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A simplified portion of the iron-carbon equilibrium diagram is shown - Leaving Cert Engineering - Question b - 2020
Question b
A simplified portion of the iron-carbon equilibrium diagram is shown. (i) Identify the regions A, B, C and D shown. (ii) For 0.5% carbon steel shown, describe in d... show full transcript
Worked Solution & Example Answer:A simplified portion of the iron-carbon equilibrium diagram is shown - Leaving Cert Engineering - Question b - 2020
Step 1
Identify the regions A, B, C and D shown.
Answer
Region A represents Liquid Steel, which is the molten state of the iron-carbon alloy.
Region B denotes the Pasty Stage, where the material is partially solid and can be shaped but is not fully solidified.
Region C indicates Austenite, a phase that occurs in steel at high temperatures, where the structure is face-centered cubic and allows for the solubility of carbon.
Region D refers to the mixture of Pearlite and Cementite, which are phases that form as the steel cools and solidifies.
Step 2
For 0.5% carbon steel shown, describe in detail the heat treatment process of annealing.
Answer
To anneal 0.5% carbon steel, the steel is first heated to a temperature that is approximately 25°-50°F above the upper critical temperature (UCT), as it is classified as hypo-eutectoid steel (with carbon content less than 0.83%).
Once the desired temperature is reached, it is soaked at this temperature for a sufficient period, allowing for uniform heating. After soaking, the steel is removed from the heat source and allowed to cool gradually in the furnace, which is essential to avoid rapid cooling that could induce hardness.
During this full annealing process, new grains are formed, and this phenomena is termed recrystallization. This step is crucial as it softens the metal, improves its ductility, refines the grain size, and eliminates any internal stresses that may have developed during prior processing.
Step 3