I don't know how true it is but I have heard that it is very bad to put metallic objects in microwave. I don't know how good the idea of heating water in a kettle (or any metallic holder) in microwave is. About superheating, the best thing to do is to make sure that water surface is not even. Ceramic objects, cups have very smooth surfaces that denies water molecules of any surface to form bubbles (same phenomenon as how dirt is needed to form rain, in which case saturated clouds need a point of contact to liquify). Heat water with a straw in it, or stir it with a spoon with your hand stretched before taking it out of microwave. I always do the latter.