Transformation martensitique et l'etat mecanique des phases

Abstract

Many hypotheses have been proposed to explain the particular nature of the martensitic transformation of steel (interruption of the transformation at a certain temperature and its subsequent start again during further cooling) (1). According to one of the most plausible hypotheses triaxial compression is said to occur during the transformation of austenite to martensite. Previous work( 2,3) has shown, however, that normal tensile stresses can only be obtained in the bulk of the new phase. The initial phase exists without the influence of tangential stress. Experimentally derived facts and functions are the basis of the present research. From experience it is known that the degree of cold work in the initial phase (austenite (4)) increases with the development of the martensitic transformation. In addition it is shown that the increase in strength in the absence of cold work in the initial phase (e.g. during the thermal stabilization of austenite) lowers the start temperature, T mc , of the martensitic transformation (5–7). Moreover, during examination of this phenomenon it was found that the difference in free energy of austenite and martensite depends on the irreversible deformation of the crystal lattice during the transformation. Calculations show an exact correlation between the expression for values of the work of deformation of the austenite crystal lattice and the free energy of this transformation. Thus the thermal conditions for the start of the martensitic transformation are revealed to be determined by the initial mechanical state of the austenite. In the present research, these various discovered dependencies are studied during the entire course of the martensitic transformation, from start to finish. On this basis it is possible to formulate the equation of the martensitic curve and to perform calculations pertaining to the martensitic diagram. The equation was obtained by comparing the variable applied thermodynamics with the changes in mechanical state of the austenite during the transformation.