Mathematical Simulation of Thermal Stress in a Bainitic Steel Railroad Rail with Accelerated Normalization

The purpose of this study was to create a mathematical model which can describe thermal-structural and stress state of a bainitic steel railroad rail in accelerated normalization. The solution of the nonlinear nonstationary problem of heat conduction and termoelasticplasticity is based on the method of finite elements. To describe the conditions of heat exchange, we used boundary conditions of the third kind. We carried out simulations of transforming austenite to pearlite and bainite under isothermal conditions according to Kolmogorov - Avrami - Mehl equation. We describe the transition from isothermal decomposition kinetics of austenite to nonisothermal conditions by the theory of isokinetic reactions involving rules additives. We also offer the option to normalization, namely through heating the rail up to the temperature of 860°C, followed by cooling the air-water mixture for 90 seconds with average speed of cooling the working surface of the head of about 4°C and then in air until its cooling. We show the results of the calculation of temperatures, structures and stresses in a railroad rail to various points of the heat treatment. Findings of the research show that when we normalize bainitic rail steel, we obtain pearlitic-bainitic structure, containing 90-98% of bainite. The martensite present in the structure only in the narrow area near the feather of the sole is 55%. The results show that the use for the manufacture of bainitic steel railroad rails and accelerated normalization as a heat treatment instead of the traditional quenching in oil can significantly reduce the level of residual stresses. We reveal the feasibility of using numerical methods of calculating thermal stresses because experimental methods do not allow us to determine the time strain. The developed software can be used to rationalize the rails heat treatment.

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