It's true, they can explode, but I've never seen one do so. You'd have to be doing some MAJOR rpms on one to make that happen. Most, as mowman racer pointed out have ratings. So as long as you stay within those limits, you should be ok. The important part is to try and not nick or damage cast iron which can mis balance it and cause stress cracks than can later cause it to explode. Same theory as caring for a bench grinder wheel.
