The effect of vanadium in as-cast ductile iron

Abstract

The effects of adding different amounts of vanadium (0.02–0.54%) to ductile irons have been investigated. Optical microscopy revealed that that the addition of vanadium led to the formation of a phase with a particle size of up to 6 μm in the as-cast microstructure and SEM—EDX microanalysis showed that this phase is vanadium-rich. In addition to these coarse particles, TEM studies using carbon extraction replicas revealed a wide range of sub-micron vanadium-containing particles. Selected area diffraction patterns of vanadium-rich particles with a size of < 100 nm showed that they were cubic and corresponded to V4C3. Image analysis and colour metallography have been used to measure the volume fraction of the coarse particles and have shown that about 75–80% of the vanadium carbide was present in the coarse form.Quantitative metallography showed that, as the vanadium content was increased to 0.54%, the average graphite nodule count decreased, the pearlit content increased by about 20–30%, the pearlite interlammellar spacing decreased and the ferrite grain size decreased. In spite of its strong tendency to promote the formation of eutectic carbide, it has been found possible to add up to 0.54% vanadium without forming eutectic carbide in sections equivalent to a 6 mm thick plate, so long as late inoculation is used to add sufficient inoculant to give a silicon increment of 0.8%.The addition of 0.54% vanadium increased the modulus of elasticity, proof strength, tensile strength and hardness but reduced the elongation.