Chuck this blank at very slow speed and carefully shape the nuts with your file. Because of its greater strength, particularly in smaller sizes, compared to mild steel, carbon steel will be one of your choices for the smallest tools you'll produce. It doesn't hold a charge of abrasive as well as other metals do, but it is easy to form and obtain. Just be sure that it is annealed before shaping. Otherwise, it will be very hard to produce the shapes needed and the abrasive will not be effective.
Cast iron works well as a tool metal.
Its drawbacks are that it is difficult to obtain in the necessary sizes and it wears readily when used with abrasives. The upside is that it has microscopic openings that tend to trap the abrasive particles. Copper has traditionally been used for very small carving points. Diamond and copper have a natural affinity for each other that makes charging these tools easy. Brass makes wonderful tools, but it should only be used with fine abrasives.