Effect of Bainite Morphology on the Mechanical Properties of a Ferrite- Bainite- Martensite Triple-Phase Steel

Dual and triple phase steels have unique properties such as continuous yielding behavior, low ratio of yield strength to tensile strength, high work hardening rate and high homogenous elongation percentage; thus, they are used widely in automotive industries. The aim of this study is to develop a triple-phase microstructure of ferrite-bainite-martensite with high strength and desirable ductility. In this research, the samples of 4140 steel were austenitized at 850°C for 1 hour. Then, the samples were kept at temperature of 720°C (the ferrite-austenite region) for 3 minutes, and transferred to a salt bath at different temperatures of 380, 400, 420 and 450°C for 4 minutes to obtain various morphologies of bainite, finally quenched in water to form martensite. The observation of the microstructure by a scanning electron microscope showed that with increasing austempering temperature from 380 to 450°C, the morphology changes from lower bainite to upper bainite. Hardness and tensile tests revealed that by increasing the austempering temperature the hardness, yield strength, ultimate tensile strength, elongation and the amounts of “n” and “k” in the Holloman equation, decrease. Fractography of the tensile tested specimens by a stereo-microscope showed that with increasing the austempering temperature, the fracture surface changes from ductile to brittle.