The thesis is solves the scientific and technical problem of increasing the structural homogeneity of the metal of contact butt welded joints of heat-strengthened railway rails made of high-carbon rail steels by improving the technology and regimes of controlled in- duction heat treatment and bringing the mechanical properties of the welded joint closer to the base metal of the rail and increasing its wear resistance in operating conditions.
A universal complex methodology has been developed, which is based on mathe- matical and physical modeling of electromagnetic, thermal and structural-phase processes using welded model samples of small geometric shapes, with a transition based on the theory of similarity to welded joints of real rails and allows determining the most favora- ble modes of induction thermal processing of welded joints of various types of railway rails with the provision of the necessary complex of improving their properties.
The kinetics of phase transformations were established and a thermokinetic diagram was constructed during cooling of a butt-welded joint of high-carbon rail steel K76F mi- croalloyed with vanadium during its heat treatment. It was found that the reduced carbon content in the welded joint of this steel at the stage of its heating and cooling shifts the be- ginning and end of structural transformations towards higher temperatures. It was estab- lished that in the process of the thermal cycle of the induction heat treatment of the welded joint of K76F steel, the critical cooling rates of the weld metal at which the martensitic structure is formed are, on average, 16 °С/s in the temperature range of 800...500 °С when cooled according to the exponential law and 10 °C/s when cooled according to the linear law. Cooling according to the exponential law makes it possible to obtain a more dispersed
structure of the metal. It was established that an increase in the cooling rate of the welded joint leads to a change in the volume fraction and dimensions of the carbide phase in pear- lite. At low cooling rates, a characteristic granular structure of pearlite is revealed, which partially transforms into a globular and lamellar structure at an increased rate, which in- creases the mechanical properties of the metal in the zone of the welded joint. According to the results of mechanical studies, it was found that at a cooling rate of 8...12 °С/s in the zone of the welded joint, the overall value of microhardness increases to the level of HV0,01 3500-3700 MPa, which corresponds to the regulated norms of technical standards for contact welded connection of heat-strengthened rails. When the speed is increased above 16 °С/s, the microhardness reaches the values of HV0,01 5400 MPa, which already exceeds the regulated limit. According to the results of physical modeling of induction maintenance, the most favorable technological modes of the process were established, namely, an average metal heating rate of 7 °С/s to temperatures of 880...900 °С, holding time at these temperatures of 70 s, and a cooling rate within 8...12 °С /s up to temperatures of 430...370 °С, at which the formation of hardening structures does not occur and the deviation of the hardness of the metal in the heat-treated zone of the welded joint from the hardness level of the base metal of the rail decreases to 15%.
The developed three-dimensional mathematical model of the induction system "in- ductor - rail" taking into account the laws of propagation of electromagnetic and thermal fields and non-linear physical characteristics of the steel rail, made it possible to create an original design of the inductor and provides uniform volumetric heating of the welded joints of thermally strengthened railway rails of the P65 type , a relatively small heating width and a small level of reactive power generation that needs compensation.
The technological modes of induction heat treatment of contact butt welded joints of heat-strengthened rails of type R65 made of high-carbon rail steel K76F proposed in the dissertation do not lead to the complete disappearance of characteristic zones with reduced hardness along the fusion line and weakening zones, but allow to reduce their size and lev- el of deviation hardness from the hardness of the base metal of the rail, to obtain a more uniform structure of the metal of the welded joint with the absence of unwanted hardening structures, to increase the grain score and wear resistance of the metal.
