When chromium is added to alloy steel, it primarily increases which property?

When chromium is added to alloy steel, it primarily increases hardness. Chromium enhances the hardness of steel by enabling the formation of hard carbide phases during heat treatment processes. These carbides are very hard, contributing significantly to the overall hardness and wear resistance of the alloy.

Additionally, chromium also facilitates the expansion of the steel's microstructure, which can improve its strength and toughness. However, while these properties are influenced by chromium, the direct and most significant effect is the increased hardness. This characteristic makes chromium-containing steels suitable for applications where wear resistance and durability are critical.

Chromium is also known to improve other properties such as corrosion resistance, but the primary focus of the question addresses hardness directly related to its role in alloy steel. Therefore, understanding the specific effect of chromium on hardness provides a clear justification for why this is the correct answer.