Carbon steel is a steel with carbon content from about 0.05 up to 2.1 percent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states:

• no minimum content is specified or required for chromium, cobalt, molybdenum, nickel, niobium, titanium, tungsten, vanadium, zirconium, or any other element to be added to obtain a desired alloying effect;
• the specified minimum for copper does not exceed 0.40%;
• or the specified maximum for any of the following elements does not exceed the percentages noted: manganese 1.65%; silicon 0.60%; copper 0.60%.^[1]

The term carbon steel may also be used in reference to steel which is not stainless steel; in this use carbon steel may include alloy steels. High carbon steel has many different uses such as milling machines, cutting tools (such as chisels) and high strength wires. These applications require a much finer microstructure, which improves the toughness.

As the carbon content percentage rises, steel has the ability to become harder and stronger through heat treating; however, it becomes less ductile. Regardless of the heat treatment, a higher carbon content reduces weldability. In carbon steels, the higher carbon content lowers the melting point.^[2]

Carbon steel is often divided into two main categories: low-carbon steel and high-carbon steel. It may also contain other elements, such as manganese, phosphorus, sulfur, and silicon, which can affect its properties. Carbon steel can be easily machined and welded, making it versatile for various applications. It can also be heat treated to improve its strength, hardness, and durability.

Carbon steel is susceptible to rust and corrosion, especially in environments with high moisture levels and/or salt. It can be shielded from corrosion by coating it with paint, varnish, or other protective material. Alternatively, it can be made from a stainless steel alloy that contains chromium, which provides excellent corrosion resistance. Carbon steel can be alloyed with other elements to improve its properties, such as by adding chromium and/or nickel to improve its resistance to corrosion and oxidation or adding molybdenum to improve its strength and toughness at high temperatures.

It is an environmentally friendly material, as it is easily recyclable and can be reused in various applications. It is energy-efficient to produce, as it requires less energy than other metals such as aluminium and copper.^{[citation needed]}

The following classification method is based on the American AISI/SAE standard. Other international standards including DIN (Germany), GB (China), BS/EN (UK), AFNOR (France), UNI (Italy), SS (Sweden) , UNE (Spain), JIS (Japan), ASTM standards, and others.

Carbon steel is broken down into four classes based on carbon content:^[1]

The purpose of heat treating carbon steel is to change the mechanical properties of steel, usually ductility, hardness, yield strength, or impact resistance. Note that the electrical and thermal conductivity are only slightly altered. As with most strengthening techniques for steel, Young's modulus (elasticity) is unaffected. All treatments of steel trade ductility for increased strength and vice versa. 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) and 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 or 1,341 °F) affects the rate at which carbon diffuses out of austenite and forms cementite. Generally speaking, cooling swiftly 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 eutectoid 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. The following is a list of the types of heat treatments possible:

Spheroidizing

Spheroidite forms when carbon steel is heated to approximately 700 °C (1,300 °F) for over 30 hours. Spheroidite can form at lower temperatures but the time needed drastically increases, as this is a diffusion-controlled process. The result is a structure of rods or spheres of cementite within primary structure (ferrite or pearlite, depending on which side of the eutectoid you are on). The purpose is to soften higher carbon steels and allow more formability. This is the softest and most ductile form of steel.^[15]

Full annealing

A hypoeutectoid carbon steel (carbon composition smaller than the eutectoid one) is heated to approximately 30 to 50 °C (86 to 120 °F) above the austenictic temperature (A₃), whereas a hypereutectoid steel is heated to a temperature above the eutectoid one (A₁) for a certain number of hours; this ensures all the ferrite transforms into austenite (although cementite might still exist in hypereutectoid steels). The steel must then be cooled slowly, in the realm of 20 °C (36 °F) per hour. Usually it is just furnace cooled, where the furnace is turned off with the steel still inside. This results in a coarse pearlitic structure, which means the "bands" of pearlite are thick.^[16] Fully annealed steel is soft and ductile, with no internal stresses, which is often necessary for cost-effective forming. Only spheroidized steel is softer and more ductile.^[17]

Process annealing

A process used to relieve stress in a cold-worked carbon steel with less than 0.3% C. The steel is usually heated to 550 to 650 °C (1,000 to 1,200 °F) for 1 hour, but sometimes temperatures as high as 700 °C (1,300 °F). The image above shows the process annealing area.

Isothermal annealing

It is a process in which hypoeutectoid steel is heated above the upper critical temperature. This temperature is maintained for a time and then reduced to below the lower critical temperature and is again maintained. It is then cooled to room temperature. This method eliminates any temperature gradient.

Normalizing

Carbon steel is heated to approximately 550 °C (1,000 °F) for 1 hour; this ensures the steel completely transforms to austenite. The steel is then air-cooled, which is a cooling rate of approximately 38 °C (100 °F) per minute. This results in a fine pearlitic structure, and a more-uniform structure. Normalized steel has a higher strength than annealed steel; it has a relatively high strength and hardness.^[18]

Quenching

Carbon steel with at least 0.4 wt% C is heated to normalizing temperatures and then rapidly cooled (quenched) in water, brine, or oil to the critical temperature. The critical temperature is dependent on the carbon content, but as a general rule is lower as the carbon content increases. This results in a martensitic structure; a form of steel that possesses a super-saturated carbon content in a deformed body-centered cubic (BCC) crystalline structure, properly termed body-centered tetragonal (BCT), with much internal stress. Thus quenched steel is extremely hard but brittle, usually too brittle for practical purposes. These internal stresses may cause stress cracks on the surface. Quenched steel is approximately three times harder (four with more carbon) than normalized steel.^[19]

Martempering (marquenching)

Martempering is not actually a tempering procedure, hence the term marquenching. It is a form of isothermal heat treatment applied after an initial quench, typically in a molten salt bath, at a temperature just above the "martensite start temperature". At this temperature, residual stresses within the material are relieved and some bainite may be formed from the retained austenite which did not have time to transform into anything else. In industry, this is a process used to control the ductility and hardness of a material. With longer marquenching, the ductility increases with a minimal loss in strength; the steel is held in this solution until the inner and outer temperatures of the part equalize. Then the steel is cooled at a moderate speed to keep the temperature gradient minimal. Not only does this process reduce internal stresses and stress cracks, but it also increases impact resistance.^[20]

Tempering

This is the most common heat treatment encountered because the final properties can be precisely determined by the temperature and time of the tempering. Tempering involves reheating quenched steel to a temperature below the eutectoid temperature and then cooling. The elevated temperature allows very small amounts spheroidite to form, which restores ductility but reduces hardness. Actual temperatures and times are carefully chosen for each composition.^[21]

Austempering

The austempering process is the same as martempering, except the quench is interrupted and the steel is held in the molten salt bath at temperatures between 205 and 540 °C (400 and 1,000 °F), and then cooled at a moderate rate. The resulting steel, called bainite, produces an acicular microstructure in the steel that has great strength (but less than martensite), greater ductility, higher impact resistance, and less distortion than martensite steel. The disadvantage of austempering is it can be used only on a few sheets of steel, and it requires a special salt bath.^[22]

Case hardening processes harden only the exterior of the steel part, creating a hard, wear-resistant skin (the "case") but preserving a tough and ductile interior. Carbon steels are not very hardenable meaning they can not be hardened throughout thick sections. Alloy steels have a better hardenability, so they can be through-hardened and do not require case hardening. This property of carbon steel can be beneficial, because it gives the surface good wear characteristics but leaves the core flexible and shock-absorbing.

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