What is the lower critical temperature of steel?

What is the lower critical temperature of steel?

• A. 1333F (723C)
• B. 1300F (704C)
• C. 1200F (649C)
• D. 1400F (815C)

Answer: (A)

The lower critical temperature is the Ac1 point—the temperature where steel begins to transform from ferrite to austenite as it’s heated. For most carbon steels, this starts around 723–727°C (about 1330–1340°F). That’s why the value near 1333°F (723°C) is the correct choice. The other temperatures lie outside this typical Ac1 range: one is a bit below, one is well below, and the last is above it (closer to Ac3), so they don’t represent the starting point of the transformation.

The lower critical temperature is the Ac1 point—the temperature where steel begins to transform from ferrite to austenite as it’s heated. For most carbon steels, this starts around 723–727°C (about 1330–1340°F). That’s why the value near 1333°F (723°C) is the correct choice. The other temperatures lie outside this typical Ac1 range: one is a bit below, one is well below, and the last is above it (closer to Ac3), so they don’t represent the starting point of the transformation.