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Effects of vanadium microalloying of 1080 steel on pearlite transformation kinetics and microstructural development
Wong, Addison
Wong, Addison
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2025
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The effects of V alloying on the pearlite transformation kinetics and microstructure in 1080 steel were evaluated. Two industrial wire rod steels, 1080 and 1080V with a 0.035 wt pct V addition, were isothermally heat treated using dilatometry to assess pearlite transformation rates. A detailed microstructural analysis, utilizing field emission scanning electron microscopy, was performed to measure pearlite ILS and colony size. Grain boundary cementite formation and morphology was evaluated to determine the influence of V in 1080 steel. Vickers microhardness testing was employed to examine the effect of the V addition on hardness. V partitioning/precipitation was analyzed through atom probe tomography (APT).
Isothermal heat treatments were conducted with a focus on achieving isothermal pearlite transformations at 550, 600, and 650 °C. Care was taken to minimize the temperature increase during pearlite transformation due to recalescence and Johnson-Mehl-Avrami-Kolmogorov fits were created using dilatometry data. V was found to retard the pearlite transformation, and induce an incubation period prior to transformation, across all transformation temperatures tested compared to the base 1080 steel. The slowed pearlite transformation resulted in a refinement of the pearlite ILS and an increase in the Vickers hardness of the 1080V steel. The 1080V steel had a finer pearlite ILS by 6-28 nm and a higher hardness by 15-20 HV, compared to the 1080 steel. Calculated yield strengths of the two steels indicated no apparent precipitation strengthening. APT revealed V clusters, with average diameters of approximately 2.6 nm, to form randomly within the ferrite lamellae in samples transformed at 650 °C. No clustering was detected by APT within samples transformed at 550 and 600 °C. Proeutectoid grain boundary ferrite formation was observed to be promoted by the addition of V and proeutectoid grain boundary cementite formation was suppressed by the presence of V, exhibiting a more discontinuous morphology at the center of the 1080V wire rod.
APT revealed V partitioning into cementite lamellae at 600 and 650 °C. No partitioning of V occurred for transformations at 550 °C. A comparable response was noted for Cr and Mn, which partitioned substantially into cementite lamellae at 600 and 650 °C, but remained homogeneous in the microstructure for transformations at 550 °C. Conversely, Si and Cu partitioned substantially into ferrite lamellae at all transformation temperatures.
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