Which kind of steel has been specially treated to resist cutting by normal means?

The correct choice is case-hardened steel. This type of steel undergoes a heat treatment process that hardens the surface while maintaining a softer, tougher core. The primary purpose of case-hardening is to enhance the surface hardness of the steel, making it more resistant to wear, abrasion, and cutting compared to untreated steel. This is particularly useful in applications where parts are subjected to high levels of stress and friction, ensuring better performance and longer service life.

The case-hardened surface can resist cutting by normal means more effectively than other types of steel, as the outer layer is significantly harder than the inner core. This strategic treatment performs well in various industrial applications, from gears to shafts, where both toughness and wear resistance are critical.

While alloy steel and tempered steel also provide various benefits, they do not inherently possess the same level of surface hardness specifically engineered to resist cutting. Ductile steel, on the other hand, is designed to deform without breaking, prioritizing toughness over hardness. Thus, while all these types of steel have unique properties that make them suitable for different applications, case-hardened steel is specifically treated to prevent cutting effectively.