Introducing Nickel Chromoly – A new material with so much additional ball speed, our engineers switched their focus to feel and stopping power. At 35% stronger than our original Hot Metal material, 4335 Nickel Chromoly enables a high-energy 8% thinner clubface. Combined with Mizuno's vibration controlling V-Chassis and deep CG design, the JPX923 Hot Metal irons are as enjoyable and controllable as they are long.