Handlebars can creak for a number of reasons. If your stem has a removable face plate, loosen all of the bolts and retighten them equally, taking up the slack evenly across the faceplate as you tighten it, kinda like the lugnuts on a car. You want equal tension at the top and bottom. Also, if the bar is a different diameter than the stem (risers on a road stem with the wrong shims), it can creak.

As for tempering, I have never heard of that.

-Rob.