Ferro-chrome (Fe-Cr) is an alloy comprised of iron (Fe) and chromium (Cr). Besides Cr and Fe, it also contains varying amounts of carbon (C) and other elements such as silicon (Si), sulphur (S), and phosphorus (P). It is used primarily in the production of stainless steel. The ratio in which the two metals (Fe and Cr) are combined can vary, with the proportion of Cr ranging between 50 % and 70 %.
Fe-Cr is frequently classified by the ratio of Cr to C it contains. The vast majority of Fe-Cr produced globally is the ‘charge chrome’. It has a lower Cr to C ratio and is most commonly produced for use in stainless steel production. The charge chrome grade was introduced to differentiate it from the conventional high carbon Fe-Cr (HC Fe-Cr). The second largest produced Fe-Cr ferro-alloy is the HC Fe-Cr which has a higher content of Cr than charge chrome and is being produced from higher grade of the chromite ore. Other grades of Fe-Cr are ‘medium carbon Fe-Cr’ (MC Fe-Cr) and ‘low carbon Fe-C’ (LC Fe-Cr). MC Fe-Cr is also known as intermediate carbon Fe-Cr and can contain upto 4 % of carbon. LC Fe-Cr typically has the Cr content of 60 % minimum with C content ranging from 0.03 % to 0.15 %. However C content in LC Fe-Cr can be upto 1 %.
In international trade, Fe-Cr is classified primarily according to its C content. The common categories of Fe-Cr used in international trade are as follows.
- Charge chrome with a base of 52 % Cr.
- HC Fe-Cr with C content ranging from 6 % to 8 %, base of 60 % Cr, and a maximum of 1.5 % Si.
- HC Fe-Cr with C content ranging from 6 % to 8 %, based on 60 % to 65 % Cr, and 2 % of Si maximum.
- HC Fe-Cr with C content ranging from 6 % to 8 % and a base of 50 % of Cr.
- HC Fe-Cr with low P, Cr – 65 % minimum, C – 7 % maximum, Si – 1 % maximum max, P – 0.015 %, and Ti – 0.05 % maximum.
- Fe-Cr with C content from 0.10 % and Cr content in the range of 60 % to 70 %.
- LC Fe-Cr with 0.05 % of C and 65 % minimum of Cr.
- LC Fe-Cr with up to 0.06 % of C and 65 % of Cr.
- LC Fe-Cr with 0.10 % of C and 62 % minimum of Cr.
- LC Fe-Cr with 0.10 % of C and 60 % to 70 % of Cr.
- LC Fe-Cr with 0.15 % of C and 60 % minimum of Cr.
HC Fe-Cr and charge chrome are normally produced by the conventional smelting process utilizing carbo-thermic reduction of chromite ore (consisting oxides of Cr and Fe) using an electric submerged arc furnace (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. The heat for this reaction can come from several forms, but typically from the electric arc formed between the tips of the electrodes in the bottom of the furnace and the furnace hearth. This arc creates temperatures of about 2,800 deg C. In the process of smelting, a large amount of electricity is consumed.
Production process for the Fe-Cr is highly electric energy intensive since all the heat needed for the endothermic reduction reactions and to achieve thermodynamic equilibrium in the furnace is supplied through electrical energy only. Thus electrical energy is the most vital input in the process.
During the production of Fe-Cr through carbo-thermic reduction, metallic Cr which is formed tends to react further with the available C to form Cr carbides (Cr3C2, Cr7C3, and Cr23C6). Similarly metallic Fe reacts with the available C to form carbides of Fe (Fe3C and Fe2C). The presence of these carbides increases the total C content of the Fe-Cr ferro-alloy beyond the specified limits since the theoretical C content of these carbides ranges from 5.5 % to 13.3 %.
Several carbides can form preferentially to the metallic Cr and Fe during the reduction process of chromite ores. Cr/Fe ratio plays a role in the determination of the C content of the Fe-Cr. As Cr has higher affinity to form carbides than Fe, a higher Cr/Fe ratio means a higher C content in the Fe-Cr.
Properties of Fe-Cr
Cr is resistant to common corrosive agents at room temperature, and is hence a fundamental constituent element for stainless steel. It also promotes the hardening of steels and the homogenization of this feature. Cr improves the heat resistant property of the steel. It may react with some acids with the evolution of hydrogen (H2). It can react with fused alkali with the formation of compounds containing hexavalent Cr. Cr has got a body centered cubic (bcc) crystal structure.
