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Properties of Materials

  • Brittleness
  • Ductility – Drawn into tension without fracture
  • Elasticity – Property of returning to original shape after deforming under force
  • Hardness – Ability to withstand abrasion, wear, scratching & indentation
    • Brinell hardness – found by pressing a ball on to the surface of the metal & then dividing the surface area of the dent by the load.
  • Strength: Ability to withstand force without rupture.
  • Toughness: Amount of energy that can be absorbed by the material before fracture. (Test: Charpy impact test).

Pure Iron (Ferrite) – Soft – Crystalline structure
Steel – Alloy of Iron & Carbon
Cast Iron – Steel + Carbon

  • Up to C 1.5% all Carbon in Steel are chemical combined with Iron (Cementite), none exist in free graphite state

When free graphite starts to exist in the solution it falls into the Cast Iron category.

Grey Cast Iron

Contains free graphite flakes. Si is added for the formation of free graphite.
2.5 - 3.8% C
1.1 - 2.8% Si
  • Cheap
  • Low Melting Point (1200oC) & Fluidity – Easy to Cast
  • Self Lubrication
  • Hard
  • Great Compression strength
  • Good vibration damping
  • Brittle
Spheroidal / Nodular Cast Iron

3.2 – 4.2% C
1.1 – 3.2% Si

          By alloying with Mg.
  • Tensile strength increased.
  • Toughness increased enabling twists & bends.

STEEL – alloy of Iron & Carbon

Figure: Microstructure of Mild Steel

Pearlite: Ferrite ( 87%) + Cementite (13%) ➝  Strength
Cementite: Fe3C (Iron Carbide)  ➝  Hardness

Figure : Microstructure of Pearlite (laminar structure)

From 0% to 0.87% C (i.e. 0.9%):    ↑Pearlite    ↓Ferrite
From 0.9% C:    ↓Pearlite    ↑Cementite

At 727O C Carbon in Pearlite starts to dissolve forming a solid solution of carbon and ferrite which is called Austenite.
Austenite: non-magnetic, solid solution of Carbon & ferrite.

Iron Carbon Phase Diagram

α – Ferrite (BCC)
𝜸 Austenite (FCC)

BCC – Body Centered Cubic structure
FCC – Face Centered Cubic structure

Heat Treatment
To soften the material.
  1. Heat Slowly to the annealing temperature.
  2. Hold the temperature to allow internal changes.
  3. Cool slowly.
Initial structure → Austenite → Pearlite + Cementite/Ferrite

When steel is worked, crystal structure is distorted. This can be corrected by normalizing.
  1. Heat Slowly to annealing temperature.
  2. Cool in air.

  1. Heat up to annealing temperature.
  2. Rapidly cool (quench) using water or oil.
Austenite → Martensite

Martensite is brittle. This can be corrected by tempering.
  1. Heat to a temperature below lower critical temperature.
  2. Cool down by air.
Partial transformation of Martensite to Pearlite.

Alloying Elements
  • Strength
  • Hardness
  • Wear resistance
  • Corrosion resistance
  • Hot hardness
  • Strength
  • Toughness
  • Corrosion resistance
  • Austenite Stabilizer
  • Heat-Resistant
  • Toughness
  • Hot hardness
  • Wear resistance
  • Strength
  • Hardness
  • Grain Refiner
  • Strength
  • Toughness

Stainless Steel
Composition: Cr 18%, Ni 8%, C 0.12%
Corrosion Resistance by the Oxide film of Chromium.
  • Corrosion Resistance
  • Low Temperature Toughness
  • Higher Ductility
  • High strength and Hardness


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