Fracture behaviour in medium-carbon Ti–V–N and V–N microalloyed ferritic-pearlitic and bainitic forging steels with enhanced machinability

The microstructure, tensile and impact behaviour of medium-carbon Ti–V–N and V–N microalloyed resulphurised forging steels with and without Cr and Ca additions have been determined before and after forging up to 75% reduction. Titanium additions refined the prior austenite grain size resulting in ductile failure becoming fully transgranular rather than mixed inter- and transgranular. However, the titanium additions also resulted in the presence of coarse (Ti,V)(C,N) particles which acted as cleavage initiation sites reducing toughness by increasing the fracture appearance transition temperature. Ductile fracture mode variation did not affect upper shelf energy, which was determined by inclusion volume fraction (reduced by Ca additions) and, to a lesser degree, by the inclusion size and shape (modified by Ca addition and forging). The addition of Cr increased the hardenability of the steels resulting in the presence of bainite in the otherwise ferrite and pearlite microstructure. A small fraction of bainite (7%) was found to improve the strength levels however a large fraction (83%) was less effective resulting in an increase in the tensile strength but a decrease in the proof strength. The amount of bainite did not affect the fracture properties of the steels with the other microstructural features of inclusion volume fraction and type having a dominant effect.