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: Find the lower boundary of austenite's existence.
Austenite is stable down to the eutectoid temperature, below that point it transforms into pearlite, and on the Fe Fe3C diagram this eutectoid line sits at about 723 degrees Celsius, so nothing below that value can be counted as austenite.
Step 2: Find the upper boundary.
On the high temperature side, austenite eventually gives way near the peritectic reaction, which occurs at roughly 1490 to 1495 degrees Celsius depending on carbon content, above this the material either becomes delta ferrite or enters the liquid region.
Step 3: Combine both limits into one range.
Putting the eutectoid floor and the peritectic ceiling together gives the full window in which austenite can exist across the diagram, that is from 723 degrees Celsius up to about 1490 degrees Celsius, which matches the option spanning both of these landmark temperatures rather than the narrower ranges that describe only pure iron or only part of the field.
\[ \boxed{723 - 1490^\circ C} \]