Figures (a) ~ (d) are metallographic photos of vanadium titanium gray cast iron with different components. It can be seen that the graphite in the metallographic structure of vanadium titanium gray cast iron is mainly A-type graphite and D-type graphite. When the content of carbon, silicon, vanadium and titanium is small, as shown in figure (a), the graphite form is typical flake A-type graphite, which is distributed uniformly without directionality. With the increase of vanadium and titanium content, as shown in figure (b), the length of flake graphite in the sample becomes shorter and the width decreases slightly. With the increase of vanadium and titanium content, as shown in figure (c), flake A-type graphite begins to grow in the direction of fine curled D-type graphite. As the content of carbon, silicon, vanadium and titanium increases at the same time, as shown in figure (d), the content of fine curled D-type graphite decreases, and coarse thick flake or block graphite, i.e. C-type graphite, begins to appear.
This is because vanadium is a strong carbide forming element, which hinders the progress of graphitization, reduces the diffusion rate of carbon atoms, promotes the precipitation of primary austenite dendrites, reduces the space for graphite nucleation and growth during Eutectic Transformation, changes the growth properties of graphite plate, and makes graphite grow in the direction of fine curled D-type graphite. At the same time, the addition of titanium increases the supercooling tendency in the solidification process of molten iron, promotes the precipitation of primary austenite, is conducive to the formation of D-type graphite, and changes the graphite morphology from A-type graphite to D-type graphite. Titanium is also a strong carbide forming element. The formation of carbide can refine graphite (when the content of titanium is > 0.1%) and promote the formation of austenite dendrite. The developed austenite dendrite makes molten iron gather in the dendrite gap, and graphite forms a large number of nuclei in a narrow small area. However, due to the poor growth conditions of graphite, fine curly D-type graphite is trapped in the austenite dendrite gap, As shown in figure (c). In figure (d), when the content of carbon, silicon, vanadium and titanium increases, thick and large pieces of graphite are formed in liquid state due to high carbon equivalent.