The figure shows the solidification cooling curve of nodular cast iron with different Nb content. It can be seen that the temperature decreases rapidly at the initial stage of solidification due to the thermal diffusion of molten iron. The carbon equivalent of the ductile iron used in the test is 4.8% ～ 5.0%, which belongs to hypereutectic cast iron. Therefore, when the temperature of the molten iron drops to the liquidus, the eutectic graphite begins to nucleate, grow up, and release the latent heat of crystallization, so that the temperature of the molten iron rises slightly and a crystallization platform appears. The corresponding temperature of the platform is the eutectic temperature of the ductile iron.
Eutectic temperature of nodular cast iron with different Nb content. It can be seen that the eutectic temperature of Nb free ductile iron is 1133 ℃. With the increase of Nb content, the eutectic temperature first increases and then decreases, but the change range is small, indicating that the effect of Nb on the eutectic temperature of ductile iron is not obvious. When the niobium content is low, niobium combines with carbon in molten iron to form niobium carbide (NBC), which becomes the core of heterogeneous nucleation of graphite, promotes the formation of graphite and increases the eutectic temperature. When the niobium content is high, the precipitation of NBC consumes the carbon element in the molten iron, which reduces the amount of primary graphite, delays the eutectic reaction and reduces the eutectic temperature, resulting in the delay of eutectic reaction and the decrease of eutectic temperature.