REVISITING THE NATURE OF SITES OF MARTENSITE NUCLEATION DURING STEEL HARDENING

Presence of microvolumes most prepared for the martensite emergence in austenite is discussed. Aming many works dealing with martensitic transformations, rare works are devoted to the location of martensite origin. This aspect of transformation is important, since it allows us to obtain new knowledge about scenarios for γ  →  α transformation development during quenching of steel. The martensite embryos are submicron austenite volumes that are most prepared for phase transition and are characterized by increased energy. Experimental results were obtained by the methods of high-temperature metallography. Steel structure observed as a result of vacuum etching was studied, as well as the surface relief caused by shear during the martensitic transformation. The resulting structural patterns made it possible to observe most of the possible places for martensite emergence: nonmetallic inclusions, twins, high-angle and small-angle grain boundaries, previously formed martensite crystals, dislocations and elements of the disclination structure. It is shown that a high dislocation density is observed in the twin area, which facilitates nucleation of martensite as a result of disappearance of part of elastic energy of the dislocation when atoms inside the embryo are rearranged. When nucleation occurs on the grain boundaries, energy is released, which is used to construct a new interphase boundary and to compensate emerging elastic energy. The relative energy of the boundaries of different types was estimated by the method of multi-beam interferometry. The depth of the grooves that were formed on the surface by thermal etching was measured. Elements of disclination structure resulting from inhomogeneous deformation were observed, which are also sites of germinal centers formation. It is noted that nanoareas with ferromagnetic order, which are present in paramagnetic austenite, may not be observed with the help of the technique used in this work. However, magnetism plays a decisive role in realization of one or another scenario of the development of phase transformation in steels. Obtaining data on the interaction of ferromagnetic areas in austenite with each other, with crystal lattice defects, the magnetic field, and data on their lifetime, number and size is an important task for future research.

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