Microstructural Study of a High Bainite Dual Phase (HBDP) Steel Austempered at Different Temperatures

4340 steel bars were austenitized at 850oC for 1 hour followed by heating at 700oC (ferrite and austenite region) for 90 min and quenching into a salt bath with different temperatures of 300, 350, 400 and 450oC. The steel bars were held for 1 hour at these temperatures before air cooling to room temperature. Various ferrite-bainite microstructures with 34% volume fraction of ferrite and different bainite morphologies were obtained. The results of SEM studies showed that by increasing the austempering temperature, the morphology of bainite varies from lower to upper bainite. According to the T-T-T diagram of the studied steel, the bainite transformation will not complete for the holding time of 1 hour at 400 and 450oC austempering temperatures and there can be 9 and 23Vol% of martensite at these temperatures respectively. Microstructural studies of specimens austempered at these temperatures showed complex microstructures of ferrite, bainite and martensite. Mechanical testing results showed reduction in yield and ultimate tensile strengths, hardness, uniform and total elongations and impact energy with increase of the austempering temperature from 300 to 400oC. But for dual phase steel austempered at 450oC, the yield and tensile strengths and hardness increased and a severe reduction in total elongation and impact energy was obtained. This brittle behaviour was related to martensite formation during cooling from this austempering temperature.