The L6 Bainite blades that howard clark makes are made of a material called L6 its a tool steel and thats not the reason why its so strong the reason why its so indestructable is because he makes the steel into a bainite crystaline structure normaly you can even have a hamon with a bainite structured blade but he manages to make every bit of the blade except for the temper line witch seems incredibly difficult, its odviouse that he has alot of experience in controling differentially tempered steel thats also probably why theres noone else that makes bainite blades.