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On hot rolled bars to
be cold drawn, the dark oxide surface is called scale.
What we see
on the surface is "scale" - a combination of oxides of iron.
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.
Scale is
- Hard
- Brittle
- 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
deal-breakers 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.
Bar Coils
Editor's Note: CR4 would like to thank Milo for contributing this blog entry, which originally appeared here.
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Re: Three Facts About Scale (Iron Oxide) Rust On Steel
