Non Metal Inclusions in Steels...

Non Metal Inclusions in Steels Non metallic inclusions are naturally occurring and typically undesired products that are formed into various types depending on their favourable thermodynamic conditions during the production of steel and in all manufacturing and treatment processes involving liquid steels. They are constituted by glass-ceramic phases embedded in steel metal matrix.All steels contain non metallic inclusions to a greater or lesser extent. The type and appearance of these non metallic inclusions depends on factors such as grade of steel, steel making process, secondary metallurgy treatments and casting of steel etc. Because of this, it is of particular significance to determine how pure the steel is. The term steel cleanness is relative one, since even steel with only 1 ppm each of oxygen and sulphide will still contains billion to trillion non metallic inclusions per ton. Various factors which influence the non metallic inclusions in steel are shown in Fig 1. Fig 1 Factors influencing the non metallic inclusions in steel  Non metallic inclusions are chemical compounds of metals (e.g. iron, manganese, aluminum, silicon, and calcium) with non metals (e.g. oxygen, sulphur, carbon, hydrogen, and nitrogen). Non metallic inclusions form separate phases. The non metallic phases containing more than one compound (e.g. different oxides, oxide + sulphide) are called complex non metallic inclusions (spinels, silicates, oxy-sulphides, carbonitrides). Despite the presence of non metallic inclusions in steels in small percentage (0.01 % to 0.02 %), they have a significant effect on the properties of steels. They are the cause for dangerous and serious material defects such as brittleness and a wide variety of crack formations. However, some of these inclusions can also have a beneficial effect on steels properties by nucleating acicular ferrite during the austenite to ferrite phase transformation especially in low carbon steels.  The...