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...

Electrical steels

Electrical steels Electrical steel is kind of special steel which is tailored to exhibit certain specific magnetic properties such as small hysteresis area (small energy dissipation per cycle or low core loss) and high permeability. It is also called lamination steel, silicon (Si) steel, silicon electrical steel or transformer steel. The steel contains specific percentage of silicon in it which is responsible for its unique property.  In mild steel there is much loss in electrical energy due to hysteresis and eddy current and hence use of mild steel is uneconomical when it is used in the electrical devices. The hysteresis loss is shown in Fig.1.  The hysteresis loss is proportional to the area of the respective loops shown in the figure.                             Fig 1 Comparison of hysteresis loss in electrical steel (left) and mild steel (right) Electrical steel is an iron alloy of iron which may have from zero to 6.5 % silicon but usually has silicon content up to 3.2 % (higher concentrations usually provoke brittleness during cold rolling). Manganese and aluminum can be added up to 0.5 %. Silicon significantly increases the electrical resistivity of the steel, which decreases the induced eddy currents and narrows the hysteresis loop of the material, thus lowering the core loss. However due to the silicon the grain structure hardens and embrittles the steel, which adversely affects the workability of the steel, especially during rolling. When alloying, the concentration levels of carbon, sulphur, oxygen and nitrogen should be kept low since these elements indicate the presence of carbides, sulphides, oxides and nitrides in the steel. These compounds, even in particles sizes as small as one micrometer in diameter, increase hysteresis losses and decrease magnetic permeability. The...