The simplest way to adjust your soldering iron's power output is to add a diode to only pass half the mains to it. This way it divides the power in half which in this case will be from 80W to 40W. This is useful with high power soldering irons which tend to burn down the beak pretty fast at maximum power, and while 80W will melt almost any soldering tin, 40W will surely do a pretty fine job. The diode can be a 1N4007 and the switch can be any just make sure that the switch metal parts aren't connected to any leads because they may become at mains potential.
