Heat treated hi-carbon steel wire. Used for rust, paint, scale, slag removal, de-burring, rough metal surfaces and metal surface prep. Multiple cup diameter options from 1-3/4" to 2-3/4". The MSFS for these brushes is 13,000 RPM. Do not use damaged or rusted brushes. Do not exceed the Max Safe Free Speed MSFS, which is the max no-load RPM. Wear eye protection, appropriate clothing (not loose fitting) and use appropriate machine guards.