Maybe somebody can point you in a direction, but the problem with
your question is simple. Which brass? Which 18kt? I know of 20
brasses just off the top of my head, and at least 1/2 dozen 18kt.
alloys, all of which have different hardnesses. In simple terms,
though, most 18kt. is softer than sterling, your typical bronze is
roughly the same as sterling, or a little harder (annealed), and just
about any brass will be as much as twice as hard as any of the others.