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Ferroalloys

• satyendra
• June 23, 2013
• 37 Comments
• alloy steels, alloying element, bulk ferroalloys, deoxidation, Ferroalloys, noble ferroalloys, SAF,

Ferroalloys

Ferroalloys are a group of alloys of iron containing one or more additional elements other than carbon. Ferroalloys have a high percentage of such elements as manganese, silicon, chromium, and aluminum etc. Ferro-alloys are mainly used as master alloys used in the iron and steel industry since it is the most economical way to introduce an alloying element into the steel melt. These alloys are incorporated into the molten stage of the steelmaking process for the purpose of producing specific properties in the steel. Ferroalloys industry is very closely related to iron and steel industry since ferroalloys are used extensively in steelmaking, and in iron or steel foundries.

Ferroalloys are the integral constituents of any steelmaking process, as these are indispensible materials for refining, deoxidation, desulphurization, and alloying to achieve the desired chemical and physical properties in steel. As such they influence the steel quality and steelmaking economics to a great extent. They are vital inputs for producing all types of steel and are used as raw material in the production of alloys steel and stainless steel. Not a single grade of steel is produced without ferro-alloys. In fact, more than 85 % of ferroalloys produced globally are consumed in the steel industry.

Ferroalloys are important additives which are used in steelmaking as deoxidants and also as alloying elements. These are added in steel production not only for de-oxidation but also for grain size control for improvement in the mechanical properties of steel. Depending upon the process of steelmaking and the type of steel being made, the requirement of different ferroalloys varies widely. The principal function of ferroalloys addition to steel increases its resistance to corrosion and oxidation, improve its hardenability, tensile strength at high temperature, wear and abrasion resistance with added carbon and increases other desired properties in the steel such as creep strength etc. The effect of the improved properties of steel by using ferro-alloys as an alloying element depend more or less on such influences as (i) a change in the chemical composition of the steel, (ii) the removal or the tying up of harmful impurities such as oxygen, nitrogen, sulphur or hydrogen, and (iii) a change in the nature of the solidification, for example, upon inoculation.

Ferroalloys have a high content of the major component, typically in the range of 50 % to 90 %, the rest being mostly iron and more or less ‘residues’ of reductants used in ferroalloy production (carbon, aluminum, and silicon depending on the process). How these components are limited in ferroalloys depends on the targeted analysis range. Such metallic impurities can have special limitations due to their influence on oxide inclusions or other precipitates (nitrides, carbides). Ferroalloys also contain small amounts of impurities sulphur, phosphorus, gases (oxygen, nitrogen, and hydrogen etc.), and moisture.

Historically, the ferro-alloy production technology used in the 19th century was developed for blast furnaces, which at that time was the main process for the production of cast iron. In a blast furnace, however, it is not possible to produce ferro-alloys with elements which have a higher affinity for oxygen or with low carbon content. This led to the development, at the beginning of the 20th century, of ferro-alloys to be manufactured (smelted) in electric furnaces. These days almost all ferro-alloys are produced in the submerged arc furnaces. These furnaces are either open or semi-open (hooded furnace allowing the entry oxygen) or closed (hermetic system) types, depending on the production requirements for the various ferro-alloys. A submerged arc furnace (SAF) is shown in Fig 1.

Fig 1 Submerged arc furnace for ferroalloy production

The basic process involves the carbo-thermic reduction of oxide ores or concentrates, in which carbon coming from a reductant in the form of coke (metallurgical coke), coal, or charcoal is normally used as a reducing agent. In the SAF, the heating process is accomplished by passing current through electrodes (Soderberg or mixed) suspended in a cup-shaped (refractory-lined) steel shell, which are progressively consumed. The carbon from the reductant (coke, coal or charcoal) captures the oxygen from the metal oxides to form carbon dioxide, while the ores are reduced to molten base metals which then combine in the solution.

The consumption of raw material mainly depends on the metal content of the ore or concentrate, the metal yield in the smelting process, and the composition of the product, as well as losses during raw material and product handling (transport, screening, etc.) and treatment (refining, solidification, crushing, packing, etc.). The product quality and process requirements impose major constraints in the choice of raw materials. Coal and coke are well-adapted reductants to both the SAF process and the high-level product quality.  The energy consumption per ton of metal differs greatly from one ferro-alloy to another.

