Types: There are a number of different grades that have some significantly different properties and machining qualities. Aluminium also forms a number of different alloys.
Types: Brass is an alloy of copper and zinc, but often other elements such as aluminium, iron, manganese, tin and lead are added. The result is a number of different types, standards and historical descriptions of brass.
Hardening: Mild steel can be case hardened to increase the surface hardness. Case hardening can be difficult to get hold of, but EKP Supplies Beta 1 compound.
There are a lot of uses for silver steel and a lot of these are because it is easy to harden. Also, silver steel is readily available in both metric and imperial round bar that is very accurate and so great for axles. Cutting speeds for silver steel, these are just an indication but they are a good starting point.