Quantitative Analysis of Precipitate in Vanadium-microalloyed Medium Carbon Steel Using Small-angle X-ray and Neutron Scattering Methods

The microstructures and chemical composition of nano-precipitates in vanadium (V) steels were investigated by the alloy contrast variation method (ACV) using small-angle X-ray scattering (SAXS) coupled with small-angle neutron scattering (SANS) at holding temperatures ranging between 600 and 700 degrees C. Both the SAXS and SANS profiles exhibited clear scattering, depending on the holding temperature, due to the presence of nano-precipitates. The scattering profiles of the precipitates are characteristic of spherical or disc-like particles. The average diameters of these precipitates increased from 0.5 nm at 600 degrees C to 23 nm at 700 degrees C, whereas the number density of the precipitates decreases with increased holding temperature. Therefore, the increasing holding temperature results in an increase in the growth rate of the precipitates. ACV analysis revealed that the chemical composition of the precipitates corresponds to NaCl-type vanadium carbide (VC) at 675 and 700 degrees C, and as VC0.9 at 625 and 650 degrees C. The formation of a different heterogeneity, non-NaCl type, was found in the sample at a holding temperature of 600 degrees C. This probably corresponds to a precursor of the NaCl phase in the initial process of precipitation.