Temperature Distribution, Phase Transformations and Residual Stresses in Heat Treatment of Grinding Balls

Abstract

The goal of this work is to model the temperature distribution, phase transformation and residual stresses induced during the heat treatment of 3 and 5 inches diameter grinding balls. In the first step, the radial distribution of temperature inside the balls was calculated and validated experimentally. During the quenching, the model considers factors such as the heating of the water and the formation of a steam layer that surrounds the balls in the beginning of the treatment. In a second step, with the temperature distribution, the CCT curves of the steel and the Koistinen- Marburger equation, the radial distribution of martensite was determined during the heat treatment. Finally, in the third step, the residual stresses field was modeled considering the temperature distribution, the force equilibrium equations and the constitutive thermo-elastic relationships, where the expansion due to the austenite – martensite transformation was included. In the temperature distribution, a good experimental-theoretical agreement was obtained, with differences at the end of the quenching no higher than 0,5 %. Respect to the residual stresses, the model indicates that the maximum tensile values occur at certain depth below the surface of the balls and the experimental evidence of the behavior of the balls in a mill simulator, as well as the measured residual stresses by means of DRX shows an acceptable agreement with the theoretical predictions.