Continuous Ferrite Network

The so-called ferrite network forms when, during cooling, ferrite forms at the grain boundaries of the former austenite and encloses them like a network. This is a normal microstructure, but its appearance depends strongly on the cooling rate and chemical composition. A coarse, continuous network can impair toughness and increase susceptibility to cracking. In practice, you want to avoid this by controlling cooling and not cooling too rapidly. The exact conditions always depend on the material; there's no one-size-fits-all recipe.