Recrystallized ferrite grain structure

Recrystallization is the reformation of a fine, low-stress structure after previous deformation or heat treatment. In welding, you take advantage of this when you do a heat treatment after welding – the grains rearrange and the weld becomes tougher. If you cool down too quickly or don't get the heat in properly, the structure stays coarse and stressed, and then it cracks faster. So: preheat properly, hold the temperature after welding and let it cool slowly, then the grain will be fine and the weld will hold.