Arc strike

Arc marks occur when the torch or electrode touches the workpiece outside the weld and strikes a short arc. This leaves a hard, often bluish spot that changes the surface structure and can cause a notch effect. Best grind it off cleanly with a flexible grinding disc or flap disc until the discoloration is gone. Work calmly and evenly so you don't grind a dip. If the mark is deep, you have to grind it out completely and weld over it, otherwise it remains a weak point.