Abstract
Repetitive drop tests were performed on Ni-hard and high chromium cast iron grinding balls with material toughnesses varied by heat treatment. Instrumented impact tests and bending fatigue tests were also performed on bar specimens with the same heat treatments, and correlations between drop strength and other strength characteristics were studied. In the drop tests from varous 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 showed that drop strength correlated better with fatigue strength and hardness than with impact toughness (KId) In both irons. With the results and the fracture morphologies, the stresses causing spalling by repeated drops were inferred to be ; (1) cyclic tension by reflection of impact stress waves, and (2)internal tensile stresses caused by surface plastic deformation. Breakage from the ball center is a different type of fracture that occurs in balls with high casting stress and in very brittle balls.
