Question

Austenite phase in Fe - \(Fe_3C\) system exists in a temperature range of

• A. 25 - 910°C
• B. 710 - 1130°C
• C. 910 - 1490°C
• D. 723 - 1490°C

Hint:

When asked for the existence range of a phase from a phase diagram, always look for the absolute lowest and absolute highest temperatures of its phase field across all compositions. For austenite, the key points are the eutectoid (lowest T) and peritectic (highest T).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
This question requires knowledge of the Iron-Iron Carbide (\(Fe-Fe_3C\)) phase diagram. Austenite (\(\gamma\)-iron) is a high-temperature, FCC phase of iron that can dissolve a significant amount of carbon. We need to identify the overall temperature range in which this phase can exist according to the diagram.

Step 2: Detailed Explanation:
Let's analyze the austenite phase field in the \(Fe-Fe_3C\) diagram:

• The

lowest temperature at which austenite is stable is the eutectoid temperature. At this point, austenite transforms into pearlite (ferrite + cementite). This temperature is approximately

723°C (or sometimes cited as 727°C).

• The

highest temperature at which austenite is stable depends on the carbon content. It is bounded by the solidus line and the delta-ferrite phase field.

• For pure iron (0% C), austenite forms from ferrite at 912°C and transforms to delta-ferrite at 1394°C.

• As carbon is added, the stability range changes. The highest temperature at which a fully austenitic phase can exist is at the peritectic reaction point, which is at

1495°C (often approximated as 1490°C in options).
Therefore, the complete temperature span across which the austenite phase can be found in the \(Fe-Fe_3C\) system is from the eutectoid temperature (723°C) up to the peritectic temperature (\(\approx\)1490°C).
Let's evaluate the options:
(A) 25 - 910°C: Incorrect. Austenite is not stable at room temperature (25°C).
(B) 710 - 1130°C: This is a partial range but not the complete range.
(C) 910 - 1490°C: This is the range for pure iron but ignores the effect of carbon, which lowers the minimum temperature to 723°C.
(D) 723 - 1490°C: This option correctly identifies the lowest temperature (eutectoid) and the approximate highest temperature (peritectic) for the existence of austenite.

Step 3: Final Answer:
The austenite phase in the Iron-Iron Carbide system exists over a broad temperature range, starting from the eutectoid temperature of 723°C up to the peritectic region around 1490°C.