This is an alloy of nickel and iron which provides a convenient way of adding nickel units during the production of stainless steel, other nickel containing steels, and foundry alloys.
Ferro-nickel typically contains 20-38% nickel with most of the balance as iron, plus traces of other elements which can include carbon, phosphorus, cobalt, copper and silicon.
Although ferro-nickel is mostly used in producing austenitic and duplex grades of stainless, it can also be present in small quantities in ferritic and martensitic grades.
This ferro-alloy is typically produced in convenient sizes and forms – such as small ingots, pellets and cones – for ease of charging to the stainless steel melting/refining process.
See also nickel, nickel pig iron and ferro-alloys. And for more detail click and see wikipedia entry.