Undercut
A narrow groove melted into the base metal at the weld bead transition caused by excessive energy density or incorrect torch positioning. It reduces the load-bearing cross-section and constitutes an unacceptable notch.
A burn notch happens when you put too much energy into one spot while welding, for example due to excessive torch power or incorrect torch positioning. The material at the edge then becomes hard and glassy, is no longer flexible, and eats away at the load-bearing cross-section of the weld. This can lead to stresses and cracks, especially hot cracks, and the weld no longer withstands the load cleanly.
To prevent this, you need to control torch power and positioning and distribute the energy evenly over the base material. If you suspect a burn notch, inspect it under a microscope and measure the diameter – if it gets too large, the weld is no longer load-bearing and you need to rework it.