by diffusion of the matrix atoms and finally coalescence of carbides. Residual austenite is transformed by immersion into liquid nitrogen, provided it has not been stabilized during a long wait at room temperature. It is shown that: the state (α + χ) corresponds to the enthalpy minimum and shows higher excess C p than (α + θ); the transformation χ → θ spreads over a long time within a broad range of temperature; and the enthalpy-hardness correlation is roughly linear. The kinetic study allowed us to determine the activation energy for the transformation η → χ and to draw the curves of reaction progress versus time at different temperatures and so to simulate different types of annealings.