Structure, processing, properties and selection

The properties of a material depend on its internal structure (atomic bonding, crystal structure, grain size, phases), and the structure is controlled by processing (casting, working, heat treatment). The final performance in service follows from the properties.

Relationship

 Processing --> Structure --> Properties --> Performance
 (heat treat,    (grain size,   (strength,     (life, cost,
  rolling)        phases)        hardness)       safety)

Example: carbon steel (0.4% C).

• Slow cooling (annealing) gives coarse ferrite + pearlite: soft and ductile, with a yield strength of about 350 MPa.
• Quenching gives martensite: very hard and brittle.
• Tempering the martensite gives fine carbides in ferrite: high strength with useful toughness.
• Cold rolling the annealed steel increases the dislocation density, so strength rises and ductility falls.

The same composition gives very different properties only because processing has changed the structure.

Factors for selecting the material of a connecting rod

1. Mechanical properties: high fatigue strength, adequate yield strength, stiffness (modulus).
2. Density: low, because the rod is a reciprocating part and its inertia loads depend on mass.
3. Toughness and resistance to impact and buckling.
4. Manufacturability: forgeability, machinability, ease of heat treatment.
5. Cost and availability: forged medium-carbon or alloy steel (e.g., 40C8, 35Mn2) is common; cast iron is used for low-duty engines and aluminium alloys for light engines.
6. Wear and service environment: bearing surfaces, operating temperature, corrosion.