Effect of Adding Carbon to Steel

Instructor: Patricia Jankowski

Patricia is an experienced registered nurse who has worked in various acute care areas as well as in legal nurse consulting. She also has a BSChE.

Steel is one of the most important construction materials in the world. This lesson is about the composition of steel in general, and, specifically, the effects of adding carbon to steel.

What is Steel?

Ever taken a field trip to a steel-making plant during the school year? If so, you might have been thrilled and terrified by the huge furnaces and hot ingots that glow far below the high catwalks enclosed in glass.

Steel is an alloy, which is a mixture of metals or metal and another chemical element. Steel is an alloy manufactured with iron and alloy elements at very high temperatures, in their molten state.

The most common element added to steel is carbon. Although many different elements such as chromium and manganese may be added to iron to make steel, carbon is the most abundant and least expensive. Let's take a closer look at steel and see how carbon is involved on the molecular level, as well as steel's properties once carbon is added.

Steel mill
steel mill

Allotropic Iron

The iron used in making steel usually comes from iron ore or from recycled steel. Iron is the component of steel that makes it so versatile and useful in terms of its many engineering properties because it is an allotropic element. This simply means that it can exist in more than one crystalline form.

At lower temperatures (723 degrees C and below), the iron crystal has a body-centered cubic structure (BCC). In this structure, the crystal particles are located at the corners and in the center of the unit cell.

Body-centered cubic structures have the crystal particles in the corners and centers of the cell
Body-centered iron crystal

At higher temperatures (910 degrees C and above), the iron crystal structure changes to face-centered cubic (FCC). In this type of cell, the crystal particles are located at each corner of the cell, as well as at the center of each cube face. This leaves the center of the cell open.

Face-centered cubic structures have the crystal particles in the corners and sides of the cell, with the center open

This more open FCC structure can accommodate a larger number of carbon atoms infiltrating the spaces between the iron atoms in the crystal, so that there is a higher percentage of carbon in the solution. This higher percentage of carbon increases the hardness of the steel.

Types and Properties of Carbon Steels

Steel has many properties that are important for a variety of uses and applications. The percentage of carbon plays a large role in determining these properties. Steels that are predominantly alloyed with carbon make up 90% of all steel. Properties include:

  • Tensile strength - the amount of load a material can withstand before breaking.
  • Ductility - the ability of a solid material to deform under stress. This property is characterized by the ability of the material to be stretched into a wire.
  • Thermal conductivity - the ability of a material to conduct heat.
  • Resistance to corrosion - the ability of a material to resist against reactions with caustic elements that can corrode or degrade it.

The final properties of the finished steel product depend upon the chemical composition of the material in the furnace, the design of the processing, and heat treatment.

Steel Classification Systems

The carbon steels are classified as low carbon (mild) steels, medium carbon steels, and high carbon steels.

The American Iron and Steel Institute (AISI) and the Society of Automotive Engineers (SAE) both have systems that are used to classify grades of steel. The systems, while similar to one another, can become complex, and are based on a four-digit designation for each grade of steel.

The last two or three numbers in the classification give the permissible range of carbon in the steel, in hundredths of a percent. For example, if the AISI-SAE number is 1010, the permissible range of carbon in the steel is 0.08 to 0.13 percent.

Low Carbon Steels

Low carbon, or 'mild', steels contain from 0.10% to 0.30% carbon. Because of the lower amount of carbon, this type of steel is more ductile and malleable because there are fewer carbon atoms embedded in the iron/carbon crystal structure.

It can be rolled into thin sheets and can be easily welded. This gives it a wide range of applications, from car body panels to home appliances. These are the most frequently manufactured of all types of steel.

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