What is the role of nickel or chromium in air hardening of steel?

In the context of air hardening of steel, the addition of nickel or chromium plays a crucial role in improving the hardenability of the steel. When small amounts of these alloying elements are added, they allow the steel to achieve a high level of hardness upon cooling in air, rather than requiring water or oil quenching, which are more aggressive cooling methods.

Nickel enhances the toughness and ductility of the steel while also contributing to its hardenability. Chromium, on the other hand, not only increases hardenability but also improves resistance to wear and corrosion. This combination allows for the formation of a harder martensitic structure during the cooling process, even without the need for rapid quenching methods.

Thus, the correct response accurately identifies the significance of these elements in facilitating the air hardening process when they are incorporated into the steel matrix in small proportions.