Grain Flow Forged HD: One-piece Grain Flow Forged HD from 1025E Pure Select mile carbon steel at Mizuno's iconic facility in Hiroshima, Japan. Where Mizuno irons have been produced since 1968. 1025 Boron : A carbon steel that featurs 30% greater strength. Harmonic Impact Technology: Fine tuned head geometry delivers ideal impact feel and feedback Hydroflow Micro Grooves: Laser etched microscopic grooves help channel away moisture for maximum spin in wet conditions Face Milling Lines : Increased surface roughness to increase spin nearer the green.