ANS:D - Ni

The preferred alloying element for low-temperature applications of steel is Ni (nickel). Nickel is commonly added to steel to improve its toughness, ductility, and resistance to brittle fracture at low temperatures. This is particularly important in applications where the steel will be exposed to sub-zero temperatures, as low-temperature conditions can increase the risk of brittle failure in unalloyed steels. By adding nickel to steel, the ductile-to-brittle transition temperature is lowered, meaning that the steel remains ductile and tough even at colder temperatures. This makes nickel-alloyed steels suitable for use in cryogenic applications, such as liquefied natural gas (LNG) storage tanks, refrigeration equipment, and aerospace components operating at low temperatures. While chromium (Cr), molybdenum (Mo), and nitrogen (N) are also commonly used as alloying elements in steel for various purposes, nickel is specifically favored for its ability to enhance low-temperature properties and prevent brittle fracture.