Carbon steel

Carbon steel is a steel with carbon content up to 2.1% by weight. American Iron and Steel Institute (AISI) definition of Carbon Steel states:

Steel is considered to be carbon steel when:

The term "carbon steel" may also be used in reference to steel which is not stainless steel; in this use carbon steel may include alloy steels.

As the carbon percentage content rises, steel has the ability to become harder and stronger through heat treating; however, it becomes less ductile. Regardless of the heat treatment, a higher carbon content reduces weldability. In carbon steels, the higher carbon content lowers the melting point.[2]

Type

Mild or low-carbon steel

Mild steel (steel containing a small percentage of carbon, strong and tough but not readily tempered), also known as plain-carbon steel and low-carbon steel, is now the most common form of steel because its price is relatively low while it provides material properties that are acceptable for many applications. Mild steel contains approximately 0.05–0.25% carbon[1] making it malleable and ductile. Mild steel has a relatively low tensile strength, but it is cheap and easy to form; surface hardness can be increased through carburizing.[3]

It is often used when large quantities of steel are needed, for example as structural steel. The density of mild steel is approximately 7.85 g/cm3 (7850 kg/m3 or 0.284 lb/in3)[4] and the Young's modulus is 200 GPa (29,000,000 psi).[5]

Low-carbon steels suffer from yield-point runout where the material has two yield points. The first yield point (or upper yield point) is higher than the second and the yield drops dramatically after the upper yield point. If a low-carbon steel is only stressed to some point between the upper and lower yield point then the surface develop Lüder bands.[6] Low-carbon steels contain less carbon than other steels and are easier to cold-form, making them easier to handle.[7]

Higher-carbon steels

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, that is, brittle and crumbly at working temperatures. Low-alloy carbon steel, such as A36 grade, contains about 0.05% sulfur and melts around 1,426–1,538 °C (2,599–2,800 °F).[8] 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.

AISI Classification

Carbon steel is broken down into four classes based on carbon content:[1]

Low-carbon steel

0.05 to 0.30% carbon content.[1]

Medium-carbon steel

Approximately 0.3–0.6% carbon content.[1] Balances ductility and strength and has good wear resistance; used for large parts, forging and automotive components.[9][10]

High-carbon steel

Approximately 0.6–1.0% carbon content.[1] Very strong, used for springs, swords, and high-strength wires.[11]

Ultra-high-carbon steel

Approximately 3.25–4.0% carbon content.[1] 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 2.5% carbon content are made using powder metallurgy.

Heat treatment

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, or impact resistance. Note that the electrical and thermal conductivity are only slightly altered. As with most strengthening techniques for steel, Young's modulus (elasticity) is unaffected. All treatments of steel trade ductility for increased strength and vice versa. Iron has a higher solubility for carbon in the austenite phase; therefore all heat treatments, except spheroidizing and process annealing, start by heating the steel to a temperature at which the austenitic phase can exist. The steel is then quenched (heat drawn out) at a moderate to low rate allowing carbon to diffuse out of the austenite forming iron-carbide (cementite) to precipitate leaving ferrite, or at a high rate, trapping the carbon within the iron thus forming martensite. The rate at which the steel is cooled through the eutectoid temperature (about 727°C) affects the rate at which carbon diffuses out of austenite and forms cementite. Generally speaking, cooling swiftly will leave iron carbide finely dispersed and produce a fine grained pearlite and cooling slowly will give a coarser pearlite. Cooling a hypoeutectoid steel (less than 0.77 wt% C) results in a lamellar-pearlitic structure of iron carbide layers with α-ferrite (nearly pure iron) between. If it is hypereutectoid steel (more than 0.77 wt% C) then the structure is full pearlite with small grains (larger than the pearlite lamella) of cementite formed on the grain boundaries. A eutectoid steel (0.77% carbon) will have a pearlite structure throughout the grains with no cementite at the boundaries. The relative amounts of constituents are found using the lever rule. The following is a list of the types of heat treatments possible:

Case hardening

Case hardening processes harden only the exterior of the steel part, creating a hard, wear resistant skin (the "case") but preserving a tough and ductile interior. Carbon steels are not very hardenable meaning they can not be hardened throughout thick sections. Alloy steels have a better hardenability, so they can through-hardened and do not require case hardening. This property of carbon steel can be beneficial, because it gives the surface good wear characteristics but leaves the core tough.

Forging temperature of steel

[20]

Steel Type Maximum forging temperature (°F / °C) Burning temperature (°F / °C)
1.5% carbon 1920 / 1049 2080 / 1140
1.1% carbon 1980 / 1082 2140 / 1171
0.9% carbon 2050 / 1121 2230 / 1221
0.5% carbon 2280 / 1249 2460 / 1349
0.2% carbon 2410 / 1321 2680 / 1471
3.0% nickel steel 2280 / 1249 2500 / 1371
3.0% nickel–chromium steel 2280 / 1249 2500 / 1371
5.0% nickel (case-hardening) steel 2320 / 1271 2640 / 1449
Chromium–vanadium steel 2280 / 1249 2460 / 1349
High-speed steel 2370 / 1299 2520 / 1385
Stainless steel 2340 / 1282 2520 / 1385
Austenitic chromium–nickel steel 2370 / 1299 2590 / 1420
Silico-manganese spring steel 2280 / 1249 2460 / 1350

See also

References

  1. 1 2 3 4 5 6 7 "Classification of Carbon and Low-Alloy Steels"
  2. Knowles, Peter Reginald (1987), Design of structural steelwork (2nd ed.), Taylor & Francis, p. 1, ISBN 978-0-903384-59-9.
  3. Engineering fundamentals page on low-carbon steel
  4. Elert, Glenn, Density of Steel, retrieved 23 April 2009.
  5. Modulus of Elasticity, Strength Properties of Metals – Iron and Steel, retrieved 23 April 2009.
  6. Degarmo, p. 377.
  7. "Low-carbon steels". efunda. Retrieved 2012-05-25.
  8. Ameristeel article on carbon steel Archived 18 October 2006 at the Wayback Machine.
  9. Nishimura, Naoya; Murase, Katsuhiko; Ito, Toshihiro; Watanabe, Takeru; Nowak, Roman. "Ultrasonic detection of spall damage induced by low-velocity repeated impact". Central European Journal of Engineering. 2 (4): 650–655. doi:10.2478/s13531-012-0013-5.
  10. Engineering fundamentals page on medium-carbon steel
  11. Engineering fundamentals page on high-carbon steel
  12. Smith, p. 388
  13. Alvarenga HD, Van de Putte T, Van Steenberge N, Sietsma J, Terryn H (Apr 2009). "Influence of Carbide Morphology and Microstructure on the Kinetics of Superficial Decarburization of C-Mn Steels". Metal Mater Trans A. doi:10.1007/s11661-014-2600-y.
  14. Smith, p. 386
  15. Smith, pp. 386–387
  16. Smith, pp. 373–377
  17. Smith, pp. 389–390
  18. Smith, pp. 387–388
  19. Smith, p. 391
  20. Brady, George S.; Clauser, Henry R.; Vaccari A., John (1997). Materials Handbook (14th ed.). New York, NY: McGraw-Hill. ISBN 0-07-007084-9.

Bibliography

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