Equiaxed crystallization
The growth of crystals with approximately equal dimensions in all spatial directions, typically promoted by strong cooling and grain refiners in the center of the casting.
Equiaxed crystals form when the melt can solidify evenly in all directions. This happens when cooling isn't too fast and the melt has enough nuclei for crystals to form on. By adding inoculants or using a calm weld, you promote a fine, uniform structure. When welding, you want this because it makes the weld tougher and prevents cracks. If you cool too fast or the weld gets too hot, the crystals instead grow elongated and coarse – that makes the weld brittle and prone to porosity. So aim for a calm, steady welding technique and let the weld cool slowly. If you reheat the weld briefly with the flame after welding, you give the crystals time to form nicely fine.