At what temperature does mild steel become fully austenetic?

At what temperature does mild steel become fully austenetic?

• A. above the upper critical (1666F or 910C)
• B. below the lower critical (near 900F)
• C. at room temperature
• D. between 600-800C

Answer: (A)

When steel becomes fully austenitic, the ferrite and pearlite have completely transformed into gamma-iron. For mild (low-carbon) steel this happens only after you pass the upper critical temperature, known as Ac3, which is about 910°C (1666°F). Above that temperature, the structure is all austenite. Below Ac3, you have a mix of phases, not a full austenitic state. So heating the steel above roughly 910°C yields a fully austenitic microstructure.

When steel becomes fully austenitic, the ferrite and pearlite have completely transformed into gamma-iron. For mild (low-carbon) steel this happens only after you pass the upper critical temperature, known as Ac3, which is about 910°C (1666°F). Above that temperature, the structure is all austenite. Below Ac3, you have a mix of phases, not a full austenitic state. So heating the steel above roughly 910°C yields a fully austenitic microstructure.