Microstructure of nodular cast iron as cast

In the process of cooling to room temperature, the microstructure of nodular cast iron may follow two paths of phase transformation, resulting in two different types of as cast microstructure. The first is the most common as cast structure of nodular cast iron, that is, the matrix structure of spherical graphite + cementite transformed according to the stable system; The other is the as cast structure of spheroidal graphite + cementite matrix formed by metastable system transformation. This kind of spheroidal graphite cast iron is often used in the manufacture of wear-resistant cast iron. At present, the most commonly used ductile iron in the automotive industry is the first type of ductile iron. Therefore, the following focuses on the as cast structure of the first type of ductile iron.

(a) Conical polycrystals form spherical graphite; (b) Surface morphology of spheroidal graphite

(1) Graphite: as the name suggests, the graphite of nodular cast iron is mainly spherical, and the core material can be observed in most graphite centers (that is, the crystalline core of graphite, including magnesium, cerium sulfide, silicon oxide and the complex of the above substances). As shown in Fig. 0001, the graphite crystals grow in the direction of single cone, and finally form many crystals in the direction of radiation, as shown in Fig. 0001. Using scanning electron microscope to observe the spherical graphite after special treatment such as hot oxygen corrosion and ion bombardment (Fig. (b)), it can be observed that the surface of spherical graphite is not flat and there are many holes and bulges.

The diameter of graphite ball of general nodular cast iron castings is between 0.015 ~ 0.5 mm. According to China’s national standard GB / t9441-2009, spherical graphite can be divided into six levels according to different sizes. The size of graphite balls is usually observed by magnifying 100 under an optical microscope.

(2) Matrix: after solidification of nodular cast iron solution, solid phase transformation will occur in the process of continuous cooling. Firstly, the supersaturated carbon in austenite precipitates in the process of temperature drop, and then eutectoid transformation occurs when the temperature drops to the eutectoid transformation temperature. The product of eutectoid transformation is the matrix structure of nodular cast iron. Because the type and structure of the products produced by this transformation are affected by the chemical composition of the casting and some process factors, several different types of matrix structures such as ferrite, pearlite (including sorbite and troostite with small sheet spacing), austenite, martensite and bainite may eventually be produced.

The ductile iron used in general production is mostly the matrix structure of ferrite and pearlite. When the volume fraction of ferrite in the matrix exceeds the volume fraction of pearlite, it is called ferrite pearlite nodular cast iron, otherwise it is called pearlite ferrite nodular cast iron. When the volume fraction of ferrite exceeds 95%, it is called ferritic nodular cast iron.