Chapter 73 takes over from Chapter 72 the moment metal becomes an article: tubes, pipe fittings, structures, tanks, cables, fasteners, household goods. The line between the two chapters is the line between a material and an object.
The trap points upward. A steel article that is also a machine part belongs to Section XVI, not here: Note 2 to that Section prevails. Fasteners of 7318 are the exception and stay in Chapter 73 even when fitted to a machine.
Every number opens on the lines beneath it, down to the eight-digit statistical code.