Study of the effects of lower bainite volume fraction on tensile and impact properties of D۶AC medium carbon low alloy ultrahigh strength steel

The effects of lower bainite volume fraction on tensile and impact properties of D۶AC ultrahigh strength steel were studied in the current work. To obtain mixed microstructures containing martensite and different volume fractions of the lower bainite, specimens were austenitized at ۹۱۰° C, then quenched in a salt bath of ۳۳۰°C for different holding times, finally quenched in oil. In order to obtain fully martensitic and bainitic microstructures, direct oil quenching and isothermal transformation heat treatment for ۲۴ hours were used respectively. All specimens were double tempered at ۲۰۰°C for ۲ hours per tempered. Microstructures were examined by optical and scanning electron microscopes. Fracture morphologies were studied by scanning electron microscopy (SEM). Results showed that both yield and ultimate tensile strength generally decreased with an increase in volume fraction of lower bainite. However, a few exceptions were observed in the mixed microstructures containing ۱۲% lower bainite, showing a higher strength than the fully martensitic microstructure. This can be explained on the basis of two factors. The first is an increase in the strength of martensite due to the partitioning of the prior austenite grains by lower bainite resulting in the refinement of martensite substructures. The second is a plastic constraint effect leading to an enhanced strength of lower bainite by the surrounding relatively rigid martensite. Charpy V-notch impact energy and ductility is improved with increasing the volume fraction of lower bainite.

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Keyword: D۶AC , Microstructure , Mechanical properties , Lower bainite , Ultrahigh strength.