Z24R - Zirconium. Tough zirconium grain. High performance cutting/notching for the toughest applications. Ferrous metals (iron, steel, welds). Stainless steel, high tensile alloys. Specifications: Max RPM - 6600, 5/8-11 Super-Lock hub. Tech Info: For use on angle grinders. Aggressive cutting performance over a wide range of applications. Super-Lock Hub on all 5/8-11 threaded arbors is the safest, vibration-free design. All sizes reinforced with the highest quality fiberglass for maximum safety and performance.