Strength evaluation of cast iron grinding balls by repeated drop tests

Abstract

Repeated drop tests were performed on Ni-hard and high-Cr cast iron grinding balls with material toughness varied by heat treatment. Instrumented impact tests and bending fatigue tests were also performed on bar specimens with the same heat treatment, and correlation between drop strength and other strength characteristics were discussed. In the drop tests from various heights, balls fractured by breakage or spalling, with longer life (Nf) at lower drop heights ( H) giving H–Nf curves similar to the S– N curves in fatigue tests. Experiments show that drop strength correlated better with fatigue strength and hardness than with impact toughness( K Id) in both irons. The stress causing spalling by repeated drops was inferred to be repeated contact stress, and internal tensile stress caused by surface plastic deformation assists the fracture. Breakage from the ball center is caused by cyclic tensile radial stress by impact body force, and is assisted by residual casting stress. Breakage type fracture is possible in as cast or very brittle balls, and with drops from high heights.