Hardening of steel is not possible, unless it is heated __________ critical point.

ANS:B - above the lowest

"Above the lowest critical point" refers to heating the steel above its lower critical temperature during the hardening process. This critical temperature is also known as the Ac1 temperature. When steel is heated above this critical temperature, its microstructure undergoes a transformation. Specifically, the steel transitions from a ferrite-pearlite or ferrite-cementite microstructure to austenite, which is a face-centered cubic (FCC) crystal structure. Austenite is a high-temperature phase of steel that has different properties compared to the lower-temperature phases. Heating the steel above the lower critical temperature allows the carbon atoms to dissolve into the austenite phase, which is essential for achieving hardness during the subsequent quenching process. After the steel has been heated to a temperature above the lower critical point and held there for sufficient time to ensure uniformity, it is then rapidly cooled (quenched) to room temperature to achieve the desired hardness.