BN-HSS - Black Nitride High Speed Steel: BN-HSS is our premium HSS with case hardened (carbon nitride) and black oxide to resist pitch build up. BN-HSS offers up to 100% tool life increases over standard HSS, without being harder to profile shape. BN-HSS features a long wearing cutting edge .010Ó deep that is more than 72 Rockwell C hardness. Much like Opti (TM) Knife. Great for long runs and stubborn hardwoods. Knife corrugations are 16 per inch at 60 degree angle.