The irwin metric tct tungsten carbide tipped drill bits offer precision drilling with stationary machines in hardened steel cast iron bronze alloys and manganese alloyed steel the c2 carbide tip will drill hardened material up to a hardness of 50 rockwell c and is four facetted ground for low drilling pressure and perfect centring the body of the drill bit is made from chrome vanadium steel hardened to 56 rockwell c a coolant should always be used when drilling with tct twist drill bits irwin su