Duktilitet (målt ved forlengelse) avtar når karboninnholdet stiger. Lav - karbonrundstål (f.eks. SAE 1018, 0,18% c) viser 25 - 30% forlengelse, noe som tillater bøyning og dannelse. Medium - karbon (SAE 1045, 0,45% c) synker til 15-20% forlengelse, egnet for begrenset forming. Høykarbon (SAE 1095, 0,95% C) har<10% elongation, making it brittle but hard. This trend occurs because carbon forms iron carbides, which restrict dislocation movement-essential for plastic deformation. For applications requiring both strength and ductility (e.g., automotive parts), medium-carbon grades with heat treatment are preferred.