Pro Series® cones are made of 4150 pre-hardened alloy steel, heat-treated with permanently infused surface hardness to Vickers 1880 (75+ Rockwell C) with diffusion zone (approx. RC 60) and an RC 28-32 core hardness. Surface hardness withstands the 72 hour salt spray corrosion test under ASTM B117 conditions, reducing surface flaking and galling. Pro Series cones are taper ground to a maximum of .0001 concentricity for long wearing spindle fit, superior accuracy and no vibration. This product is 100% made in the USA.