AbstractHigh purity 0·4C–Cr–Mo–Ni structural steel has been studied to determine the effect ?f the transformation microstructure on fracture toughness J1cof quenched and highly tempered low alloy steels. Martensitic and bainitic steels having a wide variation in the prior austenite grain size were investigated. In martensitic or lower bainitic steel, ductile fracture occurs at the crack tip and J1cis determined by intercarbide spacing. In upper bainitic steel having packets composed of well define blocks, brittle fracture occurs and J1cis controlled by block Size. In fully ductile fracture, J1ccan be correlated with carbide cracking in the process zone by which microvoids are created. Crack initiation occurs by interconnection of the crack tip and the microvoids, which are situated a characteristic distance from the crack tip. In fully brittle fracture, J1ccan be correlated with unstable cleavage cracks initiated at the boundary of a minimum microstructural unit in the process zone. Crack extension occurs by the combined action of the cleavage cracks and the main crack.MST/1012