This increases hardness (Rc 75-80) without increasing brittleness, which is measurably harder than high speed steel. The Nitro Heavy-Duty Construction Reamers have a Ferritic Nitro Carburizing surface treatment, which gives a hard, protective layer to the reamer.9/16,Spiral Flute Nut Shank NITRO Construction Reamer Body and clearance are gold and black surface treated for maximum lubricity..