Non-metallic inclusions in special steels - oxides

Non-Metallic Inclusions in Special Steels

Introduction

In steelmaking, the quality of the steel produced depends on the composition and presence of a variety of inclusions, both metallic and non-metallic. Non-metallic inclusions, also known as slag, are particularly important to consider since they are produced during the heat treatment and melting process, and can significantly impact the mechanical and dimensional properties of the alloy. In special steels, such as those used for high-performance applications like turbine engines and aerospace components, the presence of non-metallic inclusions has a particularly large impact on the performance of the steel product. This paper provides an overview of non-metallic inclusions in special steels, how they are formed, their effects on the properties of the steel, and the methods used to minimize their presence.

Formation of Non-Metallic Inclusions

Non-metallic inclusions can be classified as primary or secondary. Primary inclusions refer to those that form during the heat treatment and melting process due to the chemical composition of the steel, its impurities, and the reaction between them. Secondary inclusions are those that are introduced into the melt after melting, usually through surface oxidation of the liquefied steel. The most common types of non-metallic inclusions found in special steels are oxides, silicates, sulfides, and carbides. Oxides are formed due to the reaction between oxygen and the impurities in the steel, silicates are formed from the reaction of silicate compounds in the steel, sulfides from sulfur, and carbides from the reaction between carbon compounds in the steel and other compounds.

Effects of Non-Metallic Inclusions

In the production of special steels, non-metallic inclusions can have an adverse effect on the mechanical and dimensional properties of the steel. Their presence can result in a variety of defects, such as internal porosity, cracking, and reduced heat treatment resistance. Additionally, non-metallic inclusions can cause a reduction in fatigue strength and ductility, as well as an increase in the susceptibility to stress corrosion.

Methods for Minimizing Non-Metallic Inclusions

While the presence of non-metallic inclusions is inevitable in steelmaking, there are a variety of methods used to minimize their effects. The most common methods involve the use of deoxidizing agents, such as aluminum and silicon, and fluxes, such as calcium and magnesium. Deoxidizing agents are used to reduce the amount of oxygen in the steel melt and limit the formation of oxides, while fluxes are used to bind oxygen and other impurities and reduce their potential for forming inclusions.

In addition to these methods, special steels are also treated with a variety of cleaning and refractory techniques to further minimize the effects of non-metallic inclusions. Updated degassing techniques, such as argon stirring, argon injection, and vacuum stripping, are all used to reduce the presence of non-metallic inclusions in special steels. Additionally, slag removal techniques, such as oxidizers, arc-stirring, or physical agitation, can be used to remove existing non-metallic inclusions from the steel melt.

Conclusion

Non-metallic inclusions are an unavoidable part of the special steel-making process and can significantly impact the mechanical and dimensional properties of the resulting product. However, through the use of deoxidizing agents, fluxes, and other cleaning and refractory techniques, it is possible to reduce the effects of non-metallic inclusions and improve the quality of special steels.

Non-metallic inclusions are particles that contain elements other than pure metal and are present in all forms of steel. In order to produce a good grade of steel, these inclusions must be kept to a minimum. Oxide inclusions in steel are the result of oxidation of impurities, such as sulfur and phosphorus, during the steelmaking process. These inclusions are generally composed of oxides of iron and silicon, which are present in the iron ore used to make steel.

When steel is oxidized, iron and silicon combine with oxygen to form iron oxides and silica, which in turn form spherical inclusions. These oxides and silica can also form complex oxides of magnesium, calcium and magnesium, as well as other elements present in the steel. These inclusions often form within the steel, but can also form on the surface.

The presence of oxides in steel can have a significant effect on the mechanical properties of the metal, and can be detrimental to the end product. Poor quality grades of steel with high levels of oxide inclusions can lead to poor tensile and fatigue properties, as well as cracking or other problems. It is therefore important to control the levels of oxide inclusions during and after the steelmaking process in order to produce a steel of good quality.

Oxide inclusions can be removed from steel by a number of different methods. These include purging the steel with a gas such as nitrogen or argon to remove the oxygen, or using a process called bottom-up steeping, where the steel is slowly heated and agitated in order to break down the oxide inclusions. This can be done in a vacuum to help ensure that the heat does not cause re-oxidation of the steel. It is also possible to reduce the number of oxide inclusions by controlling the addition of alloys or other substances during the steelmaking process.

Whatever the method used, controlling the levels of oxide inclusions in steel is essential in order to produce a good quality grade of steel.