Undercutting in welding

A proper weld is produced as the rod is moved across the material, and the flame at the end of the rod cuts a path in the material and that path is refilled by the material of the rod. It flows back into the cut out filling it and joining the materials.

If the welder is too hot, or the persons technique is incorrect, or they are moving the rod to fast, the the path cut by the flame on the end of the rod is not properly filled, and the result is an undercut running along the side of the weld. This undercut causes the weld to be weak and makes it susceptible to failing.

Undercutting is easy to correct and requires practice by the welder to get the flame temperature and the speed right so as to fill the path that is cut until full. The opposite being going to slow and making a humped up weld. A proper weld will be slightly over the surface of the original part, smooth with no pits or holes, and no undercutting.