HEAT TREATMENT OF CONSTRUCTURAL STEEL 20GL, 25GFL 45G WITH HEATING IN THE INTERCRITICAL TEMPERATURE INTERVAL (ITI) TO INCREASE MECHANICAL PROPERTIES

Abstract

In the work, steels 20GL, 25GFL 45G 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.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, tempering 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. 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 border with carbide is significantly higher than at the border with ferrite. This makes it possible to obtain a multiphase dispersed micro-inhomogeneous structure after heat treatment.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.