Ferro-manganese (Fe-Mn) is a metallic ferro alloy which is added usually along with ferro-silicon (Fe-Si) as ladle addition during steelmaking. It is a ferroalloy composed principally of manganese (Mn) and iron (Fe), and normally contains much smaller proportions of minor elements, such as carbon (C), phosphorus (P), and sulphur (S).
Fe-Mn is an important additive used as a deoxidizer in the production of steel. It is a master alloy of Fe and Mn with a minimum Mn content of 65 %, and maximum Mn content of 95 %. There are two families of Mn alloys. One is called Fe-Mn while the other is known as silico-manganese (Si-Mn). Around 93 % of all the Mn produced is in the form of Mn ferroalloys consists of the Fe-Mn grades and the Si-Mn grades.
Mn plays an important role in the manufacturing of steel as deoxidizing, desulphurizing, and alloying agent. It is a mild deoxidizer than silicon (Si) but enhances the effectiveness of the latter due to the formation of stable manganese silicates and aluminates. Mn is used as an alloying element in almost all types of steel. Of particular interest is its modifying effect on the iron-carbon (Fe-C) system by increasing the hardenability of the steel.
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 Mn, although some Fe-Mn is also used as an alloying agent in the production of iron castings. Mn, which is intentionally present in nearly all steels, is used as a steel desulphurizer and deoxidizer. Mn improves the tensile strength, workability, toughness, hardness and resistance to abrasion. By removing S from steel, Mn prevents the steel from becoming brittle during the hot rolling process.
There are several grades of Fe-Mn which are divided into many groups. The three main groups are high carbon Fe-Mn (HC Fe-Mn), medium carbon Fe-Mn (MC Fe-Mn), and low carbon Fe-Mn (LC Fe-Mn). By adding the Mn as MC Fe-Mn or LC Fe-Mn instead of HC Fe-Mn, around 80 % to 93 % less C is added to the steel. Nitrided MC Fe-Mn contains a minimum of 4 % of nitrogen (N2).
HC Fe-Mn can be made in blast furnace (BF) and in electric submerged arc furnace (SAF). In SAF it is made by two different practices namely (i) high Mn slag practice, and (ii) discard slag practice. MC Fe-Mn can be produced by a de-carbonation process or through a redox (reduction-oxidation) reaction between Si in the Si-Mn alloy and Mn ores. LC Fe-Mn is produced by the reaction of Mn ore and LC Si-Mn.
The specification of Fe-Mn is normally given in various national and international standards. As per ASTM standard, Fe-Mn can be designated as four main groups in ten grades as shown in Tab 1.
Tab 1 Grades of Fe-Mn as per ASTM standard
|Sl.No.||Ferro alloy group||Grade||Mn %||C % max||Si % max||P % max||S % max||N % min|
|1||Standard HC Fe-Mn||A||78-82||7.50||1.20||0.35||0.05|
|3||Nitrided MC Fe-Mn||75-80||1.50||1.50||0.30||0.02||4.0|
Types of Fe-Mn can also be classified in another manner. There are seven types of Fe-Mn based on this classification. These seven types of Fe-Mn are given below.
- LC Fe-Mn – It is used for steels with very low content of C. In this type, C content is ranging from 0.07 % to 0.75 %. LC Fe-Mn is suitable for use in the production of 18-8 chromium-nickel (Cr-Ni) stainless steels in which a C content well below 0.10 % is required.
- MC Fe-Mn – This alloy contains 80 % to 85 % of Mn, 1.25 % to 1.50 % of C and 1.50 % maximum of Si. It is commonly used in making low C Mn steels. It is also used in the production of Hadfield Mn steel, when large amounts of return scrap are being melted.
- HC Fe-Mn – This alloy contains 75 % to 80 % of Mn, 7.5 % of C and 1.2 % of Si. It is normally used in the iron and steel industry as deoxidizer, fixer of S and alloying agent.
- Low Fe Fe-Mn – It is used for numerous applications in the Ni, aluminum (Al), and copper (Cu) industries where there is necessity of high Mn and low Fe and where low C content of pure Mn metal is not necessary. It contains 85 % to 90 % of Mn, 2 % of Fe, 3 % of Si, and 7 % of C.
- Machining screw (MS) grade Fe-Mn – It contains 80 % to 85 % of Mn, 0.35 % of Si, and 1.25 % to 1.50 % C. This low Si Fe-Mn alloy is developed to add during production of free machining screw steels.
- Drawing quality (DQ) grade Fe-Mn – It contains 86 % of Mn, 0.45 % of C, 0.40 % of Si, and 0.17 % of P. This is refined Mn product which is used as additive to steels for DQ steel sheets where both low Si and low C contents are desirable. This alloy is also used as addition agent for stainless