Photo AI
A simplified portion of the iron-carbon equilibrium diagram is shown - Leaving Cert Engineering - Question b - 2018
Question b
A simplified portion of the iron-carbon equilibrium diagram is shown. (i) Explain the transformation that occurs along line A and along line B. (ii) Name point C a... show full transcript
Worked Solution & Example Answer:A simplified portion of the iron-carbon equilibrium diagram is shown - Leaving Cert Engineering - Question b - 2018
Step 1
Explain the transformation that occurs along line A and along line B.
Answer
Along line A, the transformation is from austenite to ferrite or austenite to pearlite as the temperature decreases. This occurs when the alloy begins to solidify and the solubility of carbon decreases in austenite. At line B, the transformation involves austenite converting to pearlite and cementite as the temperature further decreases, leading to the formation of these microstructures.
Step 2
Name point C and describe the transformation which occurs at this point.
Answer
Point C is called the eutectoid point. At this point, solid austenite changes to solid pearlite at approximately 723°C corresponding to 0.83% carbon, indicating a critical composition where the eutectoid reaction takes place.
Step 3
Describe the annealing of 1.2% carbon steel under the following headings:
Answer
Temperature
For 1.2% carbon steel, the piece is heated to approximately 25°F to 50°F above the lower critical temperature (LCT), which is around 800°C.
Cooling process
The steel is soaked at this temperature to ensure uniform heating throughout the material. It is then allowed to cool gradually in a furnace to promote slower cooling rates, which leads to softer mechanical properties.
Effects on the steel
The annealing process leads to recrystallization where new grains form. This improves ductility, refines the grain size, and removes internal stresses, enhancing the overall performance and machinability of the steel.