Carbon steel
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Carbon steel is a metal, a combination of two elements, iron and carbon, where other elements are present in quantities too small to affect the properties. With a low carbon content it 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 .10% to .25% (Ex. AISI 1018 steel)
- Medium carbon steel .25% to .45% (Ex. AISI 1040 steel)
- High carbon steel. .45% to .95%
- Very high carbon steel .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.
Metallurgy
Heat-treatment is an important aspect of carbon steel processing and involves the hypoeutectoid reaction between almost pure iron (ferrite alpha phase), Cementite (iron-carbon compound), and Austenite, which is a reorganized FCC iron structure that exists only at high temperatures. Carbon has a higher degree of solubility in the austenite phase. The rate at which the steel is cooled through this eutectoid reaction affects the rate at which carbon diffuses out of austenite. Cooling through a hypoeutectoid reaction in carbon steels results in a mostly pearlitic arrangement of alternating layers of ferrite and cementite.
Carbon steels which can successfully undergo heat-treatment have a Carbon content in the range of 0.30% to 1.70% by weight. It should be noted that 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.
Heat treatment
- Annealing: Heating to a high temperature then cooling slowly. Results in a soft and ductile steel with no internal stresses, often necessary for cost-effective forming.
- Normalizing: Heating to a high temperature then cooling at a medium rate in a furnace. Results in steel that exhibits a good balance of mechanical properties, by offering high strength and a good degree of toughness
- Hardening: Heating to high temperature then cooling rapidly in water or brine. Also called quenching. Results in steel that is extremely strong but brittle containing a high degree of internal stresses. Results in formation of Martensite, a form of steel that possesses a super-saturated carbon content in a deformed crystalline structure (BCT - Body-Centered Tetragonal) with a high resistance to deformation but with extremely high internal stresses. The technique requires steel with a carbon content high enough to be hardenable.
- Case hardening, Flame hardening and Induction hardening: Only the exterior of the steel part is heated and quenched, creating a hard, wear resistant skin, but preserving a tough interior. The surface of the steel is heated to high temperature then cooling rapidly through the use of localized heating mechanisms and water cooling. 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. Case hardening requires a steel with a certain level of carbon to be effective. Low carbon steels may be case hardened only if additional carbon is introduced:
- Packing low carbon steel parts with a carbonaceous material and heating for some time diffuses carbon into the outer layers. The parts then respond to heat treatments as above. A heating period of a few hours might form a high-carbon layer about one millimeter thick.
- Carboration 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).
- Spheroidizing: Heating to a high temperature (austenitic) then cooling at an extremely slow rate through active temperature control. Results in spherically diffused carbon areas with mostly iron rich compositions, also known as spheroidite, as opposed to elongated bands of pearlite. Results in extreme softness and ductility, often only necessary when a high degree of forming is necessary.
- Tempering: Reheating hardened steel to a lower temperature then cooling. Reforms crystal structure for a combination of strength and toughness depending on temperature. Necessary when a high degree of internal stresses are present or after quenching when the material is too brittle to be viable for structural applications. Actual temperatures and times are carefully chosen for each composition.
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, but may also require exotic cooling methods such as liquid nitrogen.