Iranian Journal of Materials Science and Engineering

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The effects of lower bainite volume fraction on tensile and impact properties of D6AC 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 910° C, then quenched in a salt bath of 330°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 24 hours were used respectively. All specimens were double tempered at 200°C for 2 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 12% 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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In the present paper, the influence of cobalt additive on the sintering/infiltration behavior of W-Cu composite was studied. For this purpose, the mixed powders of tungsten and cobalt were compacted by CIP method and then sintered at 1450, 1550 and 1600 °C in a hydrogen atmosphere. The sintered specimens at 1550 °C were subsequently infiltrated with liquid copper at 1250 °C for 10, 60 and 120 min. The microstructure and composition of samples were evaluated using SEM, EDS as well as XRD techniques. The density of the sintered samples was measured by Archimedes method. Vickers indentation test was used to measurement hardness. It was found that sintering mechanism of tungsten powder depends on temperature and cobalt additive content. Also, the best infiltration behavior was observed in the samples with optimum cobalt value. In addition, it was found that the W-W contiguity as well as dihedral angle decreases as cobalt increases. Density and hardness of infiltrated specimens are attained 16.28-16.79 g.cm-3 and 220-251 VHN, respectively.