Ferroalloys are normally classified into two broad categories namely (i) bulk ferroalloys and (ii) noble or special ferroalloys. Bulk ferroalloys are produced in large quantities in electric arc furnace. T the noble ferroalloys are produced in smaller quantities but are growing in importance. Bulk ferroalloys are used in steel making and steel or iron foundries exclusively, while the use of noble ferroalloys is far more varied.

Bulk ferroalloys

Bulk ferroalloys consist of principal alloys namely ferro-manganese (Fe-Mn), silico-manganese (Si-Mn), ferro-chrome/charge-chrome (Fe-Cr / Ch-Cr) and ferro-silicon (Fe-Si). These are shown in Fig 2.

Fig 2 Bulk ferroalloys

Ferro-manganese

Ferro-manganese (Fe-Mn) is a metallic ferro alloy which is added normally along with ferro-silicon as ladle addition during steelmaking. It is a ferroalloy composed principally of manganese and iron, and normally contains much smaller proportions of minor elements, such as carbon, phosphorus, and sulphur. Fe-Mn is an important additive used as a deoxidizer in the production of steel. It is a master alloy of iron and manganese with a minimum manganese content of 65 % and maximum content of 95 %.

Fe- Mn is a ferroalloy with high content of manganese. It is produced by heating a mixture of the oxides of manganese (MnO2) and iron (Fe2O3) with carbon normally as coal and coke, in either a blast furnace or a SAF. The oxides undergo carbo thermal reduction to produce Fe- Mn. It is produced as three types of products namely (i) standard high carbon Fe-Mn, (ii) medium carbon Fe-Mn and (iii) low carbon Fe- Mn. High carbon Fe-Mn has manganese in the range of 72 % to 82 %, carbon in the range of 6 % to 8 %, and silicon in the range of around 1.5 %. Medium carbon Fe-Mn has manganese in the range of 74 % to 82 %, carbon in the range of 1 % to 3 % and silicon in the range of around 1.5 %. Low carbon Fe-Mn has manganese in the range of 80 % to 85 %, carbon in the range of 0.1 % to 0.7 % and silicon in the range of 1 % to 2 %.

Manganese plays an important role in the manufacturing of steel as deoxidizing, desulphurizing, and alloying agent. It is a mild deoxidizer than silicon but enhances the effectiveness of the latter due to the formation of stable manganese silicates and aluminates. It is used as an alloying element in almost all types of steel. Of particular interest is its modifying effect on the iron-carbon system by increasing the hardenability of the steel. By adding the manganese as medium carbon Fe-Mn or low carbon Fe-Mn instead of high carbon Fe-Mn, around 80 % to 93 % less carbon is added to the steel. Nitrided medium carbon Fe-Mn contains a minimum of 4 % of nitrogen.

Fe-Mn is produced in a number of grades and sizes and is consumed in bulk form primarily in the production of steel as a source of manganese, although some Fe-Mn is also used as an alloying agent in the production of iron castings. Manganese, which is intentionally present in nearly all steels, is used as a steel desulphurizer and deoxidizer. It improves the tensile strength, workability, toughness, hardness and resistance to abrasion. By removing sulphur from steel, manganese prevents the steel from becoming brittle during the hot rolling process.

Silico manganese

Si-Mn is a ferroalloy composed principally of manganese, silicon, and iron. It normally contains much smaller proportions of minor elements, such as carbon, phosphorus, and sulphur. The ferroalloy is also sometimes referred to as ferro-silicon-manganese. It is being used to add both silicon and manganese as ladle addition during steelmaking. Si-Mn is produced in a number of grades and sizes and is consumed in bulk form primarily in the production of steel as a source of both Si and Mn, although some Si-Mn is also used as an alloying agent in the production of iron castings. Because of its lower carbon content, it is a preferred ladle addition material during making of low carbon steels. Both manganese and silicon play an important role in the production of steel as deoxidizing, desulphurizing, and alloying agents. Si-Mn adds additional silicon in liquid steel which is a stronger deoxidizer and which also helps to improve some mechanical properties of steel.

Si-Mn with high content of manganese and silicon is produced by heating a mixture of oxides of manganese (MnO2) silicon (SiO2), and iron (Fe2O3) with carbon in a furnace. These oxides undergo a thermal decomposition reaction. The standard grade contains manganese in the range of 62 % to 68 %, silicon in the range of 12 % to 18 % and carbon in the range of around 2 %.  The low carbon grade of Si- Mn has a carbon level of 0.1 % maximum. Si-Mn is more preferred ferroalloy by the steel melting shop operators for deoxidation. The steel industry is the only consumer of these alloys.

