Eutectoid microstructural decomposition
The transformation of a solid parent phase at constant temperature into two other solid phases, as seen in the decomposition of austenite into pearlite, serving as a fundamental strengthening mechanism for steel.
Eutectoid decomposition means: Austenite transforms into pearlite during cooling, a fine mixture of ferrite and cementite. This happens in steel with a specific carbon content at a constant temperature, not over a range. In welding, this is important because the cooling rate determines how fine the structure becomes. If you cool too slowly, the pearlite becomes coarse and the steel soft. If you cool too fast, martensite can form instead, which is hard and brittle. In practice, this means you have to control the cooling. With normal structural steel, this is usually not a big issue, but with higher-grade steels, you have to pay attention to preheating and heat input to land in the desired structure range and avoid cracks.