Austenite-ferrite phase equilibrium

The austenite-ferrite balance describes the state where both austenite and ferrite are present in the microstructure. In welding, this matters because it determines the weld properties, especially toughness and hardness. If you don't control the cooling, you build up stress and risk cracks in the workpiece. So make sure the weld doesn't cool too fast, then the structure stays calm and the weld won't become brittle.