ISOTHERMAL AND ANISOTHERMAL DECOMPOSITION OF AUSTENITE INTO FERRITE IN 9CR1MO STEELS - KINETIC AND MICROSTRUCTURAL ASPECTS

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-degrees-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'' (t0) : Y(alpha) = 1-exp{-[K(t-t0)]N} with K = 1/(t63.2%-t0). The half-reaction time seems to depend on the mean austenite grain size as predicted by the CAHN relation : t1/2 = 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 (THETA(i.t.)) where the austenite to ferrite transformation is quite different : For THETA(i.t.) > 700-degrees-C : Microstructure is characterized by equiaxed grains of ferrite showing coarse ''pearlitic'' M23C6 carbide morphology. Ferrite nucleation seems to occur preferentially on austenitic comer boundaries, without early saturation of the potential nucleation sites during the isothermal ferrite growth. For THETA(i.t.) < 700-degrees-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 785-degrees-C, show that there is a quite fast carbon depletion of austenite during the early stages of the isothermal reaction at low temperatures (almost-equal-to 650-degrees-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.