Relation between CE Structure and Mechanical Properties

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Understanding Carbon Equivalent and Structural Relationships

A preliminary attempt to relate composition and structure was demonstrated in Figure 3 of the Cast Irons article, though its foundry applications remained limited. Figure 1 presents a more practical relationship between carbon equivalent values, structure, tensile strength in 30 mm diameter bars, and section size considerations.

Figure 1: Diagram relating section size, CE value, tensile strength and structure (After BCIRA)

The cooling behavior of cylindrical test bars differs significantly from flat plates of equivalent thickness, with bars cooling more rapidly. This fundamental difference necessitates expressing sections as either bar diameter or section thickness. Line H represents the boundary of unmachinable irons, while line P delineates the boundary between soft and pearlitic irons. For iron with a carbon equivalent of 4.35, the maximum thickness should not exceed 20 mm for bars or 10 mm for plates to achieve pearlitic iron structure. To prevent unmachinable chilled castings, bars should maintain a minimum diameter of 8 mm, while plates require a minimum thickness of 4 mm.

Controlling Carbon Equivalent in Foundry Operations

Melting furnaces typically produce iron with consistent carbon equivalent values, with silicon serving as the primary element for chill control. Foundries incorporate various alloying elements into cast iron to impart special properties while simultaneously managing chill characteristics.

Graphite Formation Mechanisms in Cast Iron

Flake Graphite Development, Spheroidal Graphite Formation and Temper Carbon Nodule Development

The overall stages in the flake graphite development, spheroidal graphite formation and temper carbon nodule development are shown in Figure 2a to 2c below.

Figure 2a to 2c: Graphite formation mechanisms (2a: Flake, 2b: Spheroidal, 2c: Temper carbon nodule development)

The eutectic solidification process begins at nucleation sites, each forming roughly spherical formations called eutectic cells. Within these cells, simultaneous growth of austenite and graphite occurs, with graphite maintaining continuous contact with the liquid phase.

Microscopic examination typically reveals graphite as separate flakes, but current understanding suggests each eutectic cell contains a continuous branched graphite skeleton resembling cabbage structure. Rapid radial cell growth, occurring during increased cooling rates, produces more frequent branching and undercooling, resulting in finer graphite structures visible in micrographs.

Eutectic cell diameter significantly influences mechanical properties. Higher cell density per unit volume correlates with increased tensile strength, though this adversely affects soundness. Superheating or extended holding times of molten iron reduce nuclei numbers, while inoculants such as ferro-silicon and sulfur increase nucleation sites.

Figure 2b demonstrates spherulitic graphite growth in magnesium-treated iron. Spherulitic graphite becomes rapidly surrounded by austenite layers, with spheroid growth occurring through carbon diffusion from liquid through the austenite envelope. Large diffusion distances create tendencies for remaining liquid to solidify as white iron eutectic, making inoculation highly desirable for increasing graphite formation centers.

At malleabilizing temperatures (800-950°C), solid white iron consists of eutectic matrix containing cementite, austenite, and sulfide inclusions. Graphite nucleation occurs at austenite-cementite interfaces and sulfide inclusions. Cementite gradually dissolves in austenite, with carbon diffusing to graphite nuclei. MnS tends to form flake aggregates while FeS creates spherulitic nodules (Figure 2c).

Cast Iron Microstructure Analysis

Specimen Preparation Considerations

Proper specimen preparation requires careful attention to prevent erroneous results. Graphite removal during polishing can cause cavities to become burnished over or enlarged. Various microstructure types can be classified into groups without considering phosphorus presence.

Figure 3: Medium size graphite outlining dendrites (x60)

Figure 4: Coarse graphite flakes; matrix unetched (x 60)

Figure 5: Temper carbon in a malleable iron; ferrite crystals etched (x 100)

Graphite exhibits varying sizes and forms as illustrated in Figures 3-5. Coarse flaky graphite characterizes common iron, while fine curly types, frequently outlining dendrites, appear in high-class iron, particularly when superheated before casting. Spheroidal graphite occurs in magnesium-treated irons (Figure 6), while nodular forms develop in annealed irons where cementite decomposes at 800-950°C.

Figure 6: Enlarged view of graphite spheroid; polarised light (x 600)

The ferrite present in cast iron contains significantly more impurities compared to carbon steels, affecting its properties and behavior (Fig. 7 to 9).

Figure 7: Hypo-eutectic white cast iron, cementite and pearlite (black)(x 100) BH =100

Figure 8: Hyper-eutectic white cast iron (x 100). White primary crystals of cementite in eutectic (cementite and pearlite)

Figure 9: Grey iron. High duty; pearlite and graphite (x 200)

Phosphide Eutectic Formation and Effects

Most cast irons contain phosphorus ranging from 0.03 to 1.5%, introducing additional microstructural constituents beyond previously mentioned phases. In white irons, this appears as laminated constituents (ternary eutectic) with melting points around 960°C, making it the final constituent to solidify and form islands in dendrite interstices.

Despite its brittle nature, this constituent does not significantly weaken iron in small amounts (up to 1%) because continuous cells do not form around grains. The structure, illustrated in Figure 4 from the Cast Iron article, shows phosphide eutectic alongside graphite, ferrite, and pearlite. Phosphorus forms this additional constituent in any of the previously discussed grouped structures.

Conclusion

Understanding the relationship between carbon equivalent structure and mechanical properties provides foundries with essential tools for optimizing cast iron production. The interplay between cooling rates, section sizes, and chemical composition directly influences final microstructure and performance characteristics. Proper control of these variables, combined with appropriate inoculation practices, enables production of cast iron components with desired mechanical properties and machinability characteristics.