Cement Ball Mill Grdg Media Chemical Composition. 1.Feature: 1) In general, grdg steel ball has high hardness: surface hardness to 55-68 HRC, volume hardness of 50-63 HRC. 2) impact toughness high: impact toughness more than 12 J / cm2. 3) broken rate is low: the actual broken rate is not higher than 1%.