Carbon steel (US) or non-alloy steel (Europe) 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 addition of significant amounts of other metals such as nickel, chromium, vanadium, molybdenum change the materials definition from carbon steel to alloy steel.
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.
High-carbon steel has many uses primarily applications where medium strength and low material cost are factors, these include: high strength steel wires, springs, bolts, garden and agricultural tools, wood chisels, axes etc.
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. C Low carbon steel can be easily machined and welded, making it versatile and suitable for various applications which require reasonable strength and low material cost such as car body panels, footbridges, hand rails, garden furniture/park benches, nails, gates etc . Higher carbon is less easily machined and is much less suitable for welding 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.
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 0.05âÂÂ0.30% carbon making it malleable and ductile. Mild steel has a relatively low tensile strength, but it is cheap and easy to form. Surface hardness can be increased with carburization.
The density of mild steel is approximately 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.
Steels which have a carbon content above ~0.3% can have their hardness and tensile strength modified by heat treatment. Heating to approximately 850c followed by rapid cooling/quenching in water or oil increases hardness and tensile strength, but it also reduces maleability, increases brittleness and make fractures and breakages much more likely. To reduce these brittleness problems whilst still achieving high levels of hardness and tensile strength, low or medium carbon steels are alloyed with other metals. These alloy steels also require specific heat treatment to achieve the required mechanical properties, and the tensile strength/hardness trade off vs malleability remains, but to a much lesser extent compared to low alloy carbon steel. Alloying ingredients include chromium, molybdenum, silicon, manganese, nickel, and vanadium. The relative scarcity and high cost of these metals compared to iron is primarily what restricts their use.
Carbon steels which can successfully undergo heat-treatment have a carbon content in the range of 0.30âÂÂ1.70% 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 high working temperatures. Low-alloy carbon steel, such as A36 grade, contains about 0.05% sulfur and melt 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. There are two types of higher carbon steels which are high carbon steel and the ultra high carbon steel. The reason for the limited use of high carbon steel is that it has extremely poor ductility and weldability and has a higher cost of production. The applications best suited for the high carbon steels is its use in the spring industry, farm industry, and in the production of wide range of high-strength wires.
Different classifications of non-alloy steel exist.
Carbon steel is broken down into four classes based on carbon content:
EN 10020 categorizes non-alloy steels into two main groups:
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 ) 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 ñ-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 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.