Fe-Cr is a solid which is available in a variety of forms, including small crystals, lumps and granules as well as in powder form. Its colour varies from dark metallic gray to light gray. It is odourless. It is not soluble in water. The dust particles of Fe-Cr alloy are combustible.
Chemical formula of ferro-chrome is FeCr. CAS number of Fe-Cr is 11114-46-8. The density for Fe-Cr varies in the range of 6 grams per cubic centimeters to 9 grams per cubic centimeters depending on its composition. The bulk density of Fe-Cr varies in the range of 3.3 grams per cubic centimeters to 3.7 grams per cubic centimeters. Its melting point is greater than 1500 deg C and boiling point is in the range of 2700 deg C to 3000 deg C.
Exposure to Fe-Cr can cause certain health problems. It can cause irritation to the skin. Contact of Fe-Cr with the eyes causes swelling and redness. Inhaling of Fe-Cr can result in coughing and irritation of the respiratory tract.
Fe-Cr is chemically stable under normal ambient and anticipated storage and handling conditions of temperature and pressure. It is neither classified as hazardous nor is classified as a hazardous good for its transportation.
Uses of Fe-Cr
Fe-Cr is essential for the production of stainless steel and special steels which are widely used and are of high quality. Stainless steel is defined as a steel alloy with a minimum of 10 % Cr by content, the average Cr content being 18 %. Stainless steel depends on Cr for its appearance, its corrosion resisting properties, and its low tendency to magnetization. Over 80 % of the Fe-Cr produced worldwide is used in manufacturing stainless steel. The content of Cr present in stainless steel provides stainless steel its customary appearance.
Fe-Cr is also used when more Cr is required to be added to C steel. HC Fe-Cr, produced from higher grade ore, is normally used in specialist applications such as engineering steels where a high Cr to Fe ratio and minimum levels of other elements such as S, P and titanium (Ti) are important. It is also used in the manufacture of ball-bearing steels, tool steels as well as other alloy steels. LC Fe-Cr is used during steel production to correct Cr percentages, without causing undesirable variations in the C or trace element percentages. It is used in the manufacture of acid-resistant steels. It is also a low cost alternative to metallic Cr for uses in super alloys and other special melting applications.
High nitrogen (N2) Fe-Cr is produced by the addition of 0.75 % of N2 to the different grades of Fe-Cr. This N2 rich Fe-Cr is used for manufacturing high Cr cast steel which is having a coarse crystalline structure. The N2 content produces refined grains and adds strength to the finished cast steel product.
Foundry grade Fe-Cr containing around 62 % to 66 % Cr and almost 5 % of C is used for producing cast irons.
Fe-Cr is used in the production of Ferrochrome ligno-sulfonate. Ferrochrome ligno-sulfonate is used as a drilling fluid. It is used in various water-based systems to control flow of materials at high levels of temperature as well as to reduce the ill effects of mud and clay contamination. It operates well in gypsum, fresh water, lime and salt water fluids. Fe-Cr powder is used in the field of powder metallurgy. Fe-Cr dust is used in the leather tanning industry.
Iron -chromium phase diagram
Fe-Cr phase diagram shows which phases are to be expected at equilibrium for different combinations of chromium content and temperature. The phase diagram of the Fe-Cr binary system is at Fig 1. The melting point of Fe and Cr is taken at the pressure of 1 atmosphere as 1538 deg C and 1907 deg C respectively.
The sigma phase, which is an intermetallic FeCr compound, can sometimes form in Fe-Cr alloys, such as AISI 316 or AISI 310 stainless steels. Sigma phase has harmful effects on the mechanical properties (e.g. ductility) and corrosion resistance.
In pure iron, the A4 (1394 deg C) and A3 (912 deg C) transformations take place at constant temperatures. Cr lowers the A4 and raises the A3 transformation temperatures, restricting the gamma loop in the iron-carbon phase diagram. As the binary iron-chromium phase diagram shows, the presence of Cr restricts the gamma loop (Fig 1).
The addition of C to the Fe-Cr binary system widens the alpha+gamma field and extends the gamma loop to higher Cr contents.
Fig 1 Iron-chromium binary phase diagram