The ferrite content in the matrix structure ofaffects the carbon content of the matrix, thus affecting the formation of bainite during heat treatment. The ferrite content (volume fraction) determined according to GB / T 9941-2009 is shown in Figure 1. With the increase of Nb content, the ferrite content in the matrix decreases gradually. The ferrite content of niobium free ductile iron is 6.3%, and it decreases to 2.0% when the niobium content is 0.69%.
The as cast structure of nodular cast iron with different Nb content is shown in Fig. 2, which is mainly composed of graphite ball, pearlite and ferrite, with pearlite accounting for the majority, and ferrite in the form of ox eye and fragment. In addition, the addition of niobium also reduces the white ferrite content.
As shown in Fig. 3, the pearlite lamellar spacing is reduced and the pearlite is more compact in the nodular cast iron with 0.20% Nb compared with that without NB. The pearlite lamellar spacing depends on the undercooling degree. The increase of Nb content increases the undercooling degree of ductile iron and decreases the pearlite lamellar spacing. Therefore, adding niobium can refine the pearlite structure.