Formation of microstructure in rail steel grinding balls depending on quenching medium parameters

Abstract

Studies of the formation of microstructure of grinding balls from the rejects of rail steel were carried out during their quenching in various polymer media. At the first stage, based on studies of the cooling capacity of solutions of polymers PCM and Thermovit with varying concentrations and temperatures, the authors constructed the cooling curves of grinding balls made of K76F rail steel. It was found that at concentration of these polymers in an aqueous solution of 2 and 4 %, cooling rate of grinding balls made of K76F steel is almost identical at solution temperatures of 20 and 30 °C and significantly decreases when the temperature of the polymer solution increases to 40 °C. At the same time, the most noticeable decrease in the cooling rate is characteristic of PCM polymer with its concentration at the level of 2 %. At the second stage, the authors carried out metallographic studies of the microstructure of grinding balls made of K76F rail steel, which were quenched in laboratory conditions using polymers PCM and Thermovit with concentrations of 2 – 4 % and temperature of 20 – 40 °C. As a result, it was determined that the use of the PCM solution for quenching balls provides a significantly higher quality of microstructure and hardness of heat-treated balls compared to the use of the Thermovit polymer. At the same time, varying the concentration and temperature of the PCM polymer quenching medium allows one to obtain grinding balls with different performance characteristics that determine the potential areas of their application. Thus, quenching of balls in a solution of the specified polymer with concentration of 2 % and temperature of 20 – 30 °C ensures the production of balls with high hardness (corresponding to the IV hardness group according to the state standard GOST 7524 – 2015), and the use of a solution of the same polymer with concentration of 4 % and temperature of 20 – 30 °С for quenching creates the possibility of producing balls with lower hardness, but potentially high impact resistance.