This is what a broken weld could look like. This might have been caused by a defective weld or even by inconsistent properties in the tube. Many welders, especially neophytes, like to weld too hot. That will cause the metal to become brittle around the weld and actual fail there, instead of in the weld. Once again, replace your tube with a high-quality, heavy-walled one that has been drawn over a mandrel.