Primary Grain Formation in Casting

During casting, the first crystals form as the melt cools, the so-called primary grain. How large the grains become mainly depends on the cooling rate and on whether you inoculate the melt. Fast cooling gives you fine grain, which makes the part tougher; slow cooling lets coarse grain grow, which can impair strength. For a clean structure, you have to control the cooling and choose the right inoculation, then you get a dense and durable casting.