How can heat treatment, alloying and cold working change a metal's properties?
Heat treatment (hardening, tempering, annealing, normalising), alloying and cold working (work hardening) to change properties.
A CCEA GCSE Engineering and Manufacturing answer on changing metal properties by heat treatment (hardening, tempering, annealing, normalising), alloying and cold working or work hardening.
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What this dot point is asking
CCEA Unit 3 expects you to know how a metal's properties can be changed after it is made, by heat treatment (hardening, tempering, annealing, normalising), by alloying, and by cold working (work hardening). You should be able to say what each does and why.
The answer
Heat treatment
| Treatment | What happens | Effect |
|---|---|---|
| Hardening | Heat then quench (cool quickly in water/oil) | Very hard, but brittle |
| Tempering | Reheat hardened steel to a lower temperature, cool | Removes brittleness, restores toughness (slightly less hard) |
| Annealing | Heat then cool very slowly | Soft, ductile, easy to work; relieves stress |
| Normalising | Heat then cool in still air | Refines grain, restores toughness and a uniform structure |
Alloying
Alloying mixes a metal with one or more other elements to improve its properties (for example adding chromium and nickel to steel makes stainless steel, which resists corrosion). Alloying is built in when the metal is made, unlike heat treatment which is done afterwards.
Cold working (work hardening)
Worked example: treating a chisel
Examples in context
- Example 1. A drill bit
- Hardened to keep its cutting edges sharp, then tempered so the bit does not snap under the twisting force of drilling.
- Example 2. A bent steel bracket
- After repeated bending the bend line becomes work hardened and could crack; annealing softens it so it can be reshaped.
- Example 3. Cutlery
- Made from stainless steel, an alloy of steel with chromium, so it resists corrosion from food and washing without any extra finish.
The pattern is that engineers tune metal properties three ways: by what they mix in (alloying), by heating and cooling (heat treatment), and by how they shape it cold (work hardening, reversed by annealing).
Try this
Q1. What does quenching (hardening) do to steel? [1 mark]
- Cue. It makes it very hard but brittle.
Q2. Why is tempering carried out after hardening a tool? [2 marks]
- Cue. To remove some brittleness and restore toughness so the tool does not shatter, while keeping it hard.
Q3. What is work hardening, and how is it reversed? [2 marks]
- Cue. Cold working makes a metal harder and less ductile (work hardening); annealing (heat then cool slowly) softens it again.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA style4 marksExplain the difference between hardening and tempering of steel, and why both are usually carried out on a cutting tool.Show worked answer →
Hardening heats the steel to a high temperature and then quenches it (cools it rapidly in water or oil), which makes it very hard but also very brittle.
Tempering then reheats the hardened steel to a lower temperature and cools it more slowly, which removes some of the brittleness and restores toughness, at the cost of a little hardness.
Both are done on a cutting tool so it is hard enough to keep a sharp edge (from hardening) but tough enough not to shatter in use (from tempering).
Markers reward the hardening (heat then quench, hard but brittle) and tempering (reheat lower, tougher, less brittle) steps and the reason for combining them.
CCEA style3 marksWhat is annealing, and why might it be carried out on a metal that has been bent and shaped many times?Show worked answer →
Annealing heats the metal and then lets it cool very slowly (in the furnace or in air/ash). This makes the metal soft, ductile and easy to work again, and relieves internal stress.
A metal that has been bent and shaped many times becomes work hardened (hard and brittle), so it may crack if worked further. Annealing softens it again so it can be shaped without cracking.
Markers reward the description (heat then cool slowly to soften) and the reason (to reverse work hardening so the metal can be shaped further).
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