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Material Processing

Jamie Z 2025-08-17

Hot Working

Hot working is a metal forming process in which a metal is mechanically deformed (forging, rolling, extrusion, drawing, etc.) above its recrystallization temperature but below its melting point.

At this elevated temperature:

  • The metal’s yield strength decreases → requires less force to deform.
  • Ductility increases → it can be shaped more easily without cracking.
  • Recrystallization occurs simultaneously with deformation → new, strain-free grains form, so the metal does not harden or lose ductility.
  • Strain Hardening Does not Occur
  • The material strength is isotropic (similar in all directions)

Examples of Hot Working Processes:

  • Hot rolling (used in making sheets, rails, beams).
  • Hot forging (making crankshafts, gears).
  • Hot extrusion (for rods, tubes).
  • Hot drawing (wire drawing at elevated temperatures).

Cold Working

Cold working is a metal forming process in which a metal is plastically deformed below its recrystallization temperature (usually at room temperature). Existing crystals become elongated/uneven and the grain is anisotropic (different strength in different directions)

Unlike hot working, no new grains form during deformation, so the effects of strain accumulate in the metal.

Key Characteristics of Cold Working:

  • Temperature: Done below recrystallization temperature (often room temp).
  • Increased strength & hardness due to strain hardening.
  • Reduced ductility – metal becomes less able to deform further without cracking.
  • Requires higher forces because the metal is harder at low temperatures.

Examples of Cold Working Processes:

  • Cold rolling (thin sheets, foils).
  • Wire drawing (electrical wires, cables).
  • Deep drawing (cans, cups).
  • Cold extrusion.
  • Bending, pressing, stamping.

Annealing

Annealing is a heat treatment process used on metals (and sometimes glass) to soften the material, improve ductility, and relieve internal stresses caused by cold working, machining, or welding.

  • It involves heating the metal to a specific temperature, holding it there, and then cooling it slowly, usually in the furnace.
  • Annealing results in grain growth (larger grains)

Annealing is like giving metal a "heat reset" — it softens it, restores ductility, relieves stresses, and improves workability.

  • Annealing = heat treatment (softens and restores metal).
  • Hot Working = deformation process (reshapes metal at high temperature without work hardening).
Aspect Annealing Hot Working
Definition A heat treatment process where metal is heated above recrystallization temp, held, then cooled slowly. A metal forming process where metal is mechanically deformed above recrystallization temp.
Main Purpose To soften the metal, relieve stresses, and restore ductility. To reshape the metal into desired forms (sheets, rods, forgings, etc.).
Deformation No external deformation; metal just undergoes thermal treatment. Involves mechanical deformation (rolling, forging, extrusion).
Effect on Properties Reduces hardness, increases ductility, refines grains, removes residual stresses. Prevents work hardening (since recrystallization happens continuously), allows large deformations, improves toughness.
Force Required No force required (just heating and cooling). Force is applied through hammers, presses, or rolls.
Surface Finish Better surface (since it’s usually furnace treated). Poorer surface due to oxidation and scaling at high temp.
End Result Metal becomes soft, stress-free, and ready for machining or further working. Metal is shaped into usable products like sheets, beams, gears, wires.

Extrusion

Extrusion is a metal forming process (also used in plastics and ceramics) in which a material is forced to flow through a die to create objects of a fixed cross-sectional shape.

Casting and Forging

Casting

  • Definition: Casting is a manufacturing process where molten metal is poured into a mould and allowed to solidify into the desired shape.
  • Process:
    1. Melt the metal.
    2. Pour into a mould (sand, metal, ceramic, etc.).
    3. Allow it to cool and solidify.
    4. Remove the casting and finish (machining, grinding).

Examples: Engine cylinder blocks, pump casings, turbine blades (investment casting), cookware.

Forging

  • Definition: Forging is a deformation process where solid metal is shaped by compressive forces, usually by hammering or pressing, often at high temperature (hot forging).
  • Key Points:
    • Produces stronger parts (grain flow aligns with shape).
    • Limited to simpler shapes compared to casting.
    • Can be done hot (above recrystallization) or cold (room temp).
    • More expensive for complex parts.

Examples: Crankshafts, gears, spanners, connecting rods, bolts.

Welding

  • Using heat to melt and join metals which cool to form a single solid.
  • The metal adjacent to the molten heats beyond recrystallisation temperature and cools with a different grain structure, called the heat affected zone