To cover the need for Mn and Si, the steelmaker has the choice of a blend of Si-Mn, high carbon Fe-Mn and Fe-Si governed of by specifications on carbon, silicon, and manganese. Normally earlier a mixture of high carbon Fe-Mn and Fe-Si were used, but now a trend towards more use of Si-Mn is seen at the expense of the two others. This is primarily for economic reasons.

Effects of the addition of Si-Mn to steel depend on the amount added and the combined effect with other alloying elements. Both Si and Mn have an important influence on the properties of steel since both of them have a strong affinity for oxygen, and act as deoxidizers. Deoxidation with Si-Mn results in cleaner steel, as the liquid manganese silicate formed coagulates and separates easier from the melt, compared to solid SiO2 formed during Fe-Si deoxidation. Use of Si-Mn adds less carbon to steel compared to combination of standard Fe-Si and high carbon Fe-Mn. Computational fluid dynamics calculations show that the yield of silicon from Si-Mn is higher than that of the standard Fe-Si.

Ferro silicon

Ferro-silicon (Fe-Si) is a metallic ferro-alloy having iron and silicon as its main elements. In commercial terminology, it is defined as a ferro-alloy containing 4 % or more of iron, more than 8 % but not more than 96 % of silicon, 3 % or less phosphorus, 30 % or less of manganese, less than 3 % of magnesium, and 10 % or less any other element. However, the regular grades of the ferroalloy normally contain silicon in the range of 15 % to 90 %. The normal silicon contents in the Fe-Si available in the market are 15 %, 45 %, 65 %, 75 %, and 90 %. The remainder is Fe and minor elements. The minor elements, such as aluminum, calcium, carbon, manganese, phosphorus, and sulphur are present in small percentages in Fe-Si. Fe-Si is normally used along with Fe-Mn as ladle addition during steelmaking.

Commercially, Fe-Si is differentiated by its grade and size. Fe-Si grades are defined by the percentages of silicon and minor elements contained in the product. The principal characteristic is the percentage of silicon contained in the ferro-alloy and the grades are referred to primarily by reference to that percentage. Hence 75 % Fe-Si contains around 75 % of silicon in it. Fe-Si grades are further defined by the percentages of minor elements present in the product. ‘Regular grade 75 % Fe-Si’ denote that the product containsw the indicated percentages of silicon and recognized maximum percentages of minor elements. Other grades of Fe-Si differ from regular grades by having more restrictive limits on the content of elements such as aluminum, titanium, and / or calcium in the ferro-alloy. Fe-Si is also produced in a grade which contains controlled amounts of minor elements for the purpose of adding them to steel or foundry iron using Fe-Si as the carrier. Such Fe-Si products are sometimes called ‘inoculants’.

Fe-Si is normally produced in four grades. These are (i) standard grade, (ii) low aluminum grade, (iii) low carbon grade, and (iv) high purity grade having low content of titanium. The standard grade of Fe- Si contains aluminum upto 2 % while the low aluminum grade has aluminum content of 0.5 % maximum. Fe-Si contains a high proportion of iron silicides. Fe-Si with 15 % silicon is not used for metallurgical purposes in the production of steel or cast iron. Specialty grade 15 % Fe-Si is combined with water to create a dense medium for gravity (sink / float) separation of minerals, aggregates, and metals.

Fe-Si is mainly used during steelmaking and in foundries for the production of carbon steels and stainless steels as a deoxidizing agent, for the alloying of steels, and cast iron. It is used as a reducing agent, particularly in the production of stainless steel. As a reducing agent, silicon reacts with chromium oxides to form silicon oxides, returning chromium to the liquid steel, and thus increasing the overall chromium recovery of the process.

Fe-Si is also used as the source of silicon for alloying purposes in the production of certain steel alloys, particularly silicon electrical steel, which can contain 3 % or more of silicon. Fe-Si is used by iron foundries as the source of silicon needed for alloying purposes in iron castings. During the production of cast iron, it is also used for inoculation of the iron to accelerate graphitization. In arc welding, Fe-Si can be found in some electrode coatings.

