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.