Which of the following statements best describes martensite?

Martensite is indeed best described as a structure formed when steel is quenched rapidly. This transformation occurs when austenite, the face-centered cubic phase of iron, is cooled quickly enough to prevent the carbon atoms from diffusing out as the structure changes. This rapid cooling locks the carbon in place, resulting in a body-centered tetragonal structure, which makes martensite extremely hard but also brittle.

The quenching process is crucial because it involves heating the steel to a high temperature to form austenite and then cooling it down rapidly, typically in water or oil, to form martensite. This process markedly enhances the hardness of the steel, which is an essential characteristic sought in many engineering applications where wear resistance is critical.

In contrast, other choices describe processes or characteristics that do not accurately represent martensite. For instance, martensite is significantly harder than ferrite, which is a softer phase of iron. Additionally, slow cooling of steel would lead to the formation of different microstructures, like pearlite or ferrite, rather than martensite. Lastly, martensite is not a type of non-ferrous metal, as it is derived from steel, which is an iron-based alloy.