Final answer:
Adding nickel or chromium to steel mainly creates stainless steel due to the formation of a protective oxide layer that enhances corrosion resistance, rather than specifically resulting in any of the hardening processes listed.
Step-by-step explanation:
When nickel or chromium is added in small amounts to steel, it can become what is known as stainless steel. This specific addition of chromium to steel leads to the formation of a passivating oxide layer on the surface, enhancing its resistance to corrosion. Hence, such steel is neither solely austenitized, air-hardened, case-hardened, nor flame-hardened.
Rather, adding chromium especially bestows the property of corrosion resistance through the creation of this protective layer. This characteristic is critical for many applications where steel is exposed to potentially corrosive environments, such as in kitchen appliances, medical instruments, and construction materials.