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. 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. Typical compositions of carbon are: Hot metal work from a blacksmith Look up Metal in Wiktionary, the free dictionary In chemistry, a metal (Greek: Metallon) is an element that readily forms ions (cations) and has metallic bonds, and metals are sometimes described as a lattice of positive ions (cations) surrounded by a sea of delocolised... Alloy is a combination, either in solution or compound, of two or more elements, which has a combination of at least one metal, and where the resultant material has metallic properties. ... General Name, Symbol, Number iron, Fe, 26 Chemical series transition metals Group, Period, Block 8, 4, d Appearance lustrous metallic with a grayish tinge Atomic mass 55. ... General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Atomic mass 12. ... The old steel cable of a colliery winding tower Steel is a metal alloy whose major component is iron, with carbon being the primary alloying material. ... Ductility is the physical property of being capable of sustaining large plastic deformations without fracture (in metals, such as being drawn into a wire). ...

• Mild (low carbon) steel: 0.05% to 0.26% (e.g. AISI 1018 steel)[1]
• Medium carbon steel: 0.29% to 0.54% (e.g. AISI 1040 steel)[2]
• High carbon steel: 0.55% to 0.95%[3]
• Very high carbon steel: 0.96% to 2.1%

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. Cold Work is a quality imparted on a material as a result of plastic deformation. ...

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 medium carbon contents (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. A blacksmiths forge The forge or smithy is the workplace of a smith or a blacksmith. ...

Carbon steels which can successfully undergo heat-treatment have a carbon content in the range of 0.30% to 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 2600-2800 F [4]. A chemical element, often called simply element, is a chemical substance that cannot be divided or changed into other chemical substances by any ordinary chemical technique. ... General Name, Symbol, Number sulfur, S, 16 Chemical series nonmetals Group, Period, Block 16, 3, p Appearance lemon yellow Atomic mass 32. ... Red-short is the quality possessed by carbon steel that suffers from having too much sulfur as an impurity. ...

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

Heat treatments

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

The purpose of heat treating plain-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 0.8 wt% C) steel results in a pearlitic structure with α-ferrite at the grain boundaries. If it is hypereutectoid (more than 0.8 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: Image File history File links Heat_transfer_steel_diag2. ... Image File history File links Heat_transfer_steel_diag2. ... In physical chemistry and materials science, a phase diagram is a type of graph used to show the equilibrium conditions between the thermodynamically-distinct phases. ... In solid mechanics, Youngs modulus (also known as the modulus of elasticity, elastic modulus or tensile modulus) is a measure of the stiffness of a given material. ... Eutectoid transformation occurs when a solid solution decomposes into a fixed two solid constituents at a fixed temperature. ... Pearlite is a two-phased, lamellar structure composed of alternating layers of ferrite (88 wt%) and cementite (12%) that occurs in steel. ...

• Spheroidizing: Spheroidite forms when plain-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 left shows where spheroidizing usually occurs.^[1]

• Full annealing: Plain-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 still might exist if the carbon content is greater than the eutectoid). The steel must then be cooled slowly, in the realm of 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.^[2]

• Process annealing: A process used to relieve stress in a cold-worked plain-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.^[2]

• Normalizing: Plain-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 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.^[3]

• Quenching: Plain-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 4four(high carbon content) times harder than normalized steel.^[4]
  • Martempering (Marquenching): The marquenching process is the same as quenching, but the steel is quenched in an oil or brine solution at a temperature right above the "martensite start temperature". 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. This is the quenching process used in industry to obtain martensite.^[5]

• 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. ^[6]
  • 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.^[7]

• Case hardening: Only the exterior of the steel part is heated and quenched, 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.
  • 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. The following are some examples of carburizing processes:^[8]
    1. 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.
    2. 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).
    3. Gas carburization: Parts placed into a furnace at 1700 °F containing a partial methane or carbon monoxide atmosphere. The parts are then quenched.

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. Annealing, in metallurgy and materials science, is a heat treatment wherein the microstructure of a material is altered, causing changes in its properties such as strength and hardness. ... Quenching is a general term for non-radiative de-excitation. ... A material is brittle if it is subject to fracture when subjected to stress i. ... Tempering is a heat treatment technique for metals and alloys, most often the toughening of martensitic steel. ... A semiconductor induction heater with a little inductor Induction heating is the process of heating a metal object by electromagnetic induction, where eddy currents are generated within the metal and resistance leads to Joule heating of the metal. ... Alloy is a combination, either in solution or compound, of two or more elements, which has a combination of at least one metal, and where the resultant material has metallic properties. ...

References

1. ↑ W.F. Smith and J. Hashemi, "Foundations of Materials Science and Engineering," 4th ed., McGraw-Hill, 2006, p. 388.
2. ↑ W.F. Smith and J. Hashemi, "Foundations of Materials Science and Engineering," 4th ed., McGraw-Hill, 2006, p. 386.
3. ↑ W.F. Smith and J. Hashemi, "Foundations of Materials Science and Engineering," 4th ed., McGraw-Hill, 2006, pp. 386-387.
4. ↑ W.F. Smith and J. Hashemi, "Foundations of Materials Science and Engineering," 4th ed., McGraw-Hill, 2006, pp. 373-377.
5. ↑ W.F. Smith and J. Hashemi, "Foundations of Materials Science and Engineering," 4th ed., McGraw-Hill, 2006, pp. 389-390.
6. ↑ W.F. Smith and J. Hashemi, "Foundations of Materials Science and Engineering," 4th ed., McGraw-Hill, 2006, pp. 387-388.
7. ↑ W.F. Smith and J. Hashemi, "Foundations of Materials Science and Engineering," 4th ed., McGraw-Hill, 2006, p. 391.
8. ↑ W.F. Smith and J. Hashemi, "Foundations of Materials Science and Engineering," 4th ed., McGraw-Hill, 2006, pp. 184-186.

See also

• Steel
• Heat treatment
• Cold work

The old steel cable of a colliery winding tower Steel is a metal alloy whose major component is iron, with carbon being the primary alloying material. ... Heat Treatment is a group of manufacturing techniques used to alter the hardness and toughness of a material. ... Cold Work is a quality imparted on a material as a result of plastic deformation. ...

External links

• Material properties
• A thorough discussion of tempering processes