The effect of bainite morphology on the mechanical properties of a high bainite dual phase (HBDP) steel

4340 steel bars were austenitized at 850 degrees C for 1 h followed by heating at 700 degrees C for 90 min and quenching into a salt bath at the temperature range of 300-450 degrees 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 degrees 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 degrees 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 degrees C. Finally, fractography studies showed cleavage fracture at the surface of fatigue specimens austempered at 400 degrees C, which confirms the tendency to brittle behavior. (C) 2009 Elsevier B.V. All rights reserved.