Alloy Structure of Steel

Steel is an ally of iron and carbon.

Steel is one of the most popular types of alloys in the world, with applications in nearly every industry. True alloyed steel is made by alloying iron with small amounts of carbon, but there are many different types of steel with different alloy characteristics depending on their purpose. A metal alloy has been changed on an atomic level to create a substance with the properties of two different materials.

Definition

In order to create an alloy, a parent or base metal is needed, along with an alloying agent. The parent metal for steel is iron--which in its pure form is very useful--but as steel it is lighter, stronger, and more resistant to corroding effects such as oxidation. The alloying agent used to make steel can vary, depending on the specific type of steel being made. At least a small amount of carbon is always required, but other agents are also used.

Materials

Carbon and iron may make steel, but even with one alloying agent many different types of steel can be made. Low carbon steels are only about 0.3 percent carbon based on weight, while medium carbon steels are 0.6 to 1.65 percent carbon, and high carbon steels tend to have similar amounts of carbon with additional manganese elements. The more carbon a steel alloy has, the stronger it can be, but the more brittle it becomes, and more subject to cracking or breaking, especially over time. Metal alloying agents include nickel, chromium, tungsten, and many others.

Atomic Structure

When iron is turned into an alloy, its atomic structure is combined with the atomic structure of another metal or element. This does not mean the atoms themselves change: the iron atoms always remain iron atoms. However, the structure of the metal that the iron naturally bonds into is changed, and bits and pieces are replaced with the atoms of carbon and other metals. Good alloys will have a smooth integration where steel atoms are regularly replaced throughout the structure.

Structure Types

There are two main types of alloy atom structures. The first is referred to as a substitution alloy, where the atoms of the agent actually take the place of the base metal atoms. This happens when both atoms are of similar size and in this case, the overall mass of the metal is increased, because some atoms replace others. The second type of alloy is an interstitial alloy, when the agent atoms are small enough to be compressed into the spaces between the atoms of the base metal. This is the type of alloy that steel is, since carbon atoms are small enough to fit in between the spaces of iron's atomic structure.

Considerations

While steel can harden more easily than iron, it cannot be used as easily for applications involving reheating and reforming. The hardening of the steel makes it crack more easily when heated and cooled again. Carbon-based alloys are typically heated and then cooled with care to avoid this cracking when steel is first made. This is why high carbon steels can be difficult to weld.

References

• Explain That Stuff: Alloys
• KeytoMetals: Cast steel: Structure of plain steel
• Plym: Alloy and Stainless Steels