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.
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.
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.
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.
Medium Carbon Steels
Medium carbon steels have a carbon content ranging from 0.
30% to 0.60%. As higher percentages of carbon are added, there are more carbon atoms in the crystal structure, so it becomes more brittle and less ductile.The greater strength of medium carbon steel makes it useful in applications such as gears, axles, rails, wheels, and other heavy machinery.
High Carbon Steels
High carbon steels have a carbon content that ranges from 0.60% to 1.
4%. These are among the hardest steels on the market, but the extra hardness comes at the cost of lost ductility. As with low and medium carbon steels, the properties can be changed somewhat with the addition of other alloy elements or with various types of heat treating.In general high carbon steels are used for the manufacture of cutting tools, bearings and in other applications where increased hardness is required.
Steel is an alloy, or a mixture of metals or a metal and another chemical element. Steel is an alloy manufactured with iron and alloy elements at very high temperatures.
Its versatility comes largely from the crystal structure of iron, which is allotropic, meaning it can exist in more than one crystalline form. This characteristic allows for the addition of carbon atoms to the structure at different temperatures.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.
- Low Carbon Steel (mild steel) – 0.10% to 0.30% carbon.
Highly ductile (malleable), can be rolled into thin sheets and easily welded. Used in car body panels and home appliances. Most frequently manufactured.
- Medium Carbon Steels – 0.30% to 0.
60% carbon. More brittle and less ductile. Used in gears, axles, rails, wheels, and other heavy machinery.
- High Carbon Steels – 0.60% to 1.4% carbon.
Very hard. Used in cutting tools, bearings, etc.