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A simplified portion of the iron-carbon equilibrium diagram is shown - Leaving Cert Engineering - Question b - 2021
Question b
A simplified portion of the iron-carbon equilibrium diagram is shown. (i) Identify the regions A, B, C and D shown. (ii) Identify and explain the microstructure wh... show full transcript
Worked Solution & Example Answer:A simplified portion of the iron-carbon equilibrium diagram is shown - Leaving Cert Engineering - Question b - 2021
Step 1
Identify the regions A, B, C and D shown.
Answer
Region A corresponds to Austenite, which is a face-centered cubic (FCC) phase of iron. Region B represents Austenite and Cementite, indicating a region where these two phases coexist. Region C is identified as Ferrite and Pearlite, which are two phases found in the lower carbon content steel. Finally, Region D denotes Pearlite and Cementite, marking the presence of these two phases in higher-carbon steel.
Step 2
Identify and explain the microstructure which is present at 0.83% carbon steel, up to 723°C.
Answer
At 0.83% carbon content, the microstructure is called pearlite. Pearlite is characterized by a layered structure consisting of alternate layers of ferrite and cementite. It displays a distinct appearance with a 'mother of pearl' sheen when fractured, resulting from its unique interleaved composition.
Step 3
Describe the transformation that occurs at point X, with reference to temperature, composition and phase change.
Answer
Point X represents the eutectic point in the iron-carbon phase diagram, occurring at approximately 1140°C and 4.3% carbon content. At this point, the liquid steel transitions directly into solid steel without undergoing a pasty stage. This transformation allows for the formation of a microstructure that is crucial for determining the material properties of the resulting steel.