1: Cast Irons

Overview of Cast Irons

• Cast irons are alloys of iron with a carbon content typically between 2-4%.

• In contrast, most steels (low, medium, high carbon) have a carbon content of less than 1%.

• The solubility limit for carbon in high-temperature austenite is 1.7%.

Iron-Carbon Phase Diagram

• At the eutectic temperature, austenite can hold about 1.7% carbon.

• When cooled slowly to room temperature, excess carbon precipitates as graphite inclusions.

• Rapid cooling leads to a microstructure predominantly composed of cementite (semantite), while slower cooling allows for graphite formation.

• Silicon enhances graphite precipitation in cast irons, causing it to form as rosettes or round flakes.

Families of Cast Irons

• Cast irons are categorized into four main families:

  • Gray Iron

    • Gray iron is a general-purpose cast iron characterized by a gray fractured cross-section due to graphite inclusions.

    • The matrix usually consists of steel composition, which may include ferrite, pearlite, or martensite.

    • Properties: Self-lubricating, good wear characteristics, excellent vibration damping.

  • White Iron

    • Identified by a white fractured cross-section resulting from rapid cooling, preventing graphite formation.

    • Composed mainly of free cementite and pearlite, leading to high hardness and wear resistance.

  • Malleable Iron

    • Produced from white iron through extensive heat treatment, enhancing ductility up to 20%.

    • Allows for some bending and shaping.

    • Heat treatment typically lasts over a hundred hours.

  • Ductile Iron

    • Developed in 1948, known for its ductility through alloying additions rather than extensive heat treatment.

    • Also referred to as nodular cast iron due to its microstructure.

Composition of Cast Irons

• Comparison of carbon and silicon content across different cast iron families:

  • Gray Iron: Higher carbon (2-4%) and silicon (1-3%) contents.

  • White Iron: Lower carbon and silicon contents compared to gray iron.

Mechanical Properties and Characteristics

• Elastic Modulus:

  • Cast steels have elastic moduli similar to wrought steels.

  • Gray cast iron's modulus is about two-thirds that of steel.

  • Nodular and malleable irons possess intermediate moduli between gray iron and steel.

• Impact Toughness:

  • Cast irons, especially gray cast iron, exhibit much lower toughness than steels.

  • Nodular and malleable irons provide improved toughness and impact resistance due to higher ductility.

Applications and Key Characteristics of Gray Cast Iron

• Notable properties include damping capacity due to graphite inclusions, which reduce vibration.

• Commonly used in applications such as machine bases and engine blocks due to these characteristics.