ANS:C - reduce the retained austenite in hardened steel.

Sub-zero treatment of steel is done primarily to reduce the retained austenite in hardened steel. Retained austenite is a metastable phase that remains in the microstructure of steel after it has been hardened and quenched. This retained austenite can negatively affect the mechanical properties of the steel, including its hardness, toughness, and dimensional stability. Sub-zero treatment involves subjecting the hardened steel to temperatures below room temperature (usually between -80°C and -120°C or lower) for an extended period, typically several hours. This treatment allows the transformation of some of the retained austenite into martensite, which is a harder and more stable phase of steel. By reducing the amount of retained austenite, sub-zero treatment improves the overall properties of the steel, including its hardness, wear resistance, and dimensional stability. It also helps to relieve internal stresses and increase the steel's resistance to fatigue and cracking. While sub-zero treatment may have some secondary effects on martensite transformation and tempering, its primary purpose is to reduce retained austenite in hardened steel. Therefore, the correct answer is to reduce the retained austenite in hardened steel.