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INFLUENCE OF MACRO- AND MICROSTRUCTURE OF STEEL GRINDING BALLS ON THEIR IMPACT RESISTANCE

Based on the analysis of the literature and production data, it is shown that the wear resistance of steel grinding balls, which are the main grinding medium for crushing various types of raw materials in the drum type mills, is determined not only by hardness of the surface layer, but also by the quality of their macro- and microstructure. At the same time, there is a complex nature of dependences of the balls impact resistance on the above mentioned parameters with their simultaneous impact, which determines the relevance of research in this direction. Series of experimental studies was carried out in order to obtain scientifically based dependencies of impact resistance of grinding balls on the quality of their macro- and microstructure. The grinding balls with a diameter of40 mm,50 mmand60 mmproduced by OJSC “EVRAZ ZSMK” (Novokuznetsk) and by JSC “Industrial Solutions” (Nizhny Tagil) were used. The research was carried out by metallographic, durometric and fractographic methods of analysis; tests of grinding balls on the impact resistance were performed on the impact drop machine. According to the results of metallographic studies it was determined that the volume-hardened balls are characterized by a three zone structure. The surface hardened layer with a depth from 1.9 to7.4 mmis martensite. The transition zone with thickness from 1.0 to1.7 mmhas a structure martensite + troostite. The central zone occupies the entire remaining volume of the ball and has several varieties of microstructure: ferrite + perlite, martensite + troostite + ferrite, martensite + troostite. On the basis of generalization of the obtained experimental data, it was established that at surface hardness of balls within 42 – 52 HRC the quality of macrostructure (presence of ﬂakes) is the parameter determining impact resistance. For balls with high surface hardness (57 – 61 HRC) the most significant impact on impact resistance is provided by non-uniform microstructure of metal, which causes a significant diﬀerence in hardness across the balls crosssection.