Steel Composition and Properties

Steel Composition

• Steel is an alloy composed of iron and carbon, with up to 1.5% carbon by weight
• Alloys with higher carbon content are classified as cast iron
• Other elements are added to steel for specific properties

Iron Allotropes

• Ferrite (α-iron) is the most stable form of iron at room temperature, with a body-centered cubic structure.
  • It’s a soft and ductile material.
  • It can only dissolve a small amount of carbon, up to 0.021 wt% at 910 °C.
• Austenite (γ-iron) has a face-centered cubic structure.
  • It’s also soft and metallic.
  • It can dissolve a larger amount of carbon, up to 2.04 wt% at 1146 °C.
• Cementite (Fe3C) is a stoichiometric phase with high hardness and low ductility.
  • It forms in areas with a higher carbon concentration.

Steel Structures and Properties

• Pearlite is composed of alternating layers of ferrite and cementite, with a total carbon content of 0.83%.
  • Pearlite properties vary depending on the carbon content, with a higher carbon content resulting in a mixture of pearlite and cementite grains.
• Bainite is a similar structure to pearlite, but with a different cementite morphology.
• Martensite is a chemically metastable phase with a strength up to five times higher than ferrite.
  • It has a similar unit cell structure to austenite.
• Stainless Steel is composed of large amounts of chromium and nickel (around 18% and 8%, respectively).
  • These elements form a hard oxide on the metal surface to inhibit corrosion.
• High-Speed Steel contains tungsten which interferes with cementite formation, allowing martensite to form at slower quench rates.
  • This results in high-speed steel properties.
• Brittle Elements:
  • Sulfur, nitrogen, and phosphorus increase steel brittleness, therefore requiring removal during ore processing.

Steel Properties and Alloying Elements

• Nickel & Manganese: Increase tensile strength and enhance austenite stability
• Chromium: Enhances hardness and melting temperature
• Vanadium: Boosts hardness while reducing metal fatigue

Steel Composition

• Steel is an alloy of iron and carbon, with up to 1.5% carbon by weight.
• Higher carbon content alloys are called cast iron.

Iron Allotropes

• Ferrite (α-iron) is the most stable form of iron at room temperature.
  • Body-centered cubic structure.
  • Soft and ductile.
  • Dissolves only a small amount of carbon (0.021 wt% at 910 °C).
• Austenite (γ-iron) is a face-centered cubic structure.
  • Soft and ductile.
  • Dissolves significantly more carbon (2.04 wt% at 1146 °C).

Cementite (Fe3C)

• Stoichiometric phase.
• Non-ductile and very hard.
• Forms in regions of higher carbon content.

Pearlite and Bainite

• Laminar structure of alternating ferrite and cementite layers.
• Carbon content: 0.83% by weight.
  • < 0.83% Carbon: Mixture of ferrite and pearlite grains.

  • 0.83% Carbon: Mixture of pearlite and cementite grains.

  • = 0.83% Carbon: Wholly pearlite composition (eutectoid structure).

Martensite

• Chemically metastable.
• Four to five times stronger than ferrite.
• Similar unit cell structure to austenite, but with identical chemical composition.

Steel Alloying Elements

• Nickel and manganese increase tensile strength and stabilize austenite.
• Chromium increases hardness and melting temperature.
• Vanadium increases hardness and reduces metal fatigue.

Stainless Steel

• Contains high amounts of chromium (18%) and nickel (8%).
• Forms a hard oxide layer on the surface to inhibit corrosion.

High Speed Steel

• Tungsten interferes with cementite formation, allowing martensite to form with slower quench rates.
• Results in high-speed steel.

Brittle Elements

• Sulfur, nitrogen, and phosphorus make steel brittle.
• Must be removed from ore during processing.

This quiz explores the composition and properties of steel, including its alloys and the various allotropes of iron. You'll learn about ferrite, austenite, and cementite, as well as their characteristics and structures. Test your understanding of how these elements affect the overall properties of steel.