Variation of microstructure of as cast ductile iron with casting modulus (wall thickness)

For austempered ductile iron, the carbon content in high temperature austenite and retained austenite has a significant effect on the subsequent isothermal transformation and mechanical properties of the alloy. However, the solubility of carbon atoms in austenite varies with the as cast microstructure and austenitizing process. This experiment plans to use the method of controlling casting modulus to obtain as cast ductile iron with different matrix types, and then to explore the carbon content and carbon element distribution in the matrix of as cast ductile iron with different modulus. At the same time, the influence of austenitizing process on the carbon content in austenite of as cast ductile iron with different matrix is studied, so as to optimize the austenitizing heat treatment of ductile iron Art provides a theoretical basis.

The specifications of ductile iron test bars cast in this experiment are Φ 10 mm × 300 mm, Φ 20 mm × 300 mm, Φ 30 mm × 300 mm, Φ 40 mm × 300 mm and Φ 50 mm × 300 mm. The corresponding casting modulus MS are 0.25 cm, 0.50 cm, 0.75 cm, 1.00 cm and 1.25 cm, respectively. The graphite morphology of nodular iron bars with different casting modulus is shown in Fig. 1, and its matrix structure is shown in Fig. 2.

The distribution of graphite in nodular cast iron, the number of graphite balls, the size of ball diameter, the rate of spheroidization and the amount of graphite precipitation have significant effects on the matrix structure and mechanical properties of ductile iron. Among them, the spheroidal diameter and spheroidization rate of graphite are one of the important standards to measure the nodularization quality of nodular iron. Good spheroidizing effect will obtain good performance index. The spheroidizing effect of ductile iron depends not only on its chemical composition, but also on the cooling rate of the casting, that is, the modulus ms of the casting. Therefore, according to GB / t9441-2009 metallographic examination standard of nodular cast iron, and with the help of professional image analysis software image pro plus 6.0, the graphite morphology and matrix structure of as cast ductile iron were quantitatively analyzed. Among them, the point graphite with diameter less than 5 μ m is not included in the total number of graphite balls.

With the increase of casting modulus MS, the cooling rate decreases. On the one hand, the amount of graphite precipitation in nodular iron matrix increases, the diameter of graphite ball increases, while the spheroidization rate of graphite decreases and the number of graphite balls decreases; on the other hand, the quantity of pearlite (P) and ferrite (α) increases in as cast structure. This shows that the number of ferrite in matrix can be indirectly characterized by casting modulus.

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