Subgrain Boundary
Arrangement of dislocations within a grain that separates slightly misoriented regions, formed through recovery processes.
Subgrain boundaries are nothing bad; they form normally in the microstructure when the material recovers after welding or during heat treatment. They are basically fine dislocation networks within a grain. For the workshop, this means: if you have stresses after welding or the part becomes hard and brittle, it's rarely due to the subgrain boundaries themselves. They are usually harmless. What's important is that you don't cool the part too quickly after welding, especially with higher-strength steels. Let it cool slowly or perform a stress-relief anneal, then the dislocations arrange neatly and you won't have problems with cracks or hard zones.