Mechanical treatment of steel

Why mechanical treatment is done

Mechanical treatment (mechanical working) is the shaping of steel by applying force (hammering, rolling, drawing) to obtain the required shape and size, and to improve its properties.

• It gives the required shape and size (bars, sheets, sections, wires).
• It refines the grain and removes casting defects such as blow holes and porosity, increasing strength, toughness and density.
• It improves the surface finish and dimensional accuracy.
• Cold work raises yield strength and hardness (strain hardening).

Methods

A. Hot working (above the recrystallisation temperature, about 900-1200 C for steel):

1. Rolling: red-hot steel passes between rollers to make plates, sheets, bars and structural sections.
2. Forging: hammering or pressing hot steel into shape (bolts, hooks, crankshafts).
3. Extrusion: forcing hot metal through a die to give a long section.
4. Pressing/piercing: for pipes and hollow items.

B. Cold working (at room temperature or below recrystallisation temperature):

1. Cold rolling: for thin sheets with a smooth surface.
2. Drawing: a bar is pulled through a die to make wires and rods.
3. Bending, twisting (cold twisted deformed bars) and stretching.
4. Spinning, shearing, stamping and pressing.