The two main categories of fabricated steel are plain-carbon and alloy steels. You might be wondering what the fundamental differences between them are. Let’s talk first about plain-carbon steel. Plain-carbon steel is steel in which the main alloying element is carbon, and other elements are present in quantities too small to affect the mechanical properties.

Plain-carbon steels are divided into four categories:
  • Low-carbon steel (0.005% - 0.30%C) for such items as automobile frames and bodies, structural shapes (I-beams, channels, angles) and the like.
  • Medium-carbon steel (0.30% - 0.60%C) for machined parts, tools and the like.
  • High-carbon steel (0.60% - 1.0%C) for springs, dies, railroad rail and the like.
  • Ultra-high-carbon steel (1.0 – 1.2%C) for knives, axles or punches.
Carbon is a very powerful alloying element affecting the strength and thus the hardness of the steel (Table 1) due to the formation of iron carbides (Fe3C). Manganese, another common alloying element in low-carbon steel has little impact on strength or hardness even though it is also a carbide former.