Carbon steel
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: 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. Carbon steel is a popular metal choice for knife making  due to its high amount of carbon, giving the blade more edge retention. To make the most out of this type of steel it is very important to heat treat it properly. If not the knife may end up being brittle, or too soft to hold an edge. As the carbon percentage content 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. Type. Mild or low-carbon steel. Mild  steel (iron containing a small percentage of carbon, strong and tough but not readily tempered), also known as plain-carbon steel and low-carbon steel, is now the most common form of steel because its price is relatively low while it provides material properties that are acceptable for many applications. Mild steel contains approximately 0.05–0.30% carbon making it malleable and ductile. Mild steel has a relatively low tensile strength, but it is cheap and easy to form; carburizing can increase surface hardness carburizing. In applications where large cross-sections are used to minimize deflection, failure by yield is not a  risk so low-carbon steels are the best choice, for example as structural steel. The density of mild steel is approximately 7.85 g/cm (7850 kg/m or 0.284 lb/in) and the Young's modulus is . Low-carbon steels display "yield-point runout" where the material has two yield points. The first yield point (or upper yield point) is higher than the second and the yield drops dramatically after the upper yield point. If a low-carbon steel is only stressed to some point between the upper and lower yield point then the surface develops Lüder bands. Low-carbon  steels contain less carbon than other steels and are easier to cold-form, making them easier to handle. Typical applications of low carbon steel are car parts, pipes, construction, and food cans. High-tensile steel. High-tensile steels are low-carbon, or steels at the lower end of the medium-carbon range, which have additional alloying ingredients in order to increase their strength, wear properties or specifically tensile strength. These alloying ingredients include chromium, molybdenum, silicon, manganese, nickel, and vanadium. Impurities such as phosphorus and sulfur have their maximum allowable content restricted. Higher-carbon steels. Carbon steels which can successfully  undergo heat-treatment have a carbon content in the range of 0.30–1.70% by weight. Trace impurities of various other elements can significantly affect the quality of the resulting steel. Trace amounts of sulfur in particular make the steel red-short, that is, brittle and crumbly at working temperatures. Low-alloy carbon steel, such as A36 grade, contains about 0.05% sulfur and melts around . Manganese is often added to improve the hardenability of low-carbon steels. These additions turn the material into a low-alloy steel by some definitions, but AISI's definition of carbon steel allows up to 1.65% manganese  by weight. AISI classification. Carbon steel is broken down into four classes based on carbon content: Low-carbon steel. 0.05 to 0.25% carbon (plain carbon steel) content. Medium-carbon steel. Approximately 0.3–0.5% carbon content. Balances ductility and strength and has good wear resistance; used for large parts, forging and automotive components. High-carbon steel. Approximately 0.6 to 1.0% carbon content. Very strong, used for springs, edged tools, and high-strength wires. Ultra-high-carbon steel. Approximately 1.25–2.0% carbon content. Steels that can be tempered to great hardness. Used for special purposes like (non-industrial-purpose) knives, axles, and punches. Most  steels with more than 2.5% carbon content are made using powder metallurgy. 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. 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) 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: Case hardening. 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.