STRUCTURE AND MECHANICAL PROPERTIES OF A SERIES OF STRUCTURAL STEEL AFTER HEAT TREATMENT WITH ENDURANCE IN THE INTERCRITICAL TEMPERATURE INTERVAL
Authors: Burova D. 
Published: March 2024

Purpose. To show the effectiveness of various methods of heat treatment of the studied steels with heating in ITI, as a result of which their mechanical properties are increased, which is due to the acquisition of a multiphase structure, one of the components of which in some cases is metastable austenite.
Research methods. Steels 14Г2, 20ГЛ, 20ГФЛ 45Г were investigated. Various types of heat treatment were carried out with heating in the intercritical temperature interval. In the studied steels, a multiphase structure dispersed in chemical composition was obtained. Durometric, metallographic and X-ray research methods were used. Mechanical properties under tension and impact toughness were determined. These properties were compared with those obtained from the studied steels after typical heat treatment.
Obtained results. It is shown that in the investigated steels, when performing various methods of heat treatment, there is no need to obtain a single-phase austenite structure, which is homogeneous in terms of its chemical composition, during heating. The creation of a multiphase dispersed micro-inhomogeneous structure in the studied steels by heat treatment methods, which include regulated heating and holding in ITI, allows to obtain after normalization, hardening and high or low tempering, as well as isothermal tempering, a good combination of strength and plastic properties, the level of which exceeds that obtained after the usual process.
Scientific novelty. The paper considers an alternative point of view, according to which to improve the mechanical and service properties of steels, it is necessary to obtain multiphase dispersed micro-inhomogeneous structures in them, consisting of martensite, bainite, ferrite, carbides, carbonitrides and metastable residual austenite, as well as their various combinations. In order to obtain such structures, it is advisable to use heating and holding in ITI during the heat treatment of pre-eutectoid steels, in which austenite is formed, along with ferrite and carbides. At the same time, austenite has chemical micro-heterogeneity, because the carbon concentration at its boundary with carbide is significantly higher than at the boundary with ferrite. This makes it possible to obtain a multiphase dispersed micro-inhomogeneous structure after heat treatment.
Practical value. Increasing the mechanical properties of steels makes it possible to increase the operational stability of machine parts, which is an important task of materials science. One of the directions of its solution is to obtain a multiphase structure in steels, one of the components of which is metastable austenite, in which the dynamic deformation martensitic transformation (DSMT) and the effect of self-hardening under load (SHL) occur under load. Methods of heat treatment with heating at ITI are easily implemented in production conditions and allow for energy savings. Modes of their implementation should be chosen taking into account obtaining the necessary properties, the initial structure, its defects and the cross-section of the products.