Steel is an alloy of iron and carbon, and carbon steel is a steel variant commonly used in industrial manufacturing. The carbon content of carbon steel varies between 0.05% and 2.1%, which significantly affects its mechanical properties. This article provides an overview of carbon steel and the impact of the carbon content on its strength and ductility.
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Carbon Steel and its Types
Carbon steel is an iron-carbon alloy. Although pure iron is not intrinsically strong, adding carbon imparts great strength to this material. Crude iron, which is used in steel production, has a high carbon content.
Crude iron is processed to reduce carbon and obtain several types of carbon steels with different mechanical properties. The variants of carbon steel differ in the concentration of carbon up to 2.1%, and an increase in carbon content above this level makes it a cast iron.
Manipulating the carbon content results in different types of carbon steels, as described below:
Low-Carbon Steel
This type of carbon steel is used in manufacturing automobile body components, pipes, structural shapes (I-beams, channels, and angle iron), food cans, and construction and bridge components.
The carbon content in the low-carbon steel is less than 0.25 wt.%. Instead of heat, this carbon steel type is hardened (to form martensite) via cold work. Because of its low strength, this type of carbon steel is relatively soft. Low-carbon steel is an excellent choice for machining owing to its high ductility and cost-effectiveness.
Low-carbon steel containing other elements, such as nickel, copper, vanadium, and molybdenum, is called high-strength, low-alloy steel (HSLA). HSLA steels have higher strengths than conventional low-carbon steels. Their ductility renders them easily formable and machinable. HSLA are more resistant to corrosion than plain low-carbon steel.
Medium-Carbon Steel
The carbon content in this type of carbon steel is between 0.25 and 0.60 wt.%. Additionally, it contains a manganese content between 0.60 to 1.65 wt.%.
Improving the mechanical properties of medium-carbon steel involves heat treatment via austenitizing (heating steel to a temperature at which the crystal structure changes from ferrite to austenite), followed by quenching and tempering, resulting in a martensitic microstructure.
Additional alloying of medium-carbon steel elements, such as chromium, molybdenum, and nickel, can improve the ability of this steel type to withstand high temperatures and thus harden. Although hardened medium-carbon steels have greater strength than low-carbon steels, this is at the expense of ductility and toughness.
High-Carbon Steel
The carbon and manganese contents in this type of carbon steel are in the ranges of 0.60 - 1.25 wt. % and 0.30 - 0.90 wt.%, respectively. Compared to other carbon steel types, high-carbon steel has the highest toughness and hardness but the lowest ductility. Because high-carbon steels are always tempered and hardened, they are highly wear-resistant.
A few high-carbon steel types include die and tool steels containing an additional alloying element, such as vanadium chromium, molybdenum, or tungsten, that help form wear-resistant steel.
Recent Studies
An article published in the Journal of Materials Research and Technology reported the fabrication of a novel aluminum-containing medium-manganese low-density steel. The evolution of the microstructure at multiple scales, such as the austenite characteristics and partitioning behavior of manganese, was analyzed.
The results revealed that the addition of aluminum widened the annealing processing window, which enhanced the treatment of steel and improved the effect of heating. When annealed at 780 °C and then cooled for 3 minutes, approximately half of it remained austenite, but its appearance and manganese content varied. At a longer annealing time of 20 minutes, a larger austenite piece with lower manganese content was created. However, shorter annealing times, such as 4 minutes, resulted in smaller manganese-rich pieces.
The tensile properties of the steel depend on its mechanical stability and volume fraction. The sample annealed for 4 minutes exhibited a high tensile strength of 1192 MPa and elongation of up to 33 % owing to its slow transformation-induced plasticity (TRIP) effect.
Similarly, another article published in the Journal of Materials Research and Technology investigated the effect of high pressure (2 and 3 GPa) on martensitic transformation characteristics and mechanical properties of a low-alloy medium-carbon steel (40Cr).
Electron back-scattering diffraction (EBSD), Mössbaue, and transmission electron microscope (TEM) were employed to observe the changes in the carbon steel´s microstructure. The results revealed that the high-pressure treatment resulted in a mixture of lath and plate martensite, and the plate martensite content steadily increased with pressure.
A study published in Metals reported the effect of carbon content in high-strength steel base metals on the microstructure and toughness of a specific zone (coarse-grained heat-affected zone (CGHAZ)) in simulated welding at different heat inputs.
Scanning electron microscopy (SEM) and EBSD were used to observe structural changes. The Charpy impact test revealed a zone shift from the lath bainite structure to granular bainite at 0.04 wt. % carbon and a heat input above approximately 20 kJ/cm. By contrast, at 0.12 wt. % carbon, the structure shifted from lath martensite to lath bainite and subsequently to granular bainite with increased heat input.
Owing to the parallel arrangement of structures, such as lath bundles, and the penetration of austenite into the material, the high-carbon sample with more high-angle grain boundaries (HAGBs) showed lower toughness at 20 kJ/cm than its low-carbon counterpart.
Conclusion
Overall, the wide range of carbon compositions in carbon steel highlights the material's profound influence on mechanical qualities, which shapes its application in various sectors. Low-carbon steel is a commonly used variant of carbon steel in the construction and automobile industries, owing to its affordability and malleability.
With the addition of manganese and careful heat treatment, medium-carbon steel achieves a desirable combination of strength and ductility. However, ductility is frequently sacrificed for robustness when alloying with other elements to improve resilience to harsh environments.
High-carbon steel has unmatched toughness and hardness at the expense of flexibility; however, it also offers exceptional wear resistance. Fundamentally, the relationship between the characteristics of steel and its carbon content remains critical.
These discoveries highlight the constant progress in material science by revealing not only the adaptability of carbon steel but also the complexities involved in modifying its properties to satisfy the requirements of many applications.
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References and Further Reading
Yu, C et al. (2023). High strength-high ductility combination in a low-density medium Mn steel prepared by a highly efficient route. Journal of Materials Research and Technology. https://doi.org/10.1016/j.jmrt.2023.12.020
Cui, Q et al. (2023). High-pressure quenching effect on martensitic transformation characteristics and mechanical properties of low-alloy medium-carbon steel. Journal of Materials Research and Technology, 23, 765-777. https://doi.org/10.1016/j.jmrt.2023.01.051
Wang, X., Xie, Z., Su, W., Shang, C. (2023). Role of Carbon Content on Microstructure Evolution and Impact Toughness in Coarse-Grained Heat-Affected Zone of High-Strength Steel. Metals, 13(1), 106. https://doi.org/10.3390/met13010106
Carbon Steel: Properties, Production, Examples and Applications . Accessed on 11 December 2023.
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