Abstract:In order to further improve the strength and toughness of mining mill balls, Box-Behnken design-response surface method (BBD-RSM) was adopted in this paper. Austenitizing temperature, austenitizing holding time, tempering temperature and tempering holding time were selected as independent variable influencing factors, Rockwell hardness and impact toughness of mill balls were taken as response target values, and a polynomial regression equation model was established. At the same time, OM, SEM, XRD and other analytical methods were used to study the microstructure and phase characteristics of the grinding ball under different heat treatment conditions. The results show that the significant effects of heat treatment parameters on the hardness of the ball are in the order of tempering temperature ＞ austenitizing temperature ＞ austenitizing holding time ＞ tempering holding time, and the significant effects on the impact toughness of the ball are in the order of tempering temperature ＞ tempering holding time ＞ austenitizing temperature ＞ austenitizing holding time. The optimum heat treatment parameters were obtained as follows: austenitizing temperature 885℃, austenitizing holding time 4.8h, tempering temperature 400℃, tempering holding time 3.5h, Rockwell hardness of 59.84HRC, impact toughness of 6.16J, and the relative error of verification experiment was ＜ 5%. It is proved that the response surface method is accurate and scientific in optimizing the heat treatment parameters of grinding ball. The content of alloying elements in austenite, the size, shape and distribution of carbides, the transformation behavior and proportion of residual austenite are the main reasons for the regular change of Rockwell hardness and impact work of grinding ball.
