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The teeth on the sprocket shown are surface hardened during manufacture - Leaving Cert Engineering - Question 3 - 2018
Question 3
The teeth on the sprocket shown are surface hardened during manufacture. (i) Describe, with the aid of a diagram, a suitable heat treatment process for the teeth of... show full transcript
Worked Solution & Example Answer:The teeth on the sprocket shown are surface hardened during manufacture - Leaving Cert Engineering - Question 3 - 2018
Step 1
Describe, with the aid of a diagram, a suitable heat treatment process for the teeth of the sprocket.
Answer
To surface harden the teeth of the sprocket, an induction hardening process can be employed. This involves the use of high-frequency electric currents induced in the surface of the sprocket, creating a rapid increase in temperature. This process transforms the surface layer to austenite, while the core remains hard due to the slower heating.
Diagram:
A diagram can illustrate the induction coils, the water spray for cooling, and the rotating sprocket being subjected to these coils.
- Heating: The sprocket is passed through a coil where high-frequency currents generate heat in the surface.
- Quenching: After sufficient time at the required temperature, the sprocket is rapidly cooled using water spray, transforming the heated surface back to martensite, which provides hardness.
Step 2
Outline two faults that might cause the sprocket to fail during the heat treatment process.
Answer
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Incorrect Heat Settings: If the temperature is too high or too low, it may result in insufficient hardening, leading to a brittle structure.
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Inappropriate Quenching: If the cooling process is too rapid or inconsistent, it can cause cracking or warping of the sprocket, compromising its structural integrity.
Step 3
Explain the transformation that occurs along line A and line B.
Answer
Line A represents the transformation from austenite to ferrite and cementite (graphite) as the temperature decreases. This is a crucial phase in steel hardening where the steel transitions to a structure that is less brittle and more ductile.
Line B illustrates the transformation from austenite to pearlite and cementite upon further cooling. This process enhances the toughness of the steel, allowing for more practical applications.
Step 4
Name point C and describe the transformation which occurs at this point.
Answer
Point C is known as the eutectoid point. At this point, solid austenite transforms into pearlite at approximately 723°C with a carbon content of about 0.83%. This transformation is important for adjusting the microstructure and properties of steel.
Step 5
Describe the annealing of 1.2% carbon steel under the following headings: Temperature, Cooling process, Effects on the steel.
Answer
Temperature
For 1.2% carbon steel, the annealing process typically involves heating the steel to approximately 800°C, which is about 25° to 50° above the lower critical temperature (LCT).
Cooling Process
After soaking the steel at this temperature, it is allowed to cool down gradually in the furnace or air, minimizing stress and maintaining uniformity in the structure.
Effects on the Steel
The annealing process permits the formation of new, finer grains within the steel, which helps in achieving a softer, more ductile material. This process also aids in removing internal stresses, contributing to improved mechanical performance.
Step 6
Describe, with the aid of a diagram(s), one method of measuring furnace temperature, using the following guidelines: Equipment used, Principles of operation, Accuracy of results.
Answer
A common method of measuring furnace temperature is through a thermocouple pyrometer.
Equipment Used
- Thermocouple made of two different metals joined at one end (hot junction) and connected to a galvanometer.
Principles of Operation
When the hot junction is heated, a voltage is generated that varies with temperature. This voltage is measured at the cold junction and displayed on the galvanometer, giving an accurate temperature reading.
Accuracy of Results
The accuracy depends on the calibration of the thermocouple and maintaining consistent junction temperatures. It is considered highly accurate for industrial uses.