Abnormal grain growth
The selective growth of a few individual grains at the expense of others after primary recrystallization. It produces a coarse structure with reduced impact toughness.
If you put too much heat into welding or annealing, the small grains in the structure grow together into large ones. That happens when the metal stays hot long enough – the small grains get eaten up by the larger ones. Coarse grain is always bad: the material becomes brittle, loses toughness, and can break away on you, especially in cold or under impact loading. You'll most likely have this problem in the heat-affected zone next to the weld. In multi-pass welding, each pass reheats the one below – if that gets into the critical temperature range, the grain grows. So keep the heat input low, adjust the interpass temperature, and don't anneal longer or hotter than necessary. If the material allows it, you can normalize after welding – that refines the grain again. Steels with microalloying elements like niobium or titanium are more robust; they inhibit grain growth from the start.