4340 steel bars were austenitized at 850 ◦C for 1 h followed by heating at 700 ◦C for 90 min and quenching
into a salt bath at the temperature range of 300–450 ◦C for 1 h to obtain dual structures with 34 vol.%
fraction ferrite and various bainite morphologies. SEM studies showed that by increasing the austempering
temperature, bainite morphology varies from lower to upper bainite. Tensile, impact and hardness
tests revealed that increasing the austempering temperature from 300 to 400 ◦C leads to a reduction in
yield and ultimate tensile strength, hardness, uniform and total elongation and impact energy. But in
dual phase steel produced by austempering at 450 ◦C, yield and tensile strength and hardness increased
and severe reduction in total elongation and impact energy obtained. Fractography of tensile specimens
showed brittle behavior for this austempering temperature. Fatigue test results showed that fatigue limit
decreases with increasing austempering temperature from 300 to 400 ◦C. Finally, fractography studies
showed cleavage fracture at the surface of fatigue specimens austempered at 400 ◦C, which confirms the
tendency to brittle behavior.