Materials

MATERIALS
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).

IRON & STEEL
 
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

CAST IRON
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 (1200^oC) & 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 727^O 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
Annealing
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

Normalizing
When steel is worked, crystal structure is distorted. This can be corrected by normalizing.

1. Heat Slowly to annealing temperature.
2. Cool in air.

Hardening

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

Austenite → Martensite

Tempering
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
Cr

• Strength
• Hardness
• Wear resistance
• Corrosion resistance
• Hot hardness

Ni

• Strength
• Toughness
• Corrosion resistance
• Austenite Stabilizer
• Heat-Resistant

Mo

• Toughness
• Hot hardness
• Wear resistance

Mn

• Strength
• Hardness

V

• Grain Refiner
• Strength
• Toughness

Stainless Steel
Composition: Cr 18%, Ni 8%, C 0.12%
Corrosion Resistance by the Oxide film of Chromium.
Properties

• Corrosion Resistance
• Low Temperature Toughness
• Higher Ductility
• High strength and Hardness