Vermicular Graphite Cast Iron

---

Historical Development

Cast irons, fundamentally iron-carbon alloys, have evolved significantly from traditional gray cast iron to more sophisticated variants. While gray cast iron, with its flake graphite structure, offers cost-effectiveness and excellent damping capacity, the discovery of spheroidal graphite cast iron (SG iron) in 1947 marked a significant advancement in cast iron technology.

Vermicular graphite cast iron emerged as an accidental discovery during SG iron production. Initially considered a defective form, research by Estes and Scheidewind in 1955 demonstrated its unique advantages, leading to intentional production methods and comprehensive property investigations by numerous researchers.

Microstructural Characteristics

The term "cast iron with vermicular-type graphite (GJV)" derives from the distinctive worm-like appearance of its graphite structure.

Figure 1: The microstructure of Vermicular graphite iron

Figure 2: Vermicular graphite under scanning electron microscope

The three-dimensional structure becomes particularly evident under scanning electron microscopy after matrix etching. This graphite formation represents a transitional state between lamellar and spheroidal structures, occurring through controlled or interrupted treatment during the spheroidization process.

Properties and Advantages

Vermicular graphite cast iron, standardized under ISO 16112, exhibits properties intermediate between lamellar and spheroidal graphite cast irons.

Table 1. Material grades and their properties including tensile strength, yield strength, elongation, and thermal properties

Material Grade	Temp.	GJV-300	GJV-350	GJV-400	GJV-450	GJV-500
Tensile strength Rm (N/mm²)	23°C	300 - 375	350 - 425	400 - 475	450 - 525	500 - 575
0.2% yield strength Rp0.2 (N/mm²)	23°C	210 - 260	245 - 295	280 - 330	315 - 365	350 - 400
Elongation to fracture A (%)	23°C	2.0 - 5.0	1.5 - 4.0	1.0 - 3.5	1.0 - 3.0	1.0 - 2.5
Modulus of elasticity (kN/mm²)	23°C	130 - 145	135 - 150	140 - 155	145 - 160	145 - 160
Poisson's ratio	23°C	0.26	0.26	0.26	0.26	0.26
Thermal conductivity (W/m·K)	23°C	47	43	39	38	36
Coefficient of thermal expansion (mm/m·K)	100°C	11	11	11	11	11
Specific heat capacity (J/g·K)	100°C	0.475	0.475	0.475	0.475	0.475

Compared to lamellar graphite iron, VG iron offers:

• Enhanced strength and elongation characteristics
• Superior fracture toughness
• More consistent properties across varying wall thicknesses

Advantages over spheroidal graphite iron include:

• Better thermal properties
• Improved damping capacity
• Superior casting characteristics

Industrial Applications

The material excels in applications combining thermal and mechanical stress requirements. Common applications include:

• Cylinder heads
• Exhaust manifolds
• Brake components
• Automotive transmission components
• Engine units for vehicles