Décomposition isotherme et anisotherme de l'austenite en ferrite dans des aciers de base 9Cr1Mo : aspects cinétiques et microstructuraux

Abstract
Isothermal and anisothermal (under-cooling) phase transformations from austenite have been studied in a 9Cr1Mo alloy (EM10) martensitic steel. In isothermal conditions, this steel displays typical C-curve TTT diagram for temperatures ranging from 600 to 800°C. For lower temperatures, no bainitic reaction seems to occur for isothermal anneal times up to 500h. The isothermal austenite to ferrite reaction has been described by the JOHNSON-MEHL-AVRAMI relation with the introduction of a "nucleation time" [MATH]. The half-reaction time seems to depend on the mean austenite grain size as predicted by the CAHN relation : t_1/2=Kd^m, where the experimental m values can be related to the ferrite nucleation conditions. From microstructure and kinetic considerations, we can distinguish two domains of the isothermal transformation temperature θ_i,t where the austenite to ferrite transformation is quite different :

* For θ_i,t>700°C : Microstructure is characterized by equiaxed grains of ferrite showing coarse "pearlitic" M₂₃C₆ carbide morphology. Ferrite nucleation seems to occur preferentially on austenitic corner boundaries, without early saturation of the potential nucleation sites during the isothermal ferrite growth.

* For θ_i,t<700°C : The austenite to ferrite transformation is initiated on all the austenitic grain boundaries. Nucleation occurs with "interphase" precipitation of M₂₃C₆ carbides. After about 20% of transformed austenite, M₂X carbonitride precipitation occurs. Finally, at the end of the reaction, there is no more precipitation. Complementary experiments using Martensite start temperature, micro-hardness of residual martensite and Thermo-electric Power measurements for partial transformation of austenite to ferrite at 650, 725 and 78S°C, show that there is a quite fast carbon depletion of austenite during the early stages of the isothermal reaction at low temperatures (≈650°C). Finally, the results obtained under isothermal conditions are used and to discuss the kinetics and microstructure evolutions during anisothermal under-cooling phase transformation from austenite.