Carbon steel

Carbon steel, or plain-carbon steel, is a metal alloy. It is a combination of two elements, iron and carbon. Other elements are present in quantities too small to affect its properties. The only other elements allowed in plain-carbon steel are:Mn (1.65% max), Si (0.60% max), and Cu (0.60% max).

Types of Carbon steels

There are four types of carbon steel based on the amount of carbon present in the alloy. Lower carbon steels are softer and more easily formed, and steels with a higher carbon content are harder and stronger, but less ductile, and they become more difficult to machine and weld. The different types of carbon steels are such as following:

• Low Carbon Steel – Composition of 0.05%-0.25% C and up to 0.4% Mn. Also known as mild steel, it is a low-cost material that is easy to shape. While not as hard as higher-carbon steels, carburizing can increase its surface hardness.
• Medium Carbon Steel – Composition of 0.29%-0.54% C, with 0.60%-1.65% Mn. Medium carbon steel is ductile and strong, with long-wearing properties.
• High Carbon Steel – Composition of 0.55%-0.95% C, with 0.30%-0.90% Mn. It is very strong and holds shape memory well, making it ideal for springs and wire.
• Very High Carbon Steel - Composition of 0.96%-2.1% C. Its high carbon content makes it an extremely strong material. Due to its brittleness, this grade requires special handling.

Heat treatment

The purpose of heat treating carbon steel is to change the mechanical properties of steel, usually ductility, hardness, yield strength, or impact resistance. Iron has a higher solubility for carbon in the austenite phase; therefore all heat treatments, except spheroidizing and process annealing, start by heating the steel to a temperature at which the austenitic phase can exist. The steel is then quenched (heat drawn out) at a moderate to low rate allowing carbon to diffuse out of the austenite forming iron-carbide (cementite) to precipitate leaving ferrite, or at a high rate, trapping the carbon within the iron thus forming martensite. The rate at which the steel is cooled through the eutectoid temperature (about 727°C) affects the rate at which carbon diffuses out of austenite and forms cementite. Cooling raptly will leave iron carbide finely dispersed and produce a fine grained pearlite and cooling slowly will give a coarser pearlite. Cooling a hypoeutectoid steel (less than 0.77 wt% C) results in a lamellar-pearlitic structure of iron carbide layers with α-ferrite (nearly pure iron) between. If it is hypereutectoid steel (more than 0.77 wt% C) then the structure is full pearlite with small grains (larger than the pearlite lamella) of cementite formed on the grain boundaries. A euctoid steel (0.77% carbon) will have a pearlite structure throughout the grains with no cementite at the boundaries. The relative amounts of constituents are found using the lever rule.

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