On hot rolled bars to be cold drawn, the dark oxide surface is called scale.
Scale is the name given to the oxides of iron that are formed on as wrought products as a result of mill operations (high temperature rolling or furnace treatment)
Rust is the commonly used term for iron oxide from weathering or corrosion.
- High Coefficient of Friction
So we need to get it off the steel if you are to have any chance of keeping the tool edge sharp.
There are 3 oxides of iron:
Hematite ( Fe2 O3) has a microhardness of ~ 1030 D.P.H., is red in color, and is not soluble in acid.
Magnetite (Fe3O4) has a microhardness of ~ 420-500 D.P.H., is black in color, and is not soluble in acid.
Wustite (FeO) has a microhardness of ~ 270 -350 D.P.H., is blueish in color, and is soluble in acid. Wustite is the phase that makes up the innermost scale on the bars or rods.
Hematite and Magnetite make up the outer layers of the scale, and due to their composition, make up the larger mass of scale present. Due to their hardness and quantity they are the real dealbreakers for machining as they create tool edge wear.
One of the ways that Cold Finished Steel bars aid machinability is by removing these hard abrasive oxides from the workpiece, so that they don’t destroy your tools and contaminate your cutting fluids.