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 o 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» (t 0 ): Y(α)=1-exp{-GK(t-t 0 )] N } with K=1/(t 63.2% -t 0 ). 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 o C: Microstructure is characterized by equiaxed grains of ferrite showing coarse «pearlitic» M 23 C 6 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 o C: The austenite to ferrite transformation is initiated on all the austenitic grain boundaries. Nucleation occurs with «interphase» precipitation of M 23 C 6 carbides. After about 20% of transformed austenite, M 2 X 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 785 o C, show that there is a quite fast carbon depletion of austenite during the early stages of the isothermal reaction at low temperatures (≃650 o 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