Proeutectoid Grain Boundary Ferrite

Grain boundary ferrite forms during slow cooling at the austenite grain boundaries before pearlite formation begins. In welding, this is a normal structure as long as cooling is not too fast. If you cool the workpiece too quickly, the structure can become hard and crack-prone. For a clean result, just let the heat-affected zone cool evenly after welding, then the ferrite forms in a controlled manner and the structure remains tough.