While standard carbon steel tubes are mostly ferrite, the concept becomes critical in the welding of stainless steel tubes. When TIG welding a stainless pipe, if the weld pool solidifies with zero ferrite (a purely austenitic structure), it is highly prone to "hot cracking" as it cools. Metallurgists use the Schaeffler Diagram to precisely formulate the filler wire, ensuring the final weld seam retains a Ferrite Number (FN) between 3 and 8, which acts as a microstructural "glue" to prevent cracking.