Mechanical Engineering - Engineering Materials - Discussion

The correct answer is decreases as the carbon content in steel increases.

The lower critical temperature (Ac1) is the temperature below which austenite transforms to ferrite and cementite (pearlite) in hypo-eutectoid steels. As the carbon content in steel increases:

- The lower critical temperature (Ac1) decreases.
- The upper critical temperature (Ac3) also decreases.
- The temperature range between Ac1 and Ac3 narrows.

This is because carbon stabilizes austenite, making it more difficult for the steel to transform into ferrite and pearlite. As a result, higher carbon steels require lower temperatures to achieve the same transformation.

Here's a rough estimate of the effect of carbon content on Ac1:

- Low carbon steel (0.1% C): Ac1 around 723°C (1333°F).
- Medium carbon steel (0.5% C): Ac1 around 690°C (1274°F).
- High carbon steel (1.0% C): Ac1 around 650°C (1202°F).

Note that this is a general trend and the exact values may vary depending on the specific steel composition and processing history.

The lower critical temperature is the same for all steels

Lower critical temperature for steel's austenite-to-pearlite transformation is 1333 °F (723 °Celsius). Below this temperature, austenite doesn't exist.

It should decrease with increasing Carbon content as carbon is an austenite stabiliser.

Carbon is austenite stabilizing element. So, as carbon percent increases lower critical temperature should decrease provided no effect of alloying element.

By adding soluble impurities the upper critical solution temperature increases and lower critical solution temperature decreases.