Inclusions in steel are any impurities present in the steel that are not incorporated into the molecular structure of the alloy itself. They can be chemical compounds or bits of foreign matter, usually nonmetallic in nature. Modern steelmaking processes make it possible for manufacturers to produce steel with a high degree of purity. For this reason, inclusions in steel are often measured in micrometers and make up a tiny portion of the steel as a whole, often less than 0.03%. They are classified into one of two types, endogenous and exogenous.
Steel manufacturers are concerned with inclusions in steel because even a very small number of such impurities can significantly affect the quality of the steel in many ways, including reducing its strength, flexibility, ability to hold a weld, and resistance to corrosion. By carefully manipulating the metals present in a given steel alloy and the composition of the other materials used in making the steel, the amount of indigenous inclusions can be minimized. Maintaining and monitoring equipment as well as the manufacturing process can help minimize foreign matter.
Endogenous, or indigenous, inclusions are compounds or impurities formed within the steel during the steel making process. They are the result of the reaction of substances dissolved in the molten steel. Exogenous inclusions are bits of foreign substances. They can be almost anything from bits of slag to pieces of equipment that may have flaked off into the steel during the manufacturing process.
Indigenous inclusions are unavoidable to some extent, as it is impossible to finely tune the chemistry and purity of the components to the point where inclusion content reaches 0%. Part of the reason for this is the naturally occurring impurities in the various components of a steel alloy. During the manufacturing process, these materials can react with each other, forming non-metallic compounds such as oxides, sulfides, and sulfides. These compounds, while containing atoms of metal such as iron and aluminum, are called non-metallic compounds because they exhibit properties not consistent with metals. For example, aluminum oxide is also known as the mineral corundum, which, in its natural state, is a valuable gemstone.
Exogenous inclusions in steel can come from any number of sources but are usually excess slag or bits of foreign matter that flake or break off from one of the pieces of equipment used in the casting process. Lining from the ladle, bits of the mold used to form the steel, and particles of the thermal refractory material used in furnace construction are the most common. Modern testing equipment is able to detect even minuscule numbers of inclusions in steel, allowing for increased quality control.