Carbon content influences the yield strength of steel because they fit into the interstitial crystal lattice sites of the body-centered cubic arrangement of the iron molecules. The interstitial carbon reduces the mobility of dislocations, which in turn has a hardening effect on the iron. To get dislocations to move, a high enough stress level must be applied in order for the dislocations to "break away". This is because the interstitial carbon atoms cause some of the iron BCC lattice cells to distort.
Types of carbon steel

Mild (low carbon) steel: approximately 0.05–0.29% carbon content Mild steel has a relatively low tensile strength, but it is cheap and malleable; surface hardness can be increased through carburizing   
Medium carbon steel: approximately 0.30–0.59% carbon content(e.g. AISI 1040 steel). Balances ductility and strength and has good wear resistance; used for large parts, forging and automotive components.
High carbon steel: approximately 0.6–0.99% carbon content]. Very strong, used for springs and high-strength wires.
Ultra-high carbon steel: approximately 1.0–2.0% carbon content. Steels that can be tempered to great hardness. Used for special purposes like (non-industrial-purpose) knives, axles or punches. Most steels with more than 1.2% carbon content are made using powder metallurgy and usually fall in the category of high alloy Carbon steels. Steel can be heat-treated which allows parts to be fabricated in an easily-formable soft state. If enough carbon is present, the alloy can be hardened to increase strength, wear, and impact resistance.

        Steels are often wrought by cold-working methods, which is the shaping of metal through deformation at a low equilibrium or metastable temperature.

Metallurgy

Mild steel is the most common form of steel as its price is relatively low while it provides material properties that are acceptable for many applications. Mild steel has a low Carbon content (up to 0.3%) and is therefore neither extremely brittle nor ductile. It becomes malleable when heated, and so can be forged. It is also often used where large amounts of steel need to be formed, for example as structural steel. Density of this metal is 7,861.093 kg/m³ (0.284 lb/in³), the tensile strength is a maximum of 500 MPa (72,500 psi) and it has a Young's modulus of 210 GPa.

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 have a significant effect on the quality of the resulting steel. Trace amounts of sulfur in particular make the steel red-short. Low alloy Carbon steel, such as A36 grade, contains about 0.05% sulfur and melts around 1426–1538 °C (2600–2800 °F). 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.

Hardened steel usually refers to quenched or quenched and tempered steel.

Silver steel or high-carbon bright steel, gets its name from its appearance, due to the high carbon content. It is a very-high Carbon steel, or can be thought of as some of the best high- Carbon steel. It is defined under the steel specification standards BS-1407. It is a 1%-carbon tool steel which can be ground to close tolerances. Usually the range of carbon is minimum 1.10% but as high as 1.20%. It also contains trace elements of 0.35% Mn (range 0.30–0.40%), 0.40% Cr (range 0.4–0.5%), 0.30% Si (range 0.1–0.3%), and also sometimes sulfur (max 0.035%) and phosphorus (max 0.035%). Silver steel is sometimes used for making straight razors, due to its ability to produce and hold a micro-fine edge, as those made by the French company Thiers-Issard

Case hardening

Only the exterior of the steel part is hardened, creating a hard, wear resistant skin, but preserving a tough and ductile interior.

• Flame hardening and induction hardening: The surface of the steel is heated to high temperature then cooling rapidly through the use of localized heating mechanisms and water cooling. The purpose is to create a "case" of martensite on the surface where wear resistance is needed. A carbon content of 0.4–0.6 wt% C is needed for this type of hardening. Typical uses are for the shackle of a lock, where the outer layer is hardened to be file resistant, and mechanical gears where hard gear mesh surfaces are needed to maintain a long service life while toughness is required to maintain durability and resistance to catastrophic failure.

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• Carburizing: A process used to case harden steel with a carbon content between 0.1 and 0.3 wt% C. In this process steel is introduced to a carbon rich environment and elevated temperatures for a certain amount of time. Because this is a diffusion controlled process, the longer the steel is held in this environment greater the carbon penetration will be and the higher the carbon content in these areas. The part is then quenched so that the carbon is locked in the structure. The hardness is moderately increased, but it can be hardened again through flame or induction hardening. It's possible to carburize only a portion of the part by covering it in copper plating or coating it with a commercial paste. The following are some examples of carburizing processes:

• Pack carburizing: Packing low carbon steel parts with a carbonaceous material and heating for some time diffuses carbon into the outer layers. A heating period of a few hours might form a high-carbon layer about one millimeter thick.

• Liquid carburizing: This method involves heating the part in a bath of molten barium cyanide or sodium cyanide. The surface absorbs both sodium and carbon this way.

• Gas carburization: Parts placed into a furnace at 927 °C (1700 °F) containing a partial methane or carbon monoxide atmosphere. The parts are then quenched.

• Carburization may also be accomplished with an acetylene torch set with a fuel rich flame and heating and quenching repeatedly in a carbon rich fluid (oil).

• Nitriding: This process heats the steel part to 482–621 °C (900–1150 °F) in an atmosphere of ammonia gas and dissociated ammonia. The time the part spends in this environment dictates the depth of the case. The hardness is achieved by the formation of nitrides. Nitride forming elements must be present for this method to work; these elements include chromium, molybdenum, and aluminum. The advantage of this process is it causes little distortion, so the part can be case hardened after being quench and tempered and machined.

• Cyaniding: This process heats the part in a bath of sodium cyanide to a temperature in the austenitic phase and then is quenched. This creates a very hard, yet thin case.

• Carbonitriding: This process is similar to cyaniding except a gaseous atmosphere of ammonia and hydrocarbons is used instead of sodium cyanide. If the part is to be quenched then the part is heated to 775–885 °C (1425–1625 °F); if not then the part is heated to 649–788 °C (1200–1450 °F). Trade names for the process include Tenifer, Melonite, Sursulf, Arcor, Tufftride, and Koline.

A limitation of plain carbon steel is the very rapid rate of cooling needed to produce hardening. In large pieces it is not possible to cool the inside rapidly enough and so only the surfaces can be hardened. This can be improved with the addition of other elements resulting in alloy steel.