The irwin imperial hss tct tungsten carbide tipped drill bits are designed for use in stationary machines and are suitable for drilling hard and abrasive materials such as hardened steel cast iron etc the drill body is made from chrome vanadium steel hardened to 56hrc the carbide tip will drill hardened material up to a hardness of 50 hrc the diamond ground 4 facet carbide tip is for easy centring and low drilling pressure a coolant should always be used when drilling with tct twist drill bits i