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Answer any two of the following: (i) Differentiate between the eutectic point and the eutectoid point; (ii) Describe one method of measuring furnace temperature; (iii) Compare the microstructures of martensite and ferrite; (iv) Explain the term recrystallisation in terms of heat treatments - Leaving Cert Engineering - Question a - 2008
Question a
Answer any two of the following: (i) Differentiate between the eutectic point and the eutectoid point; (ii) Describe one method of measuring furnace temperature; ... show full transcript
Worked Solution & Example Answer:Answer any two of the following: (i) Differentiate between the eutectic point and the eutectoid point; (ii) Describe one method of measuring furnace temperature; (iii) Compare the microstructures of martensite and ferrite; (iv) Explain the term recrystallisation in terms of heat treatments - Leaving Cert Engineering - Question a - 2008
Step 1
Differentiate between the eutectic point and the eutectoid point;
Answer
The eutectic point is the temperature and composition at which a liquid phase transforms directly into two solid phases at a specific composition. For the iron-carbon system, the eutectic point occurs at 1140°C with 4.3% carbon. The eutectoid point, on the other hand, refers to the transformation of a single solid phase into two different solid phases without any liquid phase. This occurs at 723°C with 0.76% carbon, resulting in the formation of pearlite from austenite.
Step 2
Describe one method of measuring furnace temperature;
Answer
One method to measure furnace temperature is through the use of an optical pyrometer. This device compares the intensity of light emitted from a heated object to that from a standard light source, adjusting the current accordingly to match temperatures. The temperature reading can be obtained from a calibrated scale, providing a reliable measure of the high temperatures present in a furnace.
Step 3
Step 4
Describe the structural changes that occur in 0.4% carbon steel as it cooled slowly from 900°C.
Answer
At 900°C, the microstructure comprises a face-centered cubic (FCC) structure of austenite. As the steel cools slowly below the upper critical temperature of 910°C, a transformation begins. Below 723°C, ferrite forms alongside layers of cementite, which is harder and more brittle. The cooling process allows for the gradual precipitation of pearlite, which consists of alternating layers of ferrite and cementite, ultimately refining the microstructure.
Step 5
Describe the principle of pack carburising.
Answer
Pack carburising is a heat treatment process that enhances the carbon content on the surface layers of steel. The object is placed in a carbon-rich medium at high temperatures, allowing carbon atoms to diffuse into the surface. The depth of this carbon penetration depends on the temperature and duration in the furnace. This process increases surface hardness and wear resistance while maintaining a tougher core.
Step 6
Outline, with the aid of a suitable diagram, the process of flame hardening.
Answer
Flame hardening involves heating the surface of a metal object to approximately 850°C using a flame. This is typically achieved with an oxy-acetylene flame. Upon reaching the desired temperature, the surface is quenched quickly, often with water or another cooling medium, which transforms the surface structure into hard martensite. This method increases wear resistance and fatigue strength of the treated area while preserving a tougher core structure.