What is the lower critical point for all steels?

• A. 600 deg. C
• B. 723 deg. C
• C. 900 deg. C
• D. 914 deg. C

Answer: (B)

The lower critical point for all steels is 723 degrees Celsius. This temperature marks an important phase change in the steel microstructure. At 723 degrees Celsius, steel undergoes a transformation from the ferrite phase to a phase called austenite, which occurs during heating. This temperature is significant in metallurgy as it helps to define the temperature range for processes such as heat treatment and quenching.

Knowing that 723 degrees Celsius is the critical point is crucial for controlling the properties of steel, as various phases can influence hardness, ductility, and strength. Understanding this transformation allows metallurgists to manipulate the steel's microstructure to achieve desired mechanical properties through processes such as annealing or normalizing.