Carbon steel

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Carbon steel is steel in which the main interstitial alloying constituent is carbon in the range of 0.12–2.0%. The American Iron and Steel Institute (AISI) defines carbon steel as the following: "Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, molybdenum, nickel, niobium, titanium, tungsten, vanadium or zirconium, or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 percent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60."[1]

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]

Types[edit]

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

Mild and low-carbon steel[edit]

Mild steel[clarification needed], also known as plain-carbon steel, is the most common form of steel because its price is relatively low while it provides material properties that are acceptable for many applications, more so than iron. Low-carbon steel contains approximately 0.05–0.320% carbon[1] making it malleable and ductile. Mild steel has a relatively low tensile strength, but it is cheap and malleable; 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 210 GPa (30,000,000 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 may 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[edit]

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.

Low carbon steel

<0.3% carbon content, see above.

Medium carbon steel

Approximately 0.30–0.59% 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 (ASTM 304)

Approximately 0.6–0.99% carbon content.[1] Very strong, used for springs and high-strength wires.[11]

Ultra-high-carbon steel

Approximately 1.0–2.0% 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 1.2% carbon content are made using powder metallurgy. Note that steel with a carbon content above 2.14% is considered cast iron.

Heat treatment[edit]

Iron-carbon phase diagram, showing the temperature and carbon ranges for certain types of heat treatments.
Main article: Heat treatment

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 high rate causing cementite to precipitate and finally the remaining pure iron to solidify. The rate at which the steel is cooled through the eutectoid temperature 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 (until the martensite critical temperature is reached) 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 (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 scattered throughout. 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[edit]

Main article: 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; therefore wide pieces cannot be through-hardened. Alloy steels have a better hardenability, so they can through-harden 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[edit]

[19]

Steel TypeMaximum forging temperature (°F / °C)Burning temperature (°F / °C)
1.5% carbon1920 / 10492080 / 1138
1.1% carbon1980 / 10822140 / 1171
0.9% carbon2050 / 11212230 / 1221
0.5% carbon2280 / 12492460 / 1349
0.2% carbon2410 / 13212680 / 1471
3.0% nickel steel2280 / 12492500 / 1371
3.0% nickel–chromium steel2280 / 12492500 / 1371
5.0% nickel (case-hardening) steel2320 / 12712640 / 1449
Chromium–vanadium steel2280 / 12492460 / 1349
High-speed steel2370 / 12992520 / 1382
Stainless steel2340 / 12822520 / 1382
Austenitic chromium–nickel steel2370 / 12992590 / 1421
Silico-manganese spring steel2280 / 12492460 / 1349

See also[edit]

References[edit]

  1. ^ a b c d e "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
  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. ^ Smith, p. 386
  14. ^ Smith, pp. 386–387
  15. ^ Smith, pp. 373–377
  16. ^ Smith, pp. 389–390
  17. ^ Smith, pp. 387–388
  18. ^ Smith, p. 391
  19. ^ 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[edit]