Carbon is by far the most important alloying addition to steel and cast iron. It is responsible, to a great extent, for the wide range of achievable mechanical properties in both wrought and powder-metallurgy materials. Carbon is often considered the most important and influential alloying element in most steels and is added to increase solid-solution strength and hardness as well as to increase hardenability.

Carbon dissolves in iron to form ferrite and austenite. Carbon also forms carbides, most notably iron carbide (Fe3C) as well as other alloy carbides. Ferrite, pearlite, bainite and martensite formation are influenced by the amount of carbon present. Within limits, increasing the carbon content increases the strength and hardness of steel while reducing its toughness and ductility. Here are a few additional facts about the influence of carbon that heat treaters need to be aware of:

  • Carbon is the principal element responsible for producing hardness in steel (due to formation of iron carbide on cooling).
  • Steel becomes more responsive to heat treatment with increasing carbon content.
  • Carbon is a strong austenite former and acts as a ferrite strengthener (in the range of 0.04-0.40% in wrought steel, up to 0.75% in cast steel).
  • The ductile-to-brittle transition temperature is increased with increasing carbon content up to approximately 0.60%. (Note: Toughness and ductility of pearlitic steels are decreased with increasing carbon content.)
  • Weldability decreases as the carbon content increases. In certain welding applications it is common to specify a “carbon equivalent” in order to limit the potential for cracking. An example of a carbon equivalent calculation is given by the Dearden-O’Neill formula (Equation 1), which is applicable to plain carbon and carbon-manganese steels but not low carbon Cr-Mo or high-strength, low-alloy microalloyed steels. Based on this equation, we can determine a steel's suitability for welding (Table 1).
  • Carbon retards intermetallic phases (e.g., sigma phase in stainless steels).
  • Carbon and nitrogen compete with one another when titanium is present, forming titanium carbide (TiC) or titanium nitride (TiN).
  • Hardness and tensile strength increases as carbon content increases up to about 0.85%.
  • Ductility generally decreases as carbon content increases, but in most cases this effect can be partially offset by heat treatment. In general, carbon improves wear and abrasion resistance but lowers ductility, toughness and machinability.
  • Carbon has a moderate tendency to segregate within an ingot.
  • The carbon content has essentially no influence on steel's corrosion resistance when exposed to hot gases, water or acids.
  • Carbon controls the hardness of the martensite. Increasing the carbon content increases the hardness of steels up to about 0.6 wt.%. At higher carbon levels, the formation of martensite is depressed to lower temperatures and the transformation from austenite to martensite may be incomplete, leading to retained austenite. This composite microstructure of martensite and austenite gives a lower overall hardness to the steel, although the microhardness of the martensite phase itself is still high.



1. "Alloy Element Effects," white paper