United Abrasives- SAIT 22600 Type 27 4-1/2-Inch x 1/4-Inch x 7/8-Inch 13300 Max RPM Z-Tech - Z24R Zirconium Depressed Center Grinding Wheels, 25-Pack Z24R Zirconium. Tough zirconium grain. High performance grinding for the toughest applications. Ferrous metals (iron, steel, welds). Stainless steel, high tensile alloys. The highest quality zirconium grain. All sizes made with uniform grain throughout, no filler grain used. Super-Lock Hub on all 5/8-11 threaded arbors in the safest, bibration-free design.