Classification of Cast Iron

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Introduction to Cast Iron Fundamentals

The term cast iron encompasses a broad spectrum of ferrous alloys, each with distinct compositional and structural characteristics. These multicomponent alloys contain major elements (iron, carbon, silicon), minor elements (<0.01%), and frequently, alloying elements (>0.01%). What distinguishes cast iron from steel is its notably higher carbon and silicon content, resulting in a rich carbon phase structure. This unique composition leads to specific solidification patterns that directly influence the material's properties and applications.

Solidification and Microstructural Development

The solidification behavior of cast iron follows two distinct pathways, determined primarily by composition, cooling rate, and melt treatment. These pathways can be either thermodynamically metastable (Fe-Fe3C system) or stable (Fe-Gr system). This differentiation is crucial because it determines the final microstructure and, consequently, the material's properties.

When following the metastable path, the rich carbon phase in the eutectic manifests as iron carbide. Conversely, when the stable solidification path is followed, graphite becomes the predominant carbon phase. While traditional definitions classify cast iron as an iron-carbon alloy containing more than 2% carbon, it's important to note that silicon and other alloying elements can significantly alter carbon's maximum solubility in austenite (γ). In some exceptional cases, alloys containing less than 2% carbon may still exhibit eutectic structures, qualifying them as cast irons.

The formation of stable or metastable eutectic structures depends on several key factors:

• The nucleation potential of the liquid
• Chemical composition
• Cooling rate

These factors collectively determine what metallurgists refer to as the "graphitization potential" of the iron. A high graphitization potential results in irons with graphite as the rich carbon phase, while a low potential leads to the formation of iron carbide.

Mechanical Properties and Eutectic Formation

The mechanical properties of cast iron are fundamentally linked to its eutectic structure. The two basic types of eutectics - austenite-graphite (stable) and austenite-iron carbide (metastable Fe3C) - exhibit markedly different mechanical characteristics. These differences manifest in several key properties:

• Strength
• Hardness
• Toughness
• Ductility

This relationship between structure and properties forms the foundation of cast iron metallurgy. Metallurgists can manipulate three primary aspects of the eutectic to achieve desired mechanical properties:

• Type of eutectic
• Amount of eutectic present
• Morphology (shape and distribution) of the eutectic

Historical Classification and Modern Identification

Cast iron classification has evolved significantly from its historical roots. The earliest classification system was based purely on fracture appearance, identifying two primary types:

White Iron:

• Characterized by a white, crystalline fracture surface
• Fracture occurs along iron carbide plates
• Results from metastable solidification (Fe3C eutectic)

Gray Iron:

• Exhibits a gray fracture surface
• Fracture occurs along graphite plates (flakes)
• Results from stable solidification (Gr eutectic)

Figure 1: Classification of special high-alloy cast iron

Modern Classification Systems

With the advancement of metallographic techniques and increased understanding of cast iron properties, classification systems have become more sophisticated, focusing on microstructural features. Modern classification considers two key aspects:

Graphite Morphology:

• Lamellar (flake) graphite (FG)
• Spheroidal (nodular) graphite (SG)
• Compacted (vermicular) graphite (CG)
• Temper graphite (TG) - formed through solid-state malleabilization

Matrix Structure:

• Ferritic
• Pearlitic
• Austenitic
• Martensitic
• Bainitic (austempered)

Commercial Classification

In practical foundry applications, a simpler commercial classification system is commonly used, dividing cast irons into two main categories:

Common Cast Irons:

• Used for general-purpose applications
• Unalloyed or low-alloyed compositions
• Cost-effective solutions for standard applications

Special Cast Irons:

• Designed for specialized applications
• Higher alloy content (>3%)
• Enhanced properties for specific requirements:
• Elevated-temperature resistance
• Corrosion resistance
• Wear resistance

Figure 2: Basic microstructures and processing for obtaining common commercial cast irons