Metallurgical Processes and Defects in Steel Products...

Metallurgical Processes and Defects in Steel Products Defects in steel products are defined as deviations in appearance, shape, dimension, macro-structure / micro-structure, and/or chemical properties when compared with the specifications given in the technical standards or any other normative documents in force. Defects are detected either through visual inspection or with the help of instruments and equipments. There are four main metallurgical processes for the manufacture of finished steel products where the steel products can pick up defects. The defects picked up during these processes are (i) casting defects, (ii) rolling defects, (iii) forging defects, and (iv) welding defects. (Fig 1).These defects are described below. Fig 1 Metallurgical processes and steel product defects Casting defects Casting is a forming process which converts liquid steel into a solid product. In foundries liquid steel is cast into complex shapes by pouring of liquid steel into a mould in which it sets to the required shape. In steel plants, liquid steel is normally continuously cast in the form of slab (either thick or thin), bloom or billets. Casting defects are defined as those characteristics which create a deficiency or imperfection exceeding quality limits imposed by design and service requirements. Defects in foundry cast steel products There are in general three broad categories of defects in the foundry cast steel products. These are (i) the major or most severe defects which result in scraping or rejection of castings, (ii) intermediate defects which permit salvaging of castings through necessary repairs, and (iii) minor defects which can be easily repaired. Common defects which generally occur in castings are given below. Porosity – It consists of the spherical holes of varying size, with bright walls, usually evenly distributed and formed due to the gases in the liquid steel. The larger holes...

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...