Brittle fracture mechanisms and toughness of model low alloy steels chemically representative of macrosegregated forgings

•Model low alloy forged and tempered steels have been manufactured with chemical compositions representative of RPV steel at nominal composition, high and very high levels of macrosegregations.•Contrary to what is observed on common carbon steels, the lowest fracture and impact toughness transition temperatures are obtained for the highest carbon content alloy.•Intergranular fracture is observed for the most enriched and brittle resistant model material.•Cleavage fracture is observed on the nominal and high carbon content materials, initiating from molybdenum/manganese enriched carbides located at grain boundaries.•The improvement of brittle fracture resistance with the level of carbon content is analyzed as the combined result of a higher hardenability of the material, that produces a smaller grain size and tougher crystalline microstructure, and of a smaller size of the carbides in the material. The influence of the chemical composition on the brittle fracture behaviour of quenched and tempered low alloy bainitic steels was investigated thanks to the laboratory elaboration of three model alloys which have underwent a heat treatment to improve their chemical homogeneity. Two model materials with chemical compositions simulating zones of large (0.29 %C) and intense (0.38 %C) positive macrosegregation were characterized and compared with the nominal composition material (0.18 %C). The ductile to brittle transition temperatures T56J and T0 of the materials with nominal and large carbon content are similar, whilst those of the material with the highest carbon content are unexpectedly significantly lower. Fractographic analyses indicate that the brittle fracture mechanisms depend also on the chemical composition. For 0.18 and 0.29 %C, cleavage is the dominant brittle fracture mechanism, whilst a mix of cleavage and intergranular failure is observed for 0.38 %C at the same levels of fracture energy or toughness. When cleavage failure is the dominant mechanism, it often originates at large molybdenum and manganese-enriched carbides located at grain boundaries likely identified as former austenitic grain ones. Microstructural analyses show variations of the large carbide population and ferritic grain size between the three alloys. Also, as carbon content increases, an evolution towards a lower bainite/martensite crystallographic microstructure is observed. The effects of these microstructural differences on the brittle fracture behavior are discussed.