Almost all Fe-Si consumed in the steel and cast iron industry contains silicon ranging from 65 % to 90 %. Fe-Si is used primarily in sized lump form. Size is important since it affects the performance of the Fe-Si in its designated use. Large lumps are normally used in primary steelmaking furnaces since they penetrate the layer of slag on top of the liquid steel more readily. Smaller lumps are more commonly used for alloying purposes to ensure rapid dissolution in liquid steel. Fines are less desirable than lumps since it is more difficult to recover the silicon content in them.

Ferro chrome / charge chrome

Fe-Cr is an alloy comprised of iron and chromium.  Besides chromium and iron, it also contains varying amounts of carbon and other elements such as silicon, sulphur, and phosphorus. It is used primarily in the production of stainless steel. The ratio in which the two metals (iron and chromium) are combined can vary, with the proportion of chromium ranging between 50 % and 70 %.

Fe-Cr is frequently classified by the ratio of chromium to carbon it contains. The vast majority of Fe-Cr produced globally is the ‘charge chrome’ (Ch-Cr). It has lower chromium to carbon ratio and is normally produced for use in stainless steel production. The Ch-Cr grade was introduced to differentiate it from the conventional high carbon Fe-Cr. The second largest produced Fe-Cr ferro-alloy is the high carbon Fe-Cr which has a higher content of chromium than the Ch-Cr grade and is being produced from higher grade of the chromite ore. Other grades of Fe-Cr are ‘medium carbon Fe-Cr and low carbon Fe-Cr. Medium carbon Fe-Cr is also known as intermediate carbon Fe-Cr and can contain upto 4 % of carbon. Lowe carbon Fe-Cr typically has the chromium content of 60 % minimum with carbon content ranging from 0.03 % to 0.15 %.  However carbon content in low carbon Fe-Cr can go upto 1 %.

In international trade, Fe-Cr is classified primarily according to its carbon content. The common categories of Fe-Cr used in international trade are (i) Ch-Cr with a base of 52 % chromium, (ii) high carbon Fe-Cr with carbon content ranging from 6 % to 8 %, base of 60 % chromium, and a maximum of 1.5 % silicon, (iii) high carbon Fe-Cr with carbon content ranging from 6 % to 8 %, based on 60 % to 65 % chromium, and 2 % of silicon maximum, (iv) high carbon Fe-Cr with carbon content ranging from 6 % to 8 % and a base of 50 % of chromium, (v) high carbon Fe-Cr with low phosphorus having composition with chromium – 65 % minimum, carbon – 7 % maximum, silicon – 1 % maximum, phosphorus – 0.015 %, and titanium – 0.05 % maximum, (vi) Fe-Cr with carbon content from 0.10 % and chromium content in the range of 60 % to 70 %, (vii) low carbon Fe-Cr with 0.05 % of carbon and 65 % minimum of chromium, (viii) low carbon Fe-Cr with upto 0.06 % of carbon and 65 % of chromium, (ix) low carbon Fe-Cr with 0.10 % of carbon and 62 % minimum of chromium, (x) low carbon Fe-Cr with 0.10 % of carbon and 60 % to 70 % of chromium, and (xi) low carbon Fe-Cr with 0.15 % of carbon and 60 % minimum of chromium.

High carbon Fe-Cr and charge chrome are normally produced by the conventional smelting process utilizing carbo-thermic reduction of chromite ore (consisting oxides of chromite and iron) using a SAF or a DC (direct current) open arc electric furnace. The carbo-thermic reduction takes place at high temperatures. Chromite ore is reduced by coal and coke to form the Fe-Cr alloy. Ch-Cr is produced from a chrome containing ore with lower chrome content.

High carbon Fe-Cr produced from higher grade ore, is normally used in specialist applications such as engineering steels where a high chromium to iron ratio and minimum levels of other elements such as sulphur, phosphorus and titanium are important. Low carbon Fe – Cr is used during steel production to correct chrome percentages, without causing undesirable variations in the carbon or trace element percentages. It is also a low cost alternative to metallic chrome for uses in super alloys and other special melting applications.

Noble ferroalloys

Noble ferroalloys (Fig  3) 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), and 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), and ferro nickel magnesium (Fe-Ni-Mg) etc.

Noble ferroalloys are the vital inputs for the production special and alloy steels. These ferroalloys are of high value and consumed in low volumes. Noble ferro alloys are mostly manufactured through alumino-thermic process. These are used in the production of steel as de-oxidant and alloying agent. Their quantum of consumption varies widely based on steel making process and grade of steel.

