Effect of ferrite volume fraction on work hardening behavior of high bainite dual phase (DP) steels

AISI 4340 steel was heated to 910 °C for 1 h then directly transferred to 750 °C and intercritically annealed for different times to obtain different ferrite volume fractions and then isothermally held at 350 °C for 40 min followed by air cooling to room temperature. Samples of these steels with dual phase ferrite–bainite structure were tensile tested at room temperature. The tensile flow stress data for this steel with different ferrite volume fraction was analyzed in term of Hollomon equation. It is seen that the two Hollomon equations can describe the flow behavior adequately and found that the work hardening takes place in two stages which each equation belong to the one of work hardening stages. In this study the effects of constituent volume fraction on Hollomon equation parameters (work hardening exponent and strength coefficient), onset strain of stage II work hardening, yield strength, ultimate tensile strength and ductility were investigated. Results showed that the yield strength, ultimate tensile strength and work hardening decrease linearly with increasing ferrite volume fraction whilst ductility increases. Finally, variations of these mechanical properties with differing ferrite volume fraction are used to rationalize the deformation mechanisms activated at different stages.