FEATURES OF THE CRACKS’ FORMATION IN HETEROPHASE INCLUSIONS OF THE “EUTECTICS OF INCLUSION – MATRIX” TYPE

Abstract

The purpose of the article − to study of crack nucleation features in heterophase inclusions of the “eutectic of inclusion-matrix” type during steel deformation. Methods. The research was conducted after deformation for a number of different grades steel samples in the temperature range of 20...1 200 °С on Instron-1195 and
IMASH-5C with special grippers, with a gripper movement speed of 1 680 mm/min. Research methods were used: petrography, micro-X-ray spectral analysis (Cameca MS-4, Nanolab-7), optical microscopy (Neophot-21). Results. It is found that the variety of phases composing the heterophase inclusions of the "eutectic of inclusion-matrix" type leads to their different behaviour under conditions of plastic deformation. It is shown that the nucleation of brittle or viscous microcracks occurs along the internal interfacial boundaries between the metal matrix and the second phase of the eutectic. It is defined that the cracks’ nature is determined by the plasticity level of the inclusion phases and the deformation temperature. It is shown that the critical degrees of samples’ deformation, at which appreciable microcracks along the internal interphase boundaries appeared, depend on the temperature and the nature of the inclusion phase “eutectic of inclusion – matrix”. It is found that the values of deformation critical degrees determine the level of cohesive strength for internal interphase boundaries in heterophase inclusions “eutectic of inclusion – matrix”. Scientific novelty. The features of microcracks nucleation in heterophase inclusions of the “eutectic of inclusion − matrix” type are determined. It is shown that the nature of the microcracks formed along the interphase boundaries depends on the temperature, plasticity level and combination conditions of brittle and plastic phases in inclusions of the “eutectic of inclusion − matrix” type as well as on the deformation temperature. It is shown that the critical degrees of steels deformation, when microcracks appeared along the internal interphase boundaries, determine the cohesive strength of these boundaries and depend on the temperature and the nature of the inclusion phase “eutectic of inclusion − matrix” type. Practical value. The use of the obtained results will make it possible to develop technologies for producing steels with regulated types of non-metallic heterophase inclusions that will allow to increase considerably their technological and operational characteristics, and also to prevent the formation of various defects during steel pressure treatment and product operation.