Vermicular Graphite Cast Iron

Cast irons, like steels, are basically alloys of iron and carbon. Gray cast iron is the oldest member of the cast iron family and the carbon in it exists in the form of graphite flakes. The main factor favoring the wide spread use of gray iron is its low cost. Even though the tensile properties of gray iron are inferior, it possesses high compression strength coupled with excellent damping capacity.

However, since its invention in 1947, spheroidal graphite cast iron (SG iron), with its high tensile strength and elongation, has been found useful in replacing certain steel components. In SG iron, the graphite is present as nodules and the continuity of the matrix is much higher as compared to that in flake graphite gray iron, resulting in superior mechanical properties.

In vermicular graphite cast irons (VG irons), graphite is present in a worm-like, stubby form with rounded edges. This material is also popularly known as compacted graphite cast iron or as CG iron. Vermicular graphite cast iron was an accidental discovery during tonnage production of SG iron castings, and was initially considered a degraded form of iron. Subsequently, it has attracted much interest, and the intentional production of VG irons was first achieved by Estes and Scheidewind (1955); various productions methods and physical and mechanical properties were investigated in different countries (Sissener et al 1972; Sofroni et al 1974; Evans et al 1976; Lalich and La Presta 1978; Green and Thomas 1979; Copper and Loper 1978; Aleksandrov et al 1976).

Vermicular graphite irons have certain advantages over the older members of the cast iron family viz. gray, ductile and malleable irons:

• The tolerable content of sulfur in VG iron is much higher than in ductile iron;
• The temperature range in which the reaction material is effective is very wide in VG iron as compared to SG iron.

Microstructure

The designation “cast iron with vermicular-type graphite (GJV)” is derived from the worm-like appearance of the graphite in a micrographic specimen. The three-dimensional structure becomes apparent in a scanning electron microscope after the metallic matrix has been removed by etching.

Vermicular graphite is a transitional form between lamellar and spheroidal graphite, which occurs when the material is deliberately given insufficient treatment or treatment is interrupted in the process of forming spheroidal graphite.

Figure 1: Microstructure of Vermicular graphite iron

Figure 2: Vermicular graphite in a scanning electron microscope

Properties

Cast iron with vermicular graphite falls between cast iron with lamellar graphite and cast iron with spheroidal graphite not only in terms of its microstructure formation but in terms of its mechanical and physical properties as well. It is standardized in ISO standard 16112.

Reference values for several key properties are shown in Table 1. Similar to cast iron materials with spheroidal graphite, these properties are heavily dependent on the ferrite-to-pearlite ratio in the basic structure.

Cast iron with vermicular graphite provides the following advantages in terms of properties compared to lamellar graphite:

• Higher strength and elongation at fracture
• Higher fracture toughness
• Properties are less dependent on the wall thickness.

Compared to cast iron with spheroidal graphite, cast iron with vermicular graphite offers the following advantages:

• Lower coefficient of thermal expansion
• Higher heat conductivity
• Lower modulus of elasticity
• Lower thermally induced internal stress level
• Better thermoshock resistance and lower tendency to distortion because of the aforementioned properties
• Better damping capacity
• Better pouring properties (shrinkage, mold-filling characteristics and fluidity).

The material is especially suitable for components which are exposed to both thermal and mechanical stresses. Examples include cylinder heads, exhaust manifolds and brake parts, but vermicular graphite is also used for the manufacture of transmission domes for cars and engine units for trucks and cars.

Table 1: Mechanical and physical properties of cast iron with vermicular graphite