This study investigated the effect of intercritical heat treatment temperature on the tensile properties, work hardening and corrosion resistance of dual phase steel. Ferrite-martensite dual phase steel with different martensite volume fractions were obtained after heat treatment at different intercritical temperatures. Microstructure, mechanical properties of steel were measured and the corrosion resistance was evaluated via polarization test. Tensile strength of the specimens increased by increasing the martensite volume fraction up to 48.2%. Further increase in martensite volume fraction led to decrease in tensile strength. Work hardening behavior analyzing showed that in DP steel with less than 50% martensite volume fraction, the work hardening took place in one stage and by increasing the martensite volume fraction two-stage work hardening behavior was observed in the Holloman analysis. The results of polarization test showed that, the corrosion resistance of dual phase steel is higher than that of plain carbon steel with ferrite-pearlite microstructure.
In this study, friction-stir spot welding is performed on DP780 dual-phase steel sheets to investigate the effect of the tool geometry and welding parameters on the static strength, the failure mode and the microstructure of the welds. First, the effects of two different parameters, the tool plunge depth and the tool holding time, were evaluated, where the tool penetration varied between 1.9-2.8 mm and the dwell time between 8-16 s. The tensile shear strength was first increased with the tool penetration and then decreased, while a longer tool holding time increased the tensile shear strength of the joints. Next, welds were made to compare the effects of two tool geometries, i.e., the conventional tapered pin and the triangular pin, on the hook shape, microstructure and static strength. The difference in the hook geometry led to differences in the tensile shear strength of the welds. The static strength of the welds made with the triangular pin was higher than that obtained using the tapered pin. Keywords: friction-stir spot welding, tool geometry, static strength, dual-phase steel, microstructure V {tudiji je izvedeno torno vrtilno to~kovno varjenje na DP780 dvofazni jekleni plo~evini, z namenom raziskave vpliva geometrije orodja in varilnih parametrov na stati~no trdnost, tip po{kodbe in mikrostrukturo zvarov. Najprej so bili ocenjeni u~inki dveh razli~nih parametrov, globine penetriranja in~asa zadr`anja orodja. Globina je variirala med 1,9 mm in 2,8 mm in zadr`evalni~as med 8 s in 16 s. Natezna stri`na trdnost se je s pove~anjem globine penetracije najprej zvi{ala, nato pa zni`ala, medtem ko je~as zadr`evanja orodja pove~al natezno stri`no trdnost zvarov. Izdelani so bili tudi zvari, na katerih se je dolo~il vpliv geometrije orodja, klasi~nega polkro`nega in trikotnega profila, na obliko, mikrostrukturo in trdnost zvarov. Stati~na trdnost zvarov, narejenih z orodjem trikotnega profila, je bila vi{ja kot tista, pridobljena s polkro`no obliko.
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