High quality M35 cobalt steel, 66-69 HRC (Rockwell hardness scale), generally has the best heat, wear and chip resistance and long service life. Three-pass sharpening, high HRC and heat resistance of the material, designed for cutting stainless steel, iron sheet, aluminum, pipe, wood, PVC and plastic. This cobalt step drill is typically capable of operating at two to three times the speed. Despite the higher price, the cost per hole will be lowest when using a cobalt drill bit because it produces more holes. [4-32mm] 15 grades (4mm, 6mm , 8mm, 10mm, 12mm, 14mm, 16mm, 18mm, 20mm, 22mm, 24mm, 26mm, 28mm, 30mm, 32mm)