AbstractThe cleavage behaviour of plain carbon steels containing from 0·2 to 0·8%C has been investigated. It is observed that each steel displays two characteristic temperatures at which a transition in the mode of fracture occurs. These are the transition temperatures for cleavage Tcand for general yielding Tg. At temperatures below Tc,the steels fail by pure cleavage. This involves the generation of a cleavage crack nucleus in a carbide particle followed by cleavage crack propagation. The cleavage fracture stressσfis independent of temperature. Between temperatures Tcand Tg, the steels fail by quasicleavage. This involves the generation of a crack nucleus by a localised fibrous process followed by cleavage crack propagation. The crack nucleation stage is shear stress controlled and therefore the quasicleavage fracture stressσqincreases with decreasing test temperature. Above temperature Tg, failure occurs at or after general yielding.MST/1045