Polygonal ferrite
A ferrite structure consisting of multi-sided, equiaxed grains formed during slow cooling. It is characterized by high ductility and lower strength.
Polygonal ferrite forms when the steel cools slowly after welding. Then you get a soft, well-formable structure, but it doesn't have much strength. For parts that have to withstand something, that's not ideal. If you cool too quickly, you instead get finer, harder structures. The problem is that if cooling is uneven, you can get internal stresses or cracks. In practice, this means: don't quench the weld with water or leave it in a draft. If you need a tough, formable component, cool slowly and keep the heat-affected zone under control. For high-strength requirements, you have to control the structure via heat management, if necessary with post-weld heat treatment.