Carbon steel hardness is influenced by carbon content, other alloy elements, and heat treatments. Low-carbon steel is softer but can be hardened through carburizing. Ultra high carbon steel is the hardest, but can be brittle. Hardening processes can increase hardness by four times, but tempering reduces it. Carburizing can make low carbon steel harder. High carbon steel can be too brittle, but hardening and tempering can increase ductility. Annealing can also increase ductility and decrease hardness.
The hardness of carbon steel can be affected by a number of different factors, including contact with the carbon, the amount and type of other elements in the alloy, and the specific processes used to create the steel. Low-carbon steel, which contains between 0.05 and 0.25% carbon, is typically the softer variety, although it can be hardened through a process known as carburizing. Ultra high carbon steel, which can contain up to 2% carbon, is typically the hardest type, although the final hardness of each product is also determined by various heat treatment processes. Hardening processes can increase the hardness of carbon steel by a factor of about four, although subsequent tempering typically reduces the hardness.
Carbon steel is an alloy consisting mainly of iron and carbon. Other elements may also be present, usually in very small quantities. There are a few different definitions of exactly what constitutes carbon steel, although elements such as silicon and copper typically account for less than 0.6% of the various alloys. Some carbon steel alloys can also contain up to 1.6% elements such as manganese. Since there are a number of different alloys that are all commonly referred to as carbon steel, each type tends to have a different hardness.
The main factor influencing the hardness of carbon steel is the amount of carbon present in the alloy. Low carbon steels tend to be the softest, while very high carbon steels can be quite hard and brittle. It is also possible to modify the physical characteristics of the carbon steel through various treatments, either to increase or decrease the hardness.
Even though low carbon steel is relatively soft, a process known as carburizing can make it harder. This heat treatment process causes the alloy to absorb additional carbon from solid, liquid and gaseous sources such as coal and carbon monoxide. The carbon is typically only absorbed into the surface of the metal, which results in a carburized outer layer and a softer, more ductile core.
High carbon steels tend to be very difficult to begin with, but hardening processes can make them even more difficult. However, if a carbon steel alloy is too hard, it will typically also be quite brittle. Some exceptionally hard alloys actually undergo hardening and tempering processes that can reduce the hardness of carbon steel and increase ductility. Other processes, such as annealing, can also effectively increase ductility and decrease the overall hardness of the steel.
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