OK. I see now. You're right. I was working with other alloys. I fast-quench my brass mostly so the heating stops, and I relied on an article extent at the time (2006 or so) that said you should. However, let me just say that with copper /tin/antimony alloys it was important to freeze those materials in an amorphous state before it could form crystalline structures. We didn't have zinc, lead, or cadmium in any of our processes, because they were too poisonous.
You aren't going to get an amorphous structure (aka "glass") by annealing. You need to go directly from the liquid state to do it, with a very high cooling rate and a number of other factors in play to get there. This document sets out some of the science:
http://simgroup.task.gda.pl/DYDAKTYKA/mudry/2-Amorphous-metallic-alloys.pdfWhile interesting, and with some emerging industrial applications, this really isn't relevant to cartridge brass.
As has been said, the main purpose of dunking cases into water after annealing is to stop you from annealing the case heads. One other factor is that too much time at temperature tends to promote grain growth, which is not a good thing, but once you remove the heat the cases cool quickly enough in air that this really won't be a factor, unless you keep the heat on them for a protracted period, and/or take them to too high a temperature.