Fig 3 Noble ferroalloys

Ferro nickel

Fe-Ni is used for alloying in the production of stainless and construction steels. It is produced in reduction furnaces from nickel concentrates (can contain up to 97.6 % Ni and be with high / low Fe content). Ni concentrates are produced from laterite ore. Laterite ore is characterized by relatively low nickel content and high moisture content together with chemically bound water in the form of hydroxide. Typical laterite ore contains 1 % to 3 % Ni and a moisture content of 5 % to 10 %. Besides laterite ore, coke and / or coal is needed as a reducing agent, since Fe-Ni production takes place by a carbo-thermic process. The production of Fe-Ni takes place in a SAF. The molten ferroalloy is tapped and granulated in water.

Fe-Ni can also be produced from secondary raw materials, such as spent catalysts and sludge from the galvanizing industry. There are several types of Fe-Ni ferroalloys which are produced from different nickel containing alloy scraps (normally also contain chromium, cobalt, molybdenum, tungsten, titanium, copper, and silicon etc, as impurities).

Ferro molybdenum

Fe-Mo is an important noble ferroalloy. Commercial grade Fe-Mo contains between 60 % and 70 %. Fe-Mo is a molybdenum based ferroalloy, produced by alumino / silico thermic reduction from technical grade molybdenum trioxide (MoO3) or in induction / EAF furnaces from Mo containing scraps.

Molybdenite concentrates are roasted to form molybdic oxide. Technical grade molybdenum trioxide (MoO3), also known as roasted molybdenum concentrate (RMC), is the main raw material in the production of Fe-Mo. Roasting carry out conversion of concentrated molybdenite (molybdenum sulphide ore) into technical grade molybdenum trioxide. This oxide is mixed with iron oxide and aluminum and is reduced in an alumino-thermic reaction to produce ferro molybdenum. Fe-Mo is used in the production of different alloy steels.

Ferro vanadium

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 carbo-thermic or a metallo-thermic (alumino thermic) reduction of vanadium oxides, assisted by the presence of iron. Since carbon is used in a carbo-thermic reduction, the carbon content of the ferroalloy is normally high. Hence the process cannot be used if there is a requirement for low carbon content. Low carbon Fe-V is normally 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. The production is carried out in an electric-arc furnace. Scrap iron is first melted, and a mixture of vanadium oxides, aluminum, and a flux such as calcium fluoride or calcium oxide is added. In the ensuing reaction, the aluminum metal is converted to alumina, forming a slag, and the vanadium oxides are reduced to ferro vanadium. When necessary, grinding, sizing, and drying of the materials are carried out prior to charging the mix to the smelting process.

Ferro tungsten

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 hence increases the hardness, wear resistance, and hot strength 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 which contain tungsten oxides, e.g. wolframite, scheelite and hubnerite. The reduction of these minerals is done either by carbo-thermic or metallo-thermic reduction as well as by a combination of both. The tungsten trioxide in these ores is reduced by silicon and /or aluminum.

Ferro tungsten contains 75 % to 85 % tungsten. Fe-W has a steel grey appearance and a fine-grained structure consisting of FeW and Fe2W.

Ferro niobium

Fe-Nb is a niobium based ferroalloys which improves the corrosion resistance and weldability of steel and prevents the inter-crystalline corrosion of stainless chrome nickel steel. Fe-Nb contains niobium in 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.

The raw materials needed to produce Fe-Nb are ores and concentrates which contain niobium and iron oxide. Basic raw material for producing ferro niobium is pyrochlore ore. From this ore niobium penta oxide (Nb2O5) is produced. Nb2O5 is mixed with iron oxide and aluminum and is then reduced by alumino-thermic reaction to produce ferro niobium.

Ferro titanium

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 the 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 produced from various raw materials such as titanium scrap, ilmenite sand, rutile, and titanium sponge. It is produced either from primary or secondary raw materials. The reduction is normally carried out by the metallo-thermic process since the carbo-thermic reduction produces a ferroalloy which contains too much carbon and hence not of much use in steelmaking. The production takes place as a batch process in a refractory lined crucible or in an electric furnace, depending on the process variation.

Ferro aluminum

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 as Ferro aluminum thermite (FAT) is an agent, which when ignited and mixed, can give off super extreme amounts of heat. Although this reactant is stable at room temperature, it will burn through an extremely intense exothermic reaction.

Ferro boron

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) and aluminum or magnesium is used as a reducing agent. The alloys can be produced by carbo-thermic or metallo-thermic reduction processes.