Cast irons

Some cast iron properties

• >2.1 wt% C but in reality 2.5 wt%C - 4 wt%C

• 1-3 wt% Si

• Fe, C, Si main elements

• brittle

• excellent castability and good machinability

• strong in compression but not tension

• low shrinkage (reduction in size naturally)

• cheaper than steel

Graphite in cast irons

• Layered hexagonal structure with covalent bonding of atoms in each layer

• Layers easily slide against each other

• Solid lubricant

• Soft and low strength

• Promoted by presence of Si and slow cooling rates

Types of cast irons

• Grey cast iron

• White cast iron

• Malleable cast iron

• Ductile/nodular/spheroidal cast iron

• Alloy cast iron (see)

• Compacted graphite iron

How to get the different cast irons:

• Fast cool - white cast iron (pearlite + cementite)

  • Reheat and hold at 700 degrees Celsius for 30+ hours

    • Fast cool - pearlitic malleable cast iron (pearlite and rosette graphite)

    • Slow cool - ferritic malleable cast iron (ferrite and rosette graphite)

• Moderate cool - Pearlitic grey cast iron (pearlite + flake graphite)

  • Adding Mg/Ce - pearlitic ductile cast iron (pearlite + nodule graphite)

• Slow cool - Ferritic grey cast iron (ferrite + flake graphite)

  • Adding Mg/Ce - ferritic ductile cast iron (ferrite + nodule graphite)

Alloying elements and impurities

• Sulfur:

  • Hardens CI by stabilising cementite

  • Promotes cementite formation with carbides

  • Embrittlement due to FeS formation

  • As high as 0.15 wt% for low quality CI

  • Impurity from fuel

• Manganese:

  • Added in small quantities to combine with S to form MnS

  • Softens CI

  • If Mn increases, stabilises cementite

  • In excess, promotes pearlite

  • % amount varies by desired matrix (0.1% ferritic and 1.2% for pearlitic)

• Phosphorus:

  • Impurity in the form of Fe3P

  • Increases embrittlement effect of CI (not good for shock loading)

  • Suitable for thin castings

  • Forms a eutectic at 950 degrees

  • Increases fluidity (liquid more fluid for more details in casting)

• Si and Al:

  • Increases graphitisation potential by increasing number of graphite particles for both eutectic and eutectoid transformation

  • Increases the ferrite to pearlite ratio leading to low strength and hardness

• Ni, Cu and Sn:

  • Increases graphitisation potential by increasing number of graphite particles for eutectic transformation but decreases it during eutectoid transformation

  • Increases the pearlite/ferrite ratio leading to high strength and hardness

• Cr, Mo, W, V:

  • Decrease graphitisation potential for both eutectic and eutectoid transformation

  • Increase carbides and pearlite

Grey cast iron properties

• Composition is selected to satisfy 3 structural requirements

  • graphite shape and distribution

  • chill free structure

  • required matrix

• C and Si exist between 2.5% - 4% and 1% to 3% respectively.

• Graphite exists in form of flakes surrounded by pearlite and ferrite

• Grey fractured surface apperance

• ADD

• Carbon equivalent

White cast iron

Malleable cast iron