Carbon steel

Carbon steel is steel in which the main interstitialalloying constituent is carbon in the range of 0.12–2.0%.The American Iron and Steel Institute (AISI) defines that:

Steel is considered to be carbon steel

when no minimum content is spec-ified or required for chromium,cobalt, molybdenum, nickel,niobium, titanium, tungsten,vanadium or zirconium, or anyother element to be added to obtaina desired alloying effect;when the specified minimumfor copper does not exceed 0.40percent;or when the maximum contentspecified for any of the followingelements does not exceed thepercentages noted: manganese1.65, silicon 0.60, copper 0.60.[1]

The term “carbon steel” may also be used in reference tosteel which is not stainless steel; in this use carbon steelmay include alloy steels.As the carbon percentage content rises, steel has the abil-ity to become harder and stronger through heat treating;however, it becomes less ductile. Regardless of the heattreatment, a higher carbon content reduces weldability. Incarbon steels, the higher carbon content lowers the melt-ing point.[2]

1 Type

See also: SAE steel grades

1.1 Mild and low-carbon steel

Mild steel, also known as plain-carbon steel, is nowthe most common form of steel because its price is rel-atively low while it provides material properties that areacceptable for many applications. Low-carbon steel con-tains approximately 0.05–0.15% carbon[1]making it mal-leable and ductile. Mild steel has a relatively low tensilestrength, but it is cheap and easy to form; surface hardnesscan be increased through carburizing.[3]

It is often used when large quantities of steel areneeded, for example as structural steel. The density ofmild steel is approximately 7.85 g/cm3 (7850 kg/m3 or0.284 lb/in3)[4] and the Young’s modulus is 210 GPa(30,000,000 psi).[5]

Low-carbon steels suffer from yield-point runout wherethe material has two yield points. The first yield point(or upper yield point) is higher than the second and theyield drops dramatically after the upper yield point. If alow-carbon steel is only stressed to some point betweenthe upper and lower yield point then the surface may de-velop Lüder bands.[6] Low-carbon steels contain less car-bon than other steels and are easier to cold-form, makingthem easier to handle.[7]

1.2 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 otherelements can have a significant effect on the quality ofthe resulting steel. Trace amounts of sulfur in particu-lar make the steel red-short, that is, brittle and crumblyat working temperatures. Low-alloy carbon steel, such asA36 grade, contains about 0.05% sulfur and melts around1,426–1,538 °C (2,599–2,800 °F).[8] Manganese is oftenadded to improve the hardenability of low-carbon steels.These additions turn the material into a low-alloy steelby some definitions, but AISI's definition of carbon steelallows up to 1.65% manganese by weight.

2 Types

See also: SAE steel grades

Carbon steel is broken down into four classes based oncarbon content:

2.1 Low-carbon steel

0.105-0.30% carbon content.[9]

2.2 Medium-carbon steel

Approximately 0.3–0.6% carbon content.[1] Bal-ances ductility and strength and has good wear resis-

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2 3 HEAT TREATMENT

tance; used for large parts, forging and automotivecomponents.[10][11]

2.3 High-carbon steel (ASTM A304)

Approximately 0.9–2.5% carbon content.[1] Very strong,used for springs and high-strength wires.[12]

2.4 Ultra-high-carbon steel

Approximately 2.5–3.0% carbon content.[1] Steels thatcan be tempered to great hardness. Used for specialpurposes like (non-industrial-purpose) knives, axles orpunches. Most steels with more than 2.5% carbon con-tent are made using powder metallurgy.

3 Heat treatment

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

Main article: Heat treatment

The purpose of heat treating carbon steel is to changethe mechanical properties of steel, usually ductility, hard-ness, yield strength, or impact resistance. Note that theelectrical and thermal conductivity are only slightly al-tered. As with most strengthening techniques for steel,Young’s modulus (elasticity) is unaffected. All treat-ments of steel trade ductility for increased strength andvice versa. Iron has a higher solubility for carbon inthe austenite phase; therefore all heat treatments, exceptspheroidizing and process annealing, start by heating thesteel to a temperature at which the austenitic phase canexist. The steel is then quenched (heat drawn out) ata high rate causing cementite to precipitate and finallythe remaining pure iron to solidify. The rate at whichthe steel is cooled through the eutectoid temperature af-fects the rate at which carbon diffuses out of austenite

and forms cementite. Generally speaking, cooling swiftlywill leave iron carbide finely dispersed and produce a finegrained pearlite (until the martensite critical temperatureis reached) and cooling slowly will give a coarser pearlite.Cooling a hypoeutectoid steel (less than 0.77 wt% C) re-sults in a lamellar-pearlitic structure of iron carbide layerswith α-ferrite (pure iron) between. If it is hypereutectoidsteel (more than 0.77 wt% C) then the structure is fullpearlite with small grains (larger than the pearlite lamella)of cementite scattered throughout. The relative amountsof constituents are found using the lever rule. The follow-ing is a list of the types of heat treatments possible:

• Spheroidizing: Spheroidite forms when carbonsteel is heated to approximately 700 °C for over 30hours. Spheroidite can form at lower temperaturesbut the time needed drastically increases, as this isa diffusion-controlled process. The result is a struc-ture of rods or spheres of cementite within primarystructure (ferrite or pearlite, depending on whichside of the eutectoid you are on). The purpose is tosoften higher carbon steels and allowmore formabil-ity. This is the softest and most ductile form of steel.The image to the right shows where spheroidizingusually occurs.[13]

• Full annealing: Carbon steel is heated to approx-imately 40 °C above Ac3 or Ac1 for 1 hour; thisensures all the ferrite transforms into austenite (al-though cementite might still exist if the carbon con-tent is greater than the eutectoid). The steel mustthen be cooled slowly, in the realm of 20°C (36°F)per hour. Usually it is just furnace cooled, wherethe furnace is turned off with the steel still inside.This results in a coarse pearlitic structure, whichmeans the “bands” of pearlite are thick.[14] Fullyannealed steel is soft and ductile, with no internalstresses, which is often necessary for cost-effectiveforming. Only spheroidized steel is softer and moreductile.[15]

• Process annealing: A process used to relieve stressin a cold-worked carbon steel with less than 0.3 wt%C. The steel is usually heated up to 550–650 °C for 1hour, but sometimes temperatures as high as 700 °C.The image rightward shows the area where processannealing occurs.

• Isothermal annealing: It is a process in which hy-poeutectoid steel is heated above the upper criticaltemperature and this temperature is maintained fora time and then the temperature is brought downbelow lower critical temperature and is again main-tained. Then finally it is cooled at room temperature.This method rids any temperature gradient.

• Normalizing: Carbon steel is heated to approxi-mately 55 °C above Ac3 or Acm for 1 hour; thisensures the steel completely transforms to austen-ite. The steel is then air-cooled, which is a cooling

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rate of approximately 38 °C (100 °F) per minute.This results in a fine pearlitic structure, and a more-uniform structure. Normalized steel has a higherstrength than annealed steel; it has a relatively highstrength and hardness.[16]

• Quenching: Carbon steel with at least 0.4 wt%C is heated to normalizing temperatures and thenrapidly cooled (quenched) in water, brine, or oil tothe critical temperature. The critical temperatureis dependent on the carbon content, but as a gen-eral rule is lower as the carbon content increases.This results in a martensitic structure; a form of steelthat possesses a super-saturated carbon content ina deformed body-centered cubic (BCC) crystallinestructure, properly termed body-centered tetragonal(BCT), with much internal stress. Thus quenchedsteel is extremely hard but brittle, usually too brittlefor practical purposes. These internal stresses causestress cracks on the surface. Quenched steel is ap-proximately three to four (with more carbon) foldharder than normalized steel.[17]

• Martempering (Marquenching): Martemperingis not actually a tempering procedure, hence theterm “marquenching”. It is a form of isothermalheat treatment applied after an initial quench of typ-ically in a molten salt bath at a temperature rightabove the “martensite start temperature”. At thistemperature, residual stresses within the materialare relieved and some bainite may be formed fromthe retained austenite which did not have time totransform into anything else. In industry, this is aprocess used to control the ductility and hardnessof a material. With longer marquenching, the duc-tility increases with a minimal loss in strength; thesteel is held in this solution until the inner and outertemperatures equalize. Then the steel is cooled ata moderate speed to keep the temperature gradientminimal. Not only does this process reduce internalstresses and stress cracks, but it also increases theimpact resistance.[18]

• Quench and tempering: This is the most com-mon heat treatment encountered, because the finalproperties can be precisely determined by the tem-perature and time of the tempering. Tempering in-volves reheating quenched steel to a temperature be-low the eutectoid temperature then cooling. Theelevated temperature allows very small amounts ofspheroidite to form, which restores ductility, but re-duces hardness. Actual temperatures and times arecarefully chosen for each composition.[19]

• Austempering: The austempering process is thesame as martempering, except the steel is held inthe molten salt bath through the bainite transforma-tion temperatures, and then moderately cooled. Theresulting bainite steel has a greater ductility, higher

impact resistance, and less distortion. The disadvan-tage of austempering is it can only be used on a fewsteels, and it requires a special salt bath.[20]

4 Case hardening

Main article: Case hardening

Case hardening processes harden only the exterior of thesteel part, creating a hard, wear resistant skin (the “case”)but preserving a tough and ductile interior. Carbon steelsare not very hardenable; therefore thick pieces cannot bethrough-hardened. Alloy steels have a better hardenabil-ity, so they can through-harden and do not require casehardening. This property of carbon steel can be benefi-cial, because it gives the surface good wear characteristicsbut leaves the core tough.

5 Forging temperature of steel[21]

6 See also

• Cold working

• Hot working

• Welding

• Forging

7 References

[1] “Classification of Carbon and Low-Alloy Steels”

[2] Knowles, Peter Reginald (1987), Design of structuralsteelwork (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 – Ironand 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] http://www.totalmateria.com/articles/Art62.htm

4 8 BIBLIOGRAPHY

[10] Nishimura, Naoya; Murase, Katsuhiko; Ito, Toshihiro;Watanabe, Takeru; Nowak, Roman. “Ultrasonic detec-tion of spall damage induced by low-velocity repeated im-pact”. Central European Journal of Engineering 2 (4):650–655. doi:10.2478/s13531-012-0013-5.

[11] Engineering fundamentals page on medium-carbon steel

[12] Engineering fundamentals page on high-carbon steel

[13] Smith, p. 388

[14] Alvarenga HD, Van de Putte T, Van Steenberge N, Si-etsma J, Terryn H (Apr 2009). “Influence of CarbideMorphology and Microstructure on the Kinetics of Su-perficial Decarburization of C-Mn Steels”. Metal MaterTrans A. doi:10.1007/s11661-014-2600-y.

[15] Smith, p. 386

[16] Smith, pp. 386–387

[17] Smith, pp. 373–377

[18] Smith, pp. 389–390

[19] Smith, pp. 387–388

[20] Smith, p. 391

[21] Brady, George S.; Clauser, Henry R.; Vaccari A., John(1997). Materials Handbook (14th ed.). New York, NY:McGraw-Hill. ISBN 0-07-007084-9.

8 Bibliography• Degarmo, E. Paul; Black, J T.; Kohser, Ronald A.(2003), Materials and Processes in Manufacturing(9th ed.), Wiley, ISBN 0-471-65653-4.

• Oberg, E. et al. (1996), Machinery’s Handbook(25th ed.), Industrial Press Inc, ISBN 0-8311-2599-3.

• Smith, William F.; Hashemi, Javad (2006), Founda-tions of Materials Science and Engineering (4th ed.),McGraw-Hill, ISBN 0-07-295358-6.

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