These gray iron wheels are less expensive than ductile and forged steel wheels. They are practical for use with light loads on wood block or hardened smooth concrete in applications where they are not exposed to any abuse. On untreated concrete they tend to cause rapid floor deterioration. The only metal wheels we recommend for applications which may involve abuse are Ductile, Forged, or Machined Steel wheels. Abuse includes things like shock loads, impact, poor floors or floor obstructions, overload, and use with mechanically powered equipment.”