I'm a novice, but am having fun learning. Sometimes - too often - when I weld (with oxy-acetylene) a butt joint the weld looks OK, but as it cools a crack will suddenly appear along the length of the weld. Today this happened as I welded two 3/16" thick 2"x2" angle irons 18 inches long together (welding them together to form a U-channel). I don't bevel the edges - just butt them together.
Any ideas what I'm doing wrong?