General Classification & Properties of Engineering Materials

Posted on Aug 20, 2024 in Geology

General Classification of Materials

For their study, materials are grouped into:

  • Metals
  • Ceramics
  • Polymers
  • Compounds

Each of these groups has general physical and chemical properties that distinguish them and make them suitable for different engineering applications.

Physical and Mechanical Properties of Engineering Materials

Physical Properties:

  • Weight (Effect of gravity)
  • Mass (Amount of matter within a volume)
  • Volume (Amount of space occupied by a body)
  • Density (Mass per unit volume)
  • Specific Weight (Weight per unit volume)
  • Temperatures: Fusion, Liquefaction, Boiling (Changes in states: Solid, Liquid, Gas)
  • Specific Heat (Quantity of heat per unit temperature)
  • Etc.

Mechanical Properties:

  • Hardness (Resistance to penetration)
  • Tensile Strength (Breaking strength)
  • Ductility (Property to be drawn into wires)
  • Malleability (Property to be formed into sheets)
  • Elasticity (Ability to recover from deformation)
  • Brittleness (Tendency to break under stress)
  • Plasticity (Ability to undergo permanent deformation)
  • Thermal Conductivity (Conduction of heat flow)
  • Electrical Conductivity (Conduction of electrical flow)
  • Acoustic Conductivity (Conduction of sound)
  • Magnetic Permeability (Ability to conduct magnetic lines of force)
  • Hardenability (Ability to be hardened by heat treatment)
  • Weldability (Ability to be joined by welding)
  • Etc.

The mechanical properties depend critically on the internal organization of matter (amorphous, crystalline, or granular), and any change or modification of the internal structure will result in a change in the properties of the material.

Mixtures and Compounds

Natural elements from the Periodic Table can be chemically combined or mixed together to form molecules of different substances or materials.

In a chemical compound, molecules are formed while preserving the same proportion and types of atoms. For example:

  • Water (H2O) = 2 atoms of Hydrogen + 1 atom of Oxygen
  • Table Salt (NaCl) = Sodium Chloride = 1 atom of Sodium + 1 atom of Chlorine

Chemical compounds follow the “Law of Definite Proportions.”

In a mixture, components can be in any proportion. For example:

  • Concrete = Cement + Sand + Gravel + Water in varying proportions.

The proportion of each element in the mixture determines its characteristics. For example, concrete can be “rich” or “lean” depending on the cement-to-sand ratio.

Metal Alloys

Mixtures of metals are termed as metal alloys. For example:

  • Bronze = Copper (Cu) + Tin (Sn) in varying proportions
  • Brass = Copper (Cu) + Zinc (Zn)
  • Steel = Iron (Fe) + Carbon (C) (Carbon varying from 0.01 to 2.0%)
  • Duralumin = Aluminum (Al) + Magnesium (Mg)

Industrial Classification of Metals

Metals are broadly classified into:

  • Ferrous Metals
  • Non-Ferrous Metals

A. Ferrous Metals

Ferrous metals are primarily composed of iron and its alloys. Iron forms chemical combinations such as:

  • Iron Oxide (Fe2O3) – Hematite
  • Iron Sulfide (FeS2) – Pyrite
  • Iron Carbide (Fe3C) – Cementite

Iron forms alloys with carbon, giving rise to steel, the most widely used metal in modern industry.

Iron-Carbon Alloys

These include:

  • Steel
  • Cast Iron
  • Pig Iron
Pig Iron

Pig iron is the product of a blast furnace and contains 4.5 to 6.7% carbon. It has limited manufacturing applications due to its properties: extremely hard, not ductile or malleable, and not weldable. It is used as a feedstock for steel production.

Blast Furnace Operation

A blast furnace is a large, cylindrical structure used to produce pig iron. It is charged with layers of coke (fuel), iron ore, and limestone (flux). Hot air is blown into the furnace, igniting the coke and generating heat to melt the iron ore. The molten iron and slag (impurities) collect at the bottom of the furnace and are tapped off separately.

Cast Iron

Cast iron contains 2 to 4.5% carbon. There are different types of cast iron, including:

  • White Cast Iron (Hard, brittle, difficult to machine)
  • Gray Cast Iron (Easy to machine, non-elastic, non-malleable)
  • Malleable Cast Iron (Machinable, deformable, difficult to weld)

Cast iron is typically produced in cupola furnaces, which are smaller than blast furnaces. Gray cast iron is widely used in the manufacture of machine parts due to its ease of machining and rigidity.

Steel

Steel contains between 0.02 to 2% carbon. It is divided into:

  • Carbon Steel
  • Alloy Steel

Carbon steel properties depend primarily on the percentage of carbon. Alloy steel contains additional elements like chromium, nickel, molybdenum, etc., to enhance specific properties.

Steel Production

Steel is produced by refining pig iron in an oxygen furnace. The process involves injecting pure oxygen into the molten pig iron to reduce the carbon content. Scrap steel is also added to control the temperature and carbon content.

Properties of Carbon Steel

The properties of carbon steel depend on the percentage of carbon. Key properties include:

  • Malleability
  • Ductility
  • Hardness
  • Tensile Strength
  • Hardenability
  • Weldability
Specifications of Carbon Steel

Organizations like ASTM (American Society for Testing Materials) and SAE (Society of Automotive Engineers) provide standards for classifying and specifying steel based on its properties and chemical composition.

Crystallography of Carbon Steels

The properties of steel are influenced by its crystalline structure. Key components include:

  • Ferrite (Pure iron)
  • Cementite (Iron carbide)
  • Pearlite (A mixture of ferrite and cementite)
  • Austenite (A high-temperature form of steel)
  • Martensite (A hardened form of steel)
Heat Treatment of Carbon Steel

Heat treatments like quenching and tempering are used to modify the properties of steel by altering its crystalline structure.

This comprehensive overview provides a detailed understanding of the classification, properties, and production of engineering materials, with a focus on ferrous metals and steel.