Improving the Working Life of Steel Grinding Balls by Optimizing their Hardness and Tenacity

Abstract

The carbon content in steel of the most produced grinding balls, 3 and 5 inch diameter, varies 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 RC in the case of the 3 inch diameter balls. The internal zone, hard and with low tenacity, produces a notorious diminution of the working life of the balls with respect to their theoretical potential. In this context, this work has as its objective to improve the quality of the grinding balls of 3 inch diameter, giving them highe r 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 (TTT) curves for the steel of the balls was determined by means of calorimetric (DSC) and metallographic analyses. Afterwards, a mathematical model (FDM) for the temperature distribution of the balls during their quenching and equalization treatment was done and experimentally validated. Along with the TTT curves and the math ematical 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 balls, with an associate hardness on this area of 53-55 RC, while the standard hardness is maintained in their external zones.