Which alloying element is most effective in reducing brittleness of steel at very low temperatures?

Nickel is highly effective in enhancing the toughness of steel at very low temperatures. It improves the ductility and impact strength of steel, which helps to prevent brittle failure in cold conditions. When nickel is added to steel, it promotes better grain structure and reduces the temperature at which the steel becomes brittle, allowing it to retain its toughness and workability.

The presence of nickel affects the steel's microstructure, leading to a more ductile behavior even at low temperatures. This is particularly important for applications such as cryogenic engineering, where materials must withstand extreme conditions without experiencing brittle fracture.

In contrast, while elements like manganese, molybdenum, and silicon can improve the overall strength and toughness of steel, they do not provide the same level of impact resistance at very low temperatures as nickel. Manganese enhances hardenability, molybdenum is beneficial for high-temperature strength, and silicon can improve the elastic modulus, but they do not specifically target low-temperature brittleness in the same effective way as nickel does.