Steel is an alloy composed primarily of iron and a small percentage of carbon. The addition of carbon transforms soft, pure iron into a material with significantly enhanced strength and hardness. This occurs because the small carbon atoms fit into the interstitial spaces within the iron’s crystal lattice structure. Depending on the final amount and distribution, carbon dictates the steel’s mechanical properties, such as its ability to resist deformation and wear. The controlled introduction of this element is a precise, multi-stage process used to tailor the alloy for specific applications.
Bulk Carbon Addition During Primary Steelmaking
The initial, large-scale introduction of carbon occurs during the primary steelmaking phase, typically within a Basic Oxygen Furnace (BOF) or an Electric Arc Furnace (EAF). This step sets the overall composition and bulk properties of the molten metal batch. In the BOF process, the starting material is carbon-saturated molten iron, or “pig iron,” which may contain carbon levels exceeding 4%. Oxygen is blown onto the metal surface to oxidize and remove the excess carbon, though direct additions are sometimes required to halt the process at the target level.
Electric Arc Furnaces (EAFs), which commonly use steel scrap and direct-reduced iron (DRI), often require a net addition of carbon. Solid carbon sources, such as graphite, coal, or petroleum coke, are added directly into the furnace. These materials dissolve into the molten metal, raising the carbon concentration to the required bulk composition. Carbon is also intentionally injected into the EAF bath to react with iron oxide in the slag, creating carbon monoxide gas that helps foam the slag layer. This foamy slag insulates the arc, protects the furnace walls, and improves energy efficiency.
The addition in this primary stage is often deliberately aimed at a slightly lower carbon content than the final specification, preparing the melt for subsequent, more accurate refinement stages. Maintaining this lower initial level prevents over-oxidation of other alloying elements, which can occur at higher temperatures. The goal is to achieve a consistent, overall carbon baseline for the entire heat of steel.
Fine-Tuning Carbon Content in Ladle Metallurgy
Once the molten steel leaves the primary furnace, it is transferred to a refractory-lined ladle for secondary refining, known as ladle metallurgy. This stage fine-tunes the chemical composition and temperature to meet tight industrial specifications. Moving the steel away from the intense, oxidizing environment allows precise additions with less material loss. The environment is often controlled using inert gas stirring, such as argon, which ensures homogeneity.
Carbon is added here to make final upward adjustments to the concentration, a process called re-carburization. This is achieved by injecting powdered carbon materials, such as graphite, deep into the liquid steel bath using a refractory-lined lance. Alternatively, a cored wire—a thin steel sheath filled with carbon powder—is fed directly into the molten metal at a controlled speed. These controlled injection methods ensure a high yield, meaning most of the material dissolves into the steel rather than being lost.
This precision control allows steelmakers to meet specifications with tolerances often measured in hundredths of a percent. Accuracy is obtained by continuously monitoring the steel’s chemistry and temperature during the treatment. Ladle metallurgy transforms the bulk composition set in the primary furnace into a finished liquid steel product ready for continuous casting.
Surface Modification Techniques
A method distinct from alloying liquid steel is adding carbon to the surface of solid steel parts, known as carburizing or case hardening. This technique creates a component with a hard, wear-resistant exterior, or “case,” while retaining a softer, tougher core. It is applied to low-carbon steel parts that cannot be fully hardened through traditional heat treatment due to their low overall carbon content.
Carburizing involves heating the solid steel part to a high temperature, typically between 850 °C and 950 °C, in a carbon-rich atmosphere. At this elevated temperature, the iron’s crystal structure allows the carbon atoms to diffuse inward from the surface. The three primary methods are:
- Pack carburizing, which uses a solid carbonaceous compound like charcoal.
- Gas carburizing, which employs carbon-bearing gases such as methane or propane.
- Liquid carburizing, which uses a molten salt bath.
The depth of the hardened case is precisely controlled by regulating the temperature and the duration of the heat treatment. Following the carbon diffusion step, the part is rapidly cooled, or quenched, to lock the newly diffused carbon atoms into the crystal structure. This final step transforms the high-carbon surface layer into a hard phase, such as martensite, while the low-carbon core remains relatively ductile.