Metals.
Enhancement of materials · Technical principles
Revision notes
Metal Enhancement
METAL ENHANCEMENTS
Metal enhancement uses controlled heating or mechanical working to change the internal structure of a metal. By changing how the metal’s crystals behave, we can make the metal harder, stronger, or easier to shape, depending on the process used.
Work hardening (cold working)
Work hardening happens when a metal is shaped without heating, for example, by bending, rolling, hammering, or pressing.
Cold working forces the metal’s crystal structure out of its original shape, which makes the metal stronger and harder in the worked area.
As the crystals become distorted, they cannot move easily, which reduces ductility and makes the metal more likely to crack if worked further.
This is why metals can become difficult to shape after repeated bending or hammering.
The effects of work hardening can be removed by annealing.
Annealing (undoing work hardening)
Annealing is used to make a work-hardened metal easier to shape again.
The metal is heated and then cooled very slowly.
Slow cooling allows the metal crystals to rearrange into a more regular structure.
This reduces internal stress, lowers hardness, and restores ductility.
As a result, the metal becomes less brittle and easier to work.
Case hardening (hard outside, tough inside)
Case hardening is used when a component needs a hard, wear-resistant surface but a tough inner core.
It is used on steels with less than 0.4% carbon content.
The surface of the steel is hardened to resist wear and indentation.
The core remains softer, allowing the part to absorb shock without breaking.
This makes the process ideal for components such as gears and shafts.
Case hardening process (step by step)
Step 1: Carburising (adding carbon to the surface)
Carburising increases the carbon content only at the surface of low-carbon steel.
The steel is placed in a sealed ceramic box packed with carbon.
The box is heated to about 930–950°C, allowing carbon atoms to diffuse into the steel’s surface.
The longer the steel remains at this temperature, the deeper the carbon-rich layer becomes.
After carburising, the steel is reheated to around 760°C to prepare it for quenching.
Step 2: Quenching (locking in hardness)
Quenching rapidly cools the steel to lock the hard surface in place.
The hot steel is quenched in water to cool the surface quickly.
Rapid cooling hardens the carbon-rich outer layer.
The inner core cools more slowly and remains tough rather than brittle.
Hardening
Used on medium- and high-carbon steels to alter the crystalline structure.
Steel is heated, held at temperature, and rapidly quenched in water, oil, or salt-water baths.
Increases hardness but also increases brittleness.
Tempering
A secondary heat treatment carried out after hardening.
Reduces excess hardness and brittleness while increasing toughness and ductility.
Metal is reheated below the critical temperature, held, then air cooled.
Tempering colour indicates the temperature reached and the amount of brittleness removed.
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