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Carbon steel, also called plain carbon steel, is a metal alloy, a combination of two elements, iron and carbon, where other elements are present in quantities too small to affect the properties. The only other alloying elements allowed in plain-carbon steel are manganese (1.65% max), silicon (0.60% max), and copper (0.60% max).^[1] Steel with a low carbon content has the same properties as iron, soft but easily formed. As carbon content rises, the metal becomes harder and stronger but less ductile and more difficult to weld. Higher carbon content lowers steel's melting point and its temperature resistance in general.

Carbon content influences the yield strength of steel because carbon molecules 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.

[edit] Types of carbon steel

Typical compositions of carbon:

• Mild (low carbon) steel: approximately 0.05–0.15% carbon content for low carbon steel and 0.16-0.29% carbon content for mild steel^[2] (e.g. AISI 1018 steel). Mild steel has a relatively low tensile strength, but it is cheap and malleable; surface hardness can be increased through carburizing.^[3]

• Medium carbon steel: approximately 0.30–0.59% carbon content^[2](e.g. AISI 1040 steel). Balances ductility and strength and has good wear resistance; used for large parts, forging and automotive components.^[4]

• High carbon steel: approximately 0.6–0.99% carbon content ^[2]. Very strong, used for springs and high-strength wires.^[5]

• Ultra-high carbon steel: approximately 1.0–2.0% carbon content ^[2]. 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.

[edit] 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).^[6] 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. It is a very-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 in the range 1.10% - 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.

[edit] Heat treatments

Iron-carbon phase diagram, showing the temperature and carbon ranges for certain types of heat treatments.

The purpose of heat treating carbon steel is to change the mechanical properties of steel, usually ductility, hardness, yield strength, and impact resistance. Note that the electrical and thermal conductivity are slightly altered. As with most strengthening techniques for steel, the modulus of elasticity (Young's modulus) is never affected. Steel has a higher solid solubility for carbon in the austenite phase, therefore all heat treatments, except spheroidizing and process annealing, start by heating to an austenitic phase. The rate at which the steel is cooled through the eutectoid reaction affects the rate at which carbon diffuses out of austenite. Generally speaking, cooling quickly will give a finer pearlite (until the martensite critical temperature is reached) and cooling slowly will give a coarser pearlite. Cooling a hypoeutectoid (less than 0.77 wt% C) steel results in a pearlitic structure with α-ferrite at the grain boundaries. If it is hypereutectoid (more than 0.77 wt% C) steel then the structure is full pearlite with small grains of cementite scattered throughout. The relative amounts of constituents are found using the lever rule. Here is a list of the types of heat treatments possible:

• Spheroidizing: Spheroidite forms when carbon steel is heated to approximately 700 °C 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. The image to the right shows where spheroidizing usually occurs.^[7]

• Full annealing: Carbon steel is heated to approximately 40 °C above Ac₃ or Ac₁ for 1 hour; this assures all the ferrite transforms into austenite (although cementite might still exist if the carbon content is greater than the eutectoid). The steel must then be cooled slowly, in the realm of 38 °C (100 °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. 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.^[8]

• Process annealing: A process used to relieve stress in a cold-worked carbon steel with less than 0.3 wt% C. The steel is usually heated up to 550–650 °C for 1 hour, but sometimes temperatures as high as 700 °C. The image to the right shows the area where process annealing occurs.

• Isothermal annealing: It is a process in which hypoeutectoid steel is heated above the upper critical temperature and this temperature is maintained for a period of time and then the temperature is brought down below lower critical temperature and is again maintained. Then finally it is cooled at room temperature. This method helps in eliminating any temperature gradient.

• Normalizing: Carbon steel is heated to approximately 55 °C above Ac₃ or Ac_m for 1 hour; this assures 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 ductility.^[9]

• 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). This crystalline structure has a very high amount of internal stress. Due to these internal stress quenched steel is extremely hard but brittle, usually too brittle for practical purposes. These internal stresses cause stress cracks on the surface. Quenched steel is approximately three (lower carbon content) to four (high carbon content) times harder than normalized steel.^[10]

• 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 of typically in an oil or brine solution at a temperature right above the "martensite start temperature". At this temperature, residual stresses within the material are relieved and some bainite may be formed from the retained ferrite 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 time, the ductility increases with a minimal loss in strength; the steel is held in this solution until the center and surface temperatures 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 the impact resistance.^[11]

• Quench and 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 then cooling. The elevated temperature allows very small amounts of spheroidite to form, which restore ductility, but reduces hardness. Actual temperatures and times are carefully chosen for each composition.^[12]

• Austempering: The austempering process is the same as martempering, except the steel is held in the brine solution through the bainite transformation temperatures, and then moderately cooled. The resulting bainite steel has a greater ductility, higher impact resistance, and less distortion. The disadvantage of austempering is it can only be used on a few steels, and it requires a special brine solution.^[13]

[edit] Case hardening

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

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.

[edit] Flame and induction hardening

Induction heating is when the surface of the steel is heated to high temperatures then cooled 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.

[edit] Carburizing

Main article: Carburization

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:^[14]

• 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.^[15]

• Gas carburizing: 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).

[edit] Nitriding

Main article: 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 aluminium. The advantage of this process is it causes little distortion, so the part can be case hardened after being quench and tempered and machined.

[edit] Cyaniding

This is a casehardening process that is fast and efficient; it is mainly used on Low carbon steels. The part is heated to 1600-1750º F in a bath of sodium cyanide and then is quenched and rinsed, in water or oil, to remove any residual cyanide.

This process produces a thin, hard shell (0.010 in. and 0.030 in.) that is harder than the one produced by carburizing and can be completed in 20 to 30 minutes compared to several hours. Advantage of this method is the short time it requires to complete the diffusion; otherwise it should be avoided because of high distortion. The major drawback of Cyaniding is that cyanide salts are poisonous.

The Typical Applications of this process are Bolts, Nuts, Screws, and Small Gears.

[edit] Carbonitriding

Main article: 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.

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