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In the present experimental work, the effects of vanadium additions on the microstructures and mechanical properties of powder metallurgy (PM) steel and microalloyed powder metallurgy (PM) steels were investigated. The microstructures of the PM steel and microalloyed PM steels were characterized by optic microscope, SEM and EDS. Experimental results showed that vanadium microalloyed steels can be produced by PM technology. The addition of vanadium limits grain growth during austenitization prior to air cooling and increases the strength in the sintered conditions. By limiting austenite grain growth, the precipitates result in significant improvement in strength.

In this work, the effects of the addition of Nb and Al on the microstructures and tensile behaviours of micro-alloyed powder metallurgy (PM) steels were investigated. The microstructure of the micro-alloyed PM steels was examined by light microscope, SEM, XRD, XRF and EDS. The results indicated that the addition of (0.1, 0.15 or 0.2) % of Nb-Al increases the yield strength (YS) and the ultimate tensile strength (UTS) of the PM sintered steels. Elongation also tends to improve with an increasing Nb and Al content. In addition, the Nb and Al limit the grain growth during austenitization. Keywords: powder metallurgy, micro-alloyed steels, microstructure V delu je bil preiskovan vpliv dodatka Nb in Al na mikrostrukturo in na natezno trdnost mikrolegiranih PM jekel, izdelanih iz prahov. Mikrostruktura mikrolegiranih PM jekel je bila preiskovana s svetlobnim mikroskopom ter s SEM, XRD, XRF in EDS. Rezultati so pokazali, da dodatek (0,1, 0,15 ali 0,2) % Nb-Al pove~a mejo plasti~nosti (YS) in natezno trdnost (UTS) sintranih PM jekel. Tudi raztezek se izbolj{a s pove~ano vsebnostjo Nb in Al. Ugotovljeno je {e, da dodatek Nb in Al omejuje rast zrn med avstenitizacijo.

In this study, the effects of heat treatment on the microstructures and mechanical properties of powder metallurgy (PM) Nb-V microalloyed steels (Fe + 0.25%C + 0.075%Nb + 0.075%V) were investigated. Argon gas was used as sintering atmosphere. The microstructure and mechanical properties of the Nb-V added PM microalloyed steel were examined by optical microscopy, scanning electron microscope (SEM) and tensile tests. Experimental results showed that microalloyed steels can be produced by PM technology and the heat treatment affects the microstructure and mechanical properties of microalloyed PM steel. Nb-V microalloyed steels were heat treated under different conditions. Microstructures and tensile strengths of the samples were compared.

The effect of different microstructures on the fatigue behaviour of a medium carbon vanadium microalloyed steel has been studied. Specimens were subjected to a controlled closed die forging followed by cooling in sand, air or oil, respectively. The hardness and fatigue properties of the microalloyed steel are determined and compared with those of ferrite-pearlite and martensite microstructures obtained by cooling with different mediums after forging. Relatively fine ferrite and pearlite increase the fatigue strength of the steel, while the martensite structure reduces the fatigue strength. Characteristics of fatigue fracture surface morphology are summarized and related to fatigue crack initiation and propagation mechanisms in the forged medium carbon microalloyed steel. The cooling rate has a remarkable effect on the microstructure, hardness, and fatigue behaviour at room temperature.

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