The hardness of steel depends on its specific composition, heat treatment, and the desired properties for a particular application. Both spring steel and carbon steel can exhibit a range of hardness levels depending on their composition and heat treatment.

Spring steel is typically designed to have a balance of hardness, toughness, and resilience to meet the requirements of spring applications. It is often high in carbon content and can be hardened to achieve the desired hardness for its intended use.

Carbon steel is a broad term that encompasses a wide range of steel alloys primarily composed of iron and carbon. The carbon content in carbon steel can vary, and different carbon steels can have different hardness levels based on their composition and heat treatment.

In general, spring steel and carbon steel can have similar hardness levels, depending on the specific grades and heat treatment processes employed. It is not accurate to categorically state that one is always harder than the other.

The hardness of steel is commonly measured using scales such as Rockwell hardness (HRC) or Brinell hardness (HB). By selecting specific grades and utilizing appropriate heat treatment techniques, both spring steel and carbon steel can achieve a wide range of hardness levels suitable for different applications.

It's important to note that while hardness is a crucial factor, other properties such as toughness, flexibility, and resilience are also important considerations when selecting the appropriate steel for a specific application.