Enhancement of materials - Metal enhancement — AQA A-Level Design and Technology: Product Design
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Enhancement of materials - Metal enhancement explained
Heat treatment changes a metal's internal structure to improve properties such as hardness, toughness or ductility.
Read the full explanation
For ferrous metals, the key methods are hardening, tempering, annealing, normalising and case hardening. Hardening involves heating steel to its critical temperature and quenching rapidly in water or oil, producing martensite, which is very hard but brittle. Tempering follows hardening: the steel is reheated to a lower temperature and cooled, reducing brittleness while retaining useful hardness. Annealing heats the metal and cools it very slowly in the furnace, softening it and relieving internal stresses. Normalising heats the metal and cools it in still air, refining the grain structure for more uniform properties. Case hardening hardens only the surface of low-carbon steel by adding carbon to the outer layer, leaving a tough core.
Your focus
- Describe the purpose and process of hardening, tempering, annealing, normalising and case hardening.
- Explain how cooling rate affects the resulting microstructure and properties.
- Select an appropriate heat treatment method for a given product requirement.
Enhancement of materials - Metal enhancement exam tips
Marking Points
- Hardening: heating steel to its critical temperature and quenching rapidly in water or oil to form martensite, increasing hardness but also brittleness.
- Tempering: reheating hardened steel to a temperature below the critical point and cooling, reducing brittleness while retaining hardness.
- Annealing: heating metal and cooling very slowly, usually in the furnace, to soften it and relieve internal stresses.
- Normalising: heating metal and cooling in still air to refine grain structure and produce more uniform mechanical properties.
- Case hardening: adding carbon to the surface of low-carbon steel and then quenching, producing a hard outer layer with a tough core.
Examiner Tips
- 💡Learn the sequence of hardening followed by tempering and the reason for each step.
- 💡Use the cooling medium to distinguish annealing (furnace), normalising (air) and hardening (quenching in water or oil).
- 💡For case hardening, remember the purpose is a hard surface with a tough core, often for gears or camshafts.
- 💡When a question asks for a heat treatment to soften a metal, choose annealing rather than hardening.
Common Mistakes
- Confusing annealing with normalising: annealing uses very slow furnace cooling, while normalising uses still air cooling.
- Thinking tempering is done before hardening: tempering always follows hardening to reduce brittleness.
- Believing case hardening hardens the entire component: it only hardens the surface, leaving the core tough.
- Assuming all heat treatment methods increase hardness: annealing softens the metal, and normalising refines grain structure rather than maximising hardness.