Knowledge

Carbon

Atomic number: 6

Density (20°C): 2.3 g/cm³

Atomic weight: 12.01

Melting point: > 3550°C (6422°F)

Boiling point: 4827°C (8721°F)

Overview

Carbon is the most important alloying element in steel and cast iron, mainly determining the wide range of mechanical properties that can be achieved in forgings and castings. When the carbon content of iron-based alloys exceeds 2.0% (except for iron alloys), they are classified as cast iron. According to metallurgical classification, common carbon steel is divided into hypo-eutectoid steel grade and hyper-eutectoid steel grade based on whether the carbon content is below or above 0.80%. The addition of other alloying elements (such as manganese, silicon, nickel, etc.) can change the carbon content at the eutectoid point, and may even eliminate this point. The limitations of carbon have led to the development of low-carbon microalloyed steel. However, this chapter only discusses steel and cast iron with carbon as the main alloying element, ignoring the presence of other elements that remain after deoxidation or are necessary to control sulfur.

Carbon is almost present in all steel from the very beginning of the steelmaking process. The raw materials for steelmaking (molten iron, pig iron, scrap steel, iron alloys, etc.) usually contain a higher carbon content than that required in the final product. During the steelmaking process, carbon is removed through oxidation reactions, and steel can be tapped when the target carbon content is reached (slightly lower than the target value if high-carbon iron alloys are to be added in the ladle). In the BOF process, it is common to "blow" the carbon content to below 0.10% and then increase it in the ladle. In the electric arc furnace production process, carbon is added by spraying to form a foaming slag, while in the ladle refining station, carbon content is precisely controlled using carbon core wires.

When high-carbon iron alloys are used, these alloys themselves become the "addition medium" for carbon. If the carbon content is too low and cannot be supplemented by iron alloys, carbon can be added using the following materials: graphite, coke, calcined petroleum coke, anthracite, and in rare cases, high-carbon scrap steel such as cast iron or cold pig iron. Special attention should be paid: adding cast iron for carbon may lead to excessive phosphorus, and low-sulfur and low-volatile coke should be selected for carbon addition.

In the production of cast iron in electric arc furnaces, a carbon addition step is often required because the raw materials are mostly low-carbon scrap steel. Although high-carbon scrap steel, high-carbon iron alloys, or even pig iron can be used as carbon sources, according to process requirements or cost considerations, special carbon additives - graphite or coke - are usually used. Natural graphite (mainly produced in Mexico) is widely used in North America, with a carbon content of 70-85% and a high impurity content, limiting its application; synthetic graphite (mostly from electric arc furnace electrode waste) has higher purity, and its crystal structure can affect the microstructure of cast iron. Metallurgical coke is low in cost but has a high ash content of up to 9%, limiting its application; calcined petroleum coke has a purity of up to 99%, but its sulfur content may exceed 1%.

Practical Carbon-Adding Operations

When carbon is added in the form of high-carbon ferroalloys, it can be done in the furnace, before tapping, or in the ladle. The conventional practice is to slightly over-decarbonize in the furnace and then supplement with ferroalloys to reach the target carbon content range. The carbon-oxygen-temperature balance at tapping is a key control factor, but the specific operation usually relies on on-site experience. Due to the high cost of in-furnace treatment, the common practice is to "open" tapping (i.e., allowing the molten steel to come into contact with air), and then complete the necessary deoxidation and composition adjustment in the ladle. Special attention should be paid: using cast iron to adjust the carbon content may lead to excessive sulfur and phosphorus. Unless the background values of these elements in the molten steel are extremely low or the final composition allows for their increase, the raw iron water should have as low phosphorus and sulfur content as possible.

The density of carbon additives (graphite, coke, anthracite) is relatively low, and they tend to float on the surface of the slag layer, causing ineffective burning loss. Therefore, they should be added at the beginning of tapping or pre-placed at the bottom of the empty ladle. Sufficient turbulence should be maintained during tapping to accelerate the dissolution of carbon. In casting production, carbon additives are also added in the ladle, following the above operation norms.

 

Rolling/Forging

 

The carbon content in steel affects the deformation processing in various ways. Generally speaking, as the carbon content increases, the processing difficulty also increases. The influence of carbon is first manifested in the soaking furnace or reheating furnace. High-carbon steel is more sensitive to thermal shock and must be heated slowly to avoid cracking. Step heating (i.e., allowing the steel billet to stay at multiple temperature platforms before reaching the rolling or forging temperature to achieve temperature uniformity) may be necessary, especially for large cross-section steels. Steel with a carbon content exceeding 0.30% is also more prone to "overburning" (deep surface oxidation), which can lead to cracks or substandard surface conditions of the final product, and the overburned billets are almost always scrapped. Therefore, high-carbon steel should be heated slowly and uniformly to avoid local overheating caused by direct flame impact.

The rolling force in both hot rolling and cold rolling increases with the increase in carbon content. In hot rolling, this effect is more significant as the final rolling temperature is approached. For example, an additional 0.15% carbon in ordinary carbon steel can increase energy consumption by up to 20% at 870°C (1600°F). The energy required for cold working is highly dependent on the carbon content, which is related to the proportion of pearlite in its microstructure. Under the same conditions, the need for intermediate annealing increases with the increase in carbon content.

It is worth noting that carbon has a strong segregation tendency in thick sections (such as steel billets) and will accumulate in the last solidified metal (along with manganese, phosphorus, and sulfur). This may lead to uneven carbon distribution in the final product, such as the "banding" often seen in hot-rolled plates (caused by phosphorus segregation: high phosphorus regions repel carbon). However, this is not necessarily harmful. For steels containing microalloying elements, the ratio of the atomic percentage of microalloying elements (MAE) to the carbon content determines the amount of MAE precipitates formed at low temperatures. At this point, cold-rolled and annealed thin sheet steels require a carbon content below 0.01%.

 

Heat treatment

Carbon increases the strength of hot-rolled steel but reduces its notch toughness, ductility and weldability. For details on the application of carbon in continuous casting and hot-rolled steel, refer to the relevant content on vanadium, niobium and titanium.

The maximum solubility of carbon in ferrite is approximately 0.025% (at 723°C/1333°F). The carbon solubility of ferrite at room temperature is less than 0.008%. The iron-carbon equilibrium diagram (Figure 1) shows three reactions and indicates that cementite (Fe3C) forms at a carbon content of 6.67%. At 1492°C (2718°F), δ-ferrite with a carbon content exceeding 0.10% undergoes a peritectic reaction with liquid metal to form austenite. Iron with a carbon content exceeding 2.0% undergoes an eutectic reaction at 1130°C (2066°F), forming ledeburite - a structure of cementite rods distributed in austenite. At 723°C (1333°F), austenite decomposes through a peritectoid reaction to form lamellar composite pearlite.

Carbon lowers the γ→α allotropic transformation temperature from 910°C (1670°F) for pure iron to the eutectoid temperature (0.80% carbon). Below the eutectoid temperature (723°C/1333°F), carbon has a significant effect on the kinetics (rate) of pearlite formation and reacts with iron to form non-equilibrium phases bainite and martensite. Pearlite forms in the high-temperature range from approximately 550°C (1020°F) to the eutectoid temperature, and its structure gradually refines as the transformation temperature decreases. Between approximately 220°C (425°F) and the lower limit of the pearlite formation range, austenite transforms into bainite. Bainite mainly has two types:

Upper bainite: Forms at higher temperatures, with a needle-like structure, and cementite particles are oriented along the boundaries of ferrite regions.

Lower bainite: Also needle-like but finer, with carbide particles distributed laterally within ferrite regions. This orientation gives it higher toughness. The temperature boundary between upper and lower bainite mainly depends on the composition (especially the carbon content). The growth rates of both types of bainite are mainly determined by the diffusion of carbon in iron.

The transformation of austenite to martensite by a diffusionless shear mechanism below about 220°C (425°F) is the most important phase transformation in commercial heat treatment. As the composition approaches eutectoid, the martensite start temperature (Ms) decreases. If a part requires a hard, wear-resistant surface and a tougher core, carburizing can be used: carbon is diffused into the surface of low-carbon steel (usually not more than a few thousandths of an inch deep). The carburizing temperature is about 925°C (1700°F), and the steel composition must be such that fine grains are maintained at this temperature. Conventional heat treatment is required after carburizing. Application

Carbon steel currently accounts for the largest tonnage in all steel sales, and its wide range of applications is clearly too extensive to list one by one. Carbon steel is used as castings and forgings, pipes and tubes, sheets and plates, wire rods, bars, rails, and structural sections. Of course, carbon steel is the cheapest iron-based alloy, and designers will prefer it unless special performance requirements necessitate the use of more expensive alloy steel grades.

Carbon steel can be classified in various ways, and classification by composition is the most intuitive method, usually following standards issued by institutions such as the Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI). The American Society for Testing and Materials (ASTM) and the American Society of Mechanical Engineers (ASME) mainly stipulate the performance indicators of steel, with composition only as supplementary information. Many standards identify the same steel grade through their respective specifications, and users can add specific requirements on the basis of general standards according to their needs. Some large users (such as automotive and construction machinery manufacturers) tend to formulate their own standards that are stricter than national standards.

Steel grades with different carbon contents have various applications: thin sheet steel usually has the lowest carbon content (less than 0.10%), ultra-low carbon steel (with carbon content below 0.02%) includes high formability thin sheet steel; low carbon steel (with carbon content ranging from 0.05% to 0.20%) covers hot-rolled strip steel, thick plates and pipes; medium carbon steel (with carbon content ranging from 0.25% to 0.55%) is mainly used for forging; high carbon steel (with carbon content over 0.6%) includes steel for rails, etc. In specific industrial applications, for instance, when choosing materials for the friction pairs of oil extraction pumps, carbon steel (such as No. 45 steel) is often enhanced in wear resistance through chromium plating or laser treatment, while the control technology of oxide scale on hot-rolled low carbon steel (such as SPHC, 510L) directly affects the surface quality.

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