BLADE-MOWER, XHT, 21-3/8″, 7 LOBE, HL, Replaces: M154062, Type: High Lift, Web Notes: XHT (Xtreme Hardness & Toughness) blades feature a unique steel that incorporates boron, carbon,, Web Notes: manganese, and other materials that ensure its strength. The blades are heat treated with a process, Web Notes: makes the blades 25% – 40% harder than standard lawnmower blades, making them the longest-lasting,, Web Notes: safest, and most impact resistant lawnmower blades on the market.