The hardness of steel primarily depends on

* Carbon's Role: The primary factor determining steel hardness is the amount of carbon it contains. More carbon generally leads to higher hardness.

* Why Not the Others?
* Percentage of alloying elements: Alloying elements (like chromium, nickel, etc.) can increase hardness, but their influence is secondary to carbon. They mostly affect other properties like corrosion resistance or strength.
* Heat treatment employed: Heat treatment (like quenching, tempering) significantly alters hardness, but it primarily works by changing the microstructure of the steel, which is influenced by the carbon content. A steel with low carbon will not achieve very high hardness even with aggressive heat treatment.
* Shape of carbides and their distribution in iron: Carbides (compounds of iron and carbon) are important, and their shape and distribution certainly affect hardness. But, the amount of carbon available to form these carbides is the fundamental control.

In summary, while other factors play a role, the percentage of carbon is the most important and direct determinant of steel hardness.