Influence of martensite volume fraction on tensile properties of triple phase ferrite–bainite–martensite steels

This paper presents the effect of martensite volume fraction (VM) on mechanical properties of AISI 4340 steel with ferrite–bainite–martensite microstructures. The steel bars were austenitized at 900 °C for 1 h followed by intercritical annealing at 740 °C for 100 min and quenching into a salt bath with temperature of 300 °C and holding at different times to obtain triple phase (TP) microstructures with 34 vol.% fraction ferrite and various martensite (or bainite) contents. Presence of three phases in adjacent to each other was confirmed by metallographic analysis and TEM studies. TEM observation also indicates higher density of dislocations within ferrite near martensite colonies compared to that near the bainitic phases. Results of the tensile and hardness tests showed that with increasing martensite volume fraction, yield strength, ultimate tensile strength and hardness increase which is related to higher martensite volume fraction, higher strength of ferrite and higher residual stresses. The increment of martensite volume fraction increased ductility in terms of uniform and total elongation and tensile fracture energy. This evidence is due to the change of martensite morphology, increase of dislocation density at interphase boundaries and increment in residual stresses of the ternary phase steel with increasing VM. Fracture surface micrographs of tensile specimens, examined by SEM and stereo optical microscopy, confirmed the reduction in ductility of TP steels with increasing VM.