AbstractThe cleavage fracture strengths and Charpy impact transition temperatures of plain C–Mn steels (0·12%C and 1–1·4%Mn) having ferrite–pearlite microstructures have been determined for ranges of grain size and grain boundary carbide thickness. Using appropriate heat treatments, ferrite grain size and carbide thickness were varied independently. Refining the ferrite grain size or grain boundary carbide thickness increased the fracture strength and decreased the impact transition temperature. Of the current theories for brittle fracture, an equation recently derived by Petch was found to give the most satisfactory agreement with the experimental data for cleavage strength.MST/1424