Improving the lifespan of Grinding Mill Balls(made by skew rolling machine)

The carbon content of steel which are produced for grinding mill steel balls, 3 and 5 inch diameter, carbon content between 0.6% and 1.1% and, due to the presence of other addition elements, after their standard heat treatment, all the ball zones –external and central –reach high hardness values over 60 HRC in the case of the 3 inch diameter balls. The internal zone, hard and with low tenacity, produces a notorious diminution of the lifespan of the grinding steel balls with respect to their theoretical potential. In this context, this work has as its objective to improve the quality of the grinding steel ball of 3 inch diameter, giving them higher superficial hardness and a central zone with less martensite, and as a consequence, with moderate hardness and adequate tenacity. To perform this, the temperature time transformation curves for the grinding steel ball was determined by means of calorimetric and metallographic analyses. Afterwards, a mathematical model for the temperature distribution of the grinding steel ball during its quenching and equalization treatment was done and experimentally validated. Along with the curves and the mathematical temperature model, new heat treatment conditions were established: a reduced ball quenching time of 55 seconds (instead of the current 80 seconds), with a final central temperature of 500°C (instead of 273°C) and an equalization time of 200 seconds (instead of 40 seconds), and a final ball temperature of 203°C (instead of 139°C). With this new treatment, less martensite was obtained in the central zones of the grinding steel ball, with an associate hardness on this area of 53-55HRC, while the standard hardness is maintained in their external zones.

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