Lecture Notes on Grain Structure of Metals

Introduction to Metal Grain Structure

• Safety barriers around junctions are zinc-coated.
• Zinc surface shows patches, which are crystals or grains.
• Metals typically consist of grains but are not always visible on the surface.

Etching Process

• Revealing Grains:
  • Use a mirror-like finish on metals like aluminum.
  • Treat with a powerful acid, wash, and apply a second chemical treatment (etching).
  • Different chemicals are used based on the metal.
• Observation of Grains:
  • Etching reveals similar sized grains in aluminum.
  • Grains can vary in size and shape in different samples (e.g., copper, zinc).

Formation of Metal Grains

• Grains form when metals are molten and solidify.
• Casting Process:
  • Molten aluminum is poured into molds to form slabs.
• Crystal Growth:
  • Tiny crystals begin forming in the liquid metal.
  • Fully grown crystals are called grains.

Cold Working of Metals

• Rolling Process:
  • Aluminum is rolled to reduce thickness, undergoing cold-working.
• Effect on Grain Structure:
  • Grains become elongated and distorted during rolling.
  • Change in grain structure affects mechanical properties:
    • Hardness and tensile strength increase, ductility decreases.

Recrystallization Process

• Heating cold-worked metals restores grain structure.
• Temperature Impact:
  • At around 350°C, new grains form at grain boundaries, replacing distorted structure (recrystallization).
• Mechanical Properties Post-Recrystallization:
  • Restored softness and ductility, decreased tensile strength.
• Caution on Temperature:
  • Excessive heating can lead to grain growth, affecting material properties negatively.

Grain Structure in Steel

• Types of Grains in Steel:
  • Ferrite (light grains) provides ductility.
  • Perlite (dark grains) consists of iron and carbon, providing hardness and strength.
• Effect of Carbon Content:
  • Varying carbon content changes the proportion of ferrite and perlite.

Heat Treatment of Steel

• Heat Treatment Process:
  • Involves controlled heating and cooling to modify grain structure and mechanical properties.
• Normalizing:
  • Heating steel to 720°C and cooling in air reduces grain size and increases uniformity.
• Toughness Measurement:
  • Toughness increases with heat treatment, fine grain structure results in better impact resistance.
• Quenching:
  • Rapid cooling increases hardness but can make steel brittle.
• Tempering:
  • Heating quenched steel modifies structure, reducing brittleness while retaining hardness.

Conclusion

• Understanding grain structure and treatment processes is critical for engineering applications.
• Tailoring the mechanical properties of metals requires careful control of grain size through processes like etching, cold working, and heat treatment.