Mizuno JPX 923 Hot Metal Iron Set (Left-Handed)Mizuno JPX 923 Hot Metal Iron Set Introducing Nickel Chromoly A new material with so much additional ball speed, Mizuno 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 Mizunos vibration controlling V Chassis and deep CG design, the JPX923 Hot Metal irons are as enjoyable and controllable as they are long.