Ferroalloys are a group of materials which are alloys of iron that contain a high percentage of one or more non ferrous metals as alloying elements. These alloys are used for the addition of these other elements into liquid metal. They are normally used as addition agents. More than 85 % of ferroalloys produced are used primarily in the manufacture of steel.
Ferroalloys are usually classified into two main categories namely (i) bulk ferroalloys and (ii) noble or special ferroalloys. Noble ferroalloys are of high value and consumed in low proportions. These ferroalloys are one of the vital inputs required for the production of special types of steels and are used as additive inputs especially in the production of alloy and special steels.
Noble ferroalloys are ferro nickel (Fe-Ni), ferro molybdenum (Fe-Mo), ferro vanadium (Fe-V), ferro tungsten (Fe-W), ferro niobium (Fe-Nb), ferro titanium (Fe-Ti), ferro aluminum (Fe-Al), ferro boron (Fe-B). There are some noble ferroalloys which are having more than one non ferrous metal as alloying elements. Examples are ferro silico magnesium (Fe-Si-Mg), ferro silico zirconium (Fe-Si-Zr), ferro nickel magnesium (Fe-Ni-Mg) etc.
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Fe-Ni is used for alloying in the production of stainless and construction steels. The production of Fe-Ni from primary raw material is carried out exclusively by the rotary kiln – electric furnace process. The first step of the process is a drying operation. Drying normally takes place in a directly fired rotary drier where the moisture content can be reduced.
Fe-Ni can also be produced from secondary raw materials such as residues that contain nickel. These residues, mostly spent catalysts from the grease production, are burnt in a rotary kiln in order to concentrate the Ni content as Ni oxide in the flue-dust. The off-gas is cleaned in a bag filter, where the collected dust is used as the raw material for the smelting process. The production of Fe-Ni then takes place in a submerged electric arc furnace (SAF). The molten ferroalloy is tapped and granulated in water.
Fe-Mo is used in the production of different alloy steels. Commercial Fe-Mo contains between 60 % and 70 %.Fe- Mo is a molybdenum (Mo) based ferroalloy, produced by alumino/silico thermic reduction from technical grade molybdenum trioxide ( MoO3) or in induction / EAF furnaces from Mo containing scraps.
The metallothermic reduction is the most common process used for the production of Fe-Mo. Due to the higher Gibbs free energy of aluminum compared with silicon, Fe-Si is the preferred reducing agent in order to ensure safe process conditions. However, to obtain the required heat balance for the reaction, it is necessary to use a small amount of aluminum. Since the reaction is done by metals instead of carbon, the exothermic reaction does not generate CO or CO2. The smelting process produces a ferroalloy batch that weighs between 2.5 tons to 4 tons. After the slag is separated, the liquid ferroalloy batch is cooled either by natural air cooling or by quenching in water. The cooled ferroalloy batch is then broken and screened to produce the product of the desired size.
Fe-V is vanadium based ferroalloy used for the modification of the microstructure of steel and for increasing the tensile strength, hardness and high temperature strength of steel. It is used in the high speed steels. Vanadium content in Fe-V ranges from 35 % to 80 %.
Fe-V is produced by a carbothermic or a metallothermic (alumino thermic) reduction of vanadium oxides, assisted by the presence of iron. Since carbon is used in a carbothermic reduction, the carbon content of the ferroalloy is normally high. Hence the process cannot be used if there is a requirements for a low carbon content. low carbon Fe-V is usually produced by an alumino thermic reduction.The basic raw materials for the production of Fe-V are vanadium oxides (V2O3, V2O4, and V2O5) with lime, aluminum and iron or steel scrap used as additives.
Fine grained materials of Fe-V are pulverized to powdery materials and are used in the production ofwelding electrodes.
Fe-W is a tungsten based ferroalloy which is used for the production of special steels. Tungsten as an alloying element forms stable carbides and therefore increases the hot strength and wear resistance of steels. Such steels (high speed steels) are needed to produce high speed cutting tools that can be used up to temperatures of around 600 deg C. Tungsten also improves a number of other properties of the steel, such as the hardness, yield strength and the ultimate tensile strength.
Fe-W is produced from different raw materials that contain tungsten oxides, e.g. wolframite, scheelite and hübnerite. The reduction of these minerals is done either by carbothermic or metallothermic reduction as well as by a combination of both.
Fe-Nb as an alloying agent improves the corrosion resistance and weldability of steel and prevents, especially, the inter crystalline corrosion of stainless chrome nickel steel. Fe-Nb contains niobium in the range of 60 % to 70 %.
Fe-Nb is used as alloying additive in heat resistant and stainless steels to improve their corrosion resistance, plasticity and welding properties. Fe-Nb addition to construction steels prevents welded joint from corrosion. It is also used for micro alloying in high strength low alloy steels. It is used in specialty alloyed steels. Vacuum grade Fe-Nb is used for super alloys additions in turbine blade applications in jet engines and land-based turbines, inconel family of alloys, and super alloys for the aerospace industry.
Fe-Ti is produced in two grades containing titanium in the range of 35 % to 35 % and 65 % to 75 %. This alloy is used for production of construction and stainless steels, and welding electrodes. Fe-Ti when added to steel, increases yield strength of steel and reduces its cracking tendency. In the production of stainless steel with a high chrome and nickel content, Fe-Ti is used to bond the sulphur. Fe-Ti is manufactured from various raw materials such as titanium scrap, ilmenite sand, rutile and titanium sponge. It is produced either from primary or secondary raw materials.
Fe-B is mainly used as an additive in steelmaking to increase the hardenability, creep resistance and hot workability since steels alloyed with boron are oxidation resistant up to 900 deg C.
The raw materials needed to produce Fe-B are boric oxides and boric acid. Carbon (charcoal), aluminum or magnesium are used as a reducing agent. The alloys can be produced by carbothermic or metallothermic reduction processes.
Fe-Al is a ferroalloy composed of iron and aluminum with the content of the aluminum ranging from 30 % to 75 %. It is primarily used as a deoxidation agent for steel, as well as for moulding in combination with scrap copper and carbon steel.
Fe-Al is used for deoxidization, welding electrode manufacture and hard limited facing applications. In addition, Fe-Al is used in FAT that is used in turn to cut or to weld metal. FAT is made from 75 % iron oxide plus 25 % aluminum oxide. The iron oxide of FAT is not rust (Fe2O3) but iron scale (Fe3O4) oxide. The chemicals are thoroughly mixed together and then compressed into a suitable container. A first fire mix is then used to ignite the mixture. This reaction is used in welding applications, like the one used to join rail tracks in situ.
