Effect of vanadium alloying on the precipitation reactions during pearlite formation in medium and high carbon steels

Vanadium microalloyed medium carbon steels are currently used in the automotive industry due to the improved tensile and impact properties that are achieved with moderate additions of vanadium. So far, from results published in the literature, the improvement in mechanical properties is attributed to the precipitation of VC within the proeutectoid ferrite that constitutes the major microstructural phase of these steels, while the pearlitic component has received relatively less attention. The purpose of the present study is to elaborate on the precipitation of vanadium carbide occurring during the isothermal pearlitic reaction in vanadium alloyed medium and high carbon steels. Since during this isothermal transformation the formation of proeutectoid ferrite precedes that of pearlite, the precipitation of VC within the proeutectoid ferrite was also studied, in order to verify existing literature results and to establish a basis of comparison with the VC precipitation occurring within the pearlitic ferrite during the subsequent pearlite formation. Three vanadium alloyed steels were used during the present research. These included two experimentally produced alloys, and a commercially available steel. The present study confirmed that, vanadium carbide interphase precipitates always exist within the pearlitic ferrite component of the lamellar and within non-lamellar pearlite. In the case of the lamellar pearlite, aligned arrays of interphase VC precipitates are present within the pearlitic ferrite, whereas in the non-lamellar pearlitic ferrite random distributions are most frequently observed. No partitioning of vanadium was observed into the adjacent pearlitic cementite regions even in cases of prolonged ageing at the high temperatures of the isothermal transformation. Morever, the experimental results show clearly that interphase VC precipitation exists not only within pearlitic ferrite but also within the proeutectoid ferrite,that forms in low carbon steels, in a form of well defined linear or curved arrays of regular shaped and uniformly sized particles.