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Ferritic spheroidal graphite (SG) cast irons are used to make parts that require high ductility and fracture toughness and good high-cycle fatigue resistance. Ferritising anneals have an all-ferritic matrix favouring high fracture toughness and ductility but this does not improve fatigue strength, and even lowers it in some cases. The purpose of the present study was therefore to optimise these heat treatments for the best compromise between fracture toughness and fatigue life. To attain this, the fatigue behaviour of cast irons was studied in the as cast and heat-treated conditions. A comparison of the lives and mechanisms of ferritisation of these metallurgical conditions made it possible to evaluate the influence of various parameters (characteristics of the initiation sites, properties of the matrix). From the present study, a method is proposed to optimise the heat treatments to produce ferrite in association with a stronger phase in the zones where microfissures are initiated. A study of heat treatments favouring the formation of a matrix made up of ferrite and bainite yielded encouraging results. The presence of bainite near the graphite spheroids and zones of porosity can increase resistance to crack initiation. This leads to a substantial improvement of fatigue life of the SG cast iron without at the same time embrittling the material.IJC/578

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