Effect of long duration intercritical heat treatment on the mechanical properties of AISI 4340 steel

Ferrite-bainite dual phase (FBDP) steels are a class of steels characterized by a microstructure consisting of a soft ferrite matrix with hard bainite islands. The holding time in the dual phase (alpha + gamma) region is an important parameter in the intercritical annealing at constant temperature and influencing mechanical properties of this steel. Dual phases with various ferrite volume fractions (45-65 vol. %) were fabricated by changing the holding time. Samples of these steels with ferrite-bainite structure were tensile tested at room temperature. Results showed that as the ferrite volume fraction increased, the yield strength decreased but a different type of behaviour was observed for ultimate tensile strength (UTS) tests. UTS increased when the ferrite volume fraction (V(f)) increased above ca. 55%. The tensile flow stress data for this steel, obtained from samples with various ferrite volume fractions, were analyzed in terms of the Hollomon equation. Two or three Hollomon equations can describe the flow behaviour adequately, and it was found that with increasing V(f) the work hardening occurs in three stages, each equation belonging to one of these stages. Finally, variations of Hollomon equation parameters were used to explain the deformation mechanisms activated at various stages.