Product Description: XHT (Xtreme Hardness & Toughness) blades feature a unique steel that incorporates boron, carbon,manganese, and other materials that ensure its strength. The blades are heat treated with thepatented and independently tested Marbain heat-treating process. This process makes the blades 25% - 40% harder than standard lawnmower blades, making them the longest-lasting, safest, and most impact resistant lawnmower blades on the market.