3.1 All heat-treatment processes involve the heating and cooling of metals according to a time-temperature cycle - NSC Mechanical Technology Welding and Metalwork - Question 3 - 2021 - Paper 1
Question 3
3.1 All heat-treatment processes involve the heating and cooling of metals according to a time-temperature cycle. Describe this cycle.
3.2 Name any THREE types of q... show full transcript
Worked Solution & Example Answer:3.1 All heat-treatment processes involve the heating and cooling of metals according to a time-temperature cycle - NSC Mechanical Technology Welding and Metalwork - Question 3 - 2021 - Paper 1
Step 1
Describe this cycle.
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Answer
The time-temperature cycle in heat-treatment processes is composed of three main stages:
Heating: The metal is gradually heated to a specific temperature to relieve any internal stresses.
Soaking: The metal is kept at the target temperature for a predetermined period to ensure a uniform temperature throughout its volume.
Cooling: The metal is then cooled at a controlled rate to room temperature to set the required properties.
Step 2
Name any THREE types of quenching mediums used during the heat-treatment processes.
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Answer
Water: A common quenching medium that provides rapid cooling.
Oil: Slower cooling rate than water, reducing the risk of cracking.
Brine: A solution of salt in water that enhances the cooling effect.
Step 3
Give ONE reason why steel is annealed.
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Answer
Annealing is performed primarily to relieve internal stresses within the steel, which improves its machinability and overall ductility.
Step 4
Name the THREE groups.
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Answer
Low carbon steel: Contains a carbon content up to 0.3%.
Medium carbon steel: Contains a carbon content between 0.3% and 0.6%.
High carbon steel: Contains a carbon content above 0.6%.
Step 5
Label the diagram as indicated, from A to D.
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Answer
A - Percentage carbon / carbon content
B - Temperature in °C
C - AC3 line / Higher critical temperature
D - AC1 line / Lower critical temperature
