Pearlite Interlamellar Spacing
Average distance between adjacent cementite lamellae in the eutectoid microstructure, which is inversely proportional to the degree of undercooling.
Pearlite forms during cooling from the austenite region when cementite and ferrite arrange in lamellae. The spacing of the lamellae depends on how fast you cool: rapid cooling gives a finer structure, slow cooling gives a coarser one. In welding, this means: if you cool too slowly, you get a coarse structure and the steel becomes tough but softer. If you cool too fast, hard, brittle martensite can form instead, which tends to crack. In practice, you need to control the cooling to get a medium, fine pearlite structure. Depending on the steel grade and plate thickness, this means adjusting the heat input and preheating so the weld doesn't cool too fast but also doesn't stay hot too long.