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:

• Mild steel 0.10% to 0.25% (e.g., AISI 1018 steel)
• Medium carbon steel 0.25% to 0.45% (e.g., AISI 1040 steel)
• High carbon steel 0.45% to 0.95%

Very high carbon steel 0.95% to 2.1%

Steel with sufficient carbon compositions can be heat-treated, allowing parts to be fabricated in an easily-formable soft state then made harder for structural applications. Steels are often wrought by cold-working methods, which is the shaping of metal through deformation at a low equilibrium or metastable temperature.

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 is also often used where large amounts of steel need to be formed, for example as structural steel.

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].

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

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 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 Ł\-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:

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• Spheroidizing: Spheroidite forms when plain-carbon steel is heated to approximately 700 ˘XC for over 30 hours. Spheroidite can form at lower temperatures but the time needed drasticly increases, as this is a diffusion controlled process. The result is a structure of rods or spheres of cementite within primary sturcture (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 ususal occurs.

• Full Annealing: Plain-carbon steel is heated to approximately 40 ˘XC above Ac₃ or Ac₁ for 1 hour; this assures all the ferrite transforms into austentite (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 ˘XF 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 ˘Xstructure, 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.

• 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 ˘XC for 1 hour, but sometimes temperatures as high as 700 ˘XC. The image to the right shows the area where process annealling occurs.^[2]

• Normalizing: Plain-carbon steel is heated to approximately 55 ˘XC above Ac₃ or Ac_m for 1 hour; this assures the steel completely transforms to austentite. The steel is then air cooled, which is a cooling rate of approximately 100 ˘XF 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.

• 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 ammount 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 3 (lower carbon content) to 4 (high carbon content) time harder than normalized steel.
  • 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 the center and surface temperatures equilize. 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 the impact resistance. This is the quenching process used in industry to obtain martensite.

• 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.
  • 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.

• 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:
    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 ˘XF 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.