Inclusions in steel are impurities that are not part of the molecular structure of the alloy, classified as endogenous or exogenous. They can reduce the quality of steel, but modern steelmaking processes minimize their presence. Endogenous inclusions are formed during the steelmaking process, while exogenous inclusions come from foreign substances. Inclusions are unavoidable to some extent due to naturally occurring impurities. Modern test equipment can detect even small amounts of inclusions, allowing for better quality control.
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 fragments of foreign bodies, usually of a non-metallic nature. Modern steelmaking processes allow manufacturers to produce steel with a high degree of purity. For this reason, inclusions in steel are often measured in micrometres and make up a very small portion of the steel as a whole, often less than 0.03%. They are classified into one of two types, endogenous and exogenous.
Steelmakers are concerned about 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 corrosion resistance . By carefully manipulating the metals present in a given steel alloy and the composition of the other materials used in steelmaking, the amount of indigenous inclusions can be minimized. Maintaining and monitoring your equipment, as well as your manufacturing process, can help keep foreign bodies to a minimum.
Endogenous, or indigenous, inclusions are compounds or impurities formed within the steel during the steelmaking process. They are the result of the reaction of substances dissolved in molten steel. Exogenous inclusions are fragments of foreign substances. They can be almost anything from pieces of slag to pieces of equipment that may have flaked off the steel during the manufacturing process.
Indigenous inclusions are unavoidable to some extent, as it is impossible to fine-tune component chemistry and purity to the point where the inclusion content reaches 0%. Part of the reason for this is 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 sulphides. These compounds, while containing metal atoms such as iron and aluminum, are called non-metallic compounds because they have properties that are inconsistent with metals. For example, aluminum oxide is also known as the mineral corundum, which, in its natural state, is a precious gemstone.
Exogenous inclusions in the steel can come from any number of sources, but are usually excess slag or foreign body fragments that flake or detach from one of the tools used in the casting process. The ladle lining, the mold pieces used to form the steel, and the particles of the thermal refractory material used in furnace construction are the most common. Modern test equipment can detect even a minute number of inclusions in the steel, allowing for greater quality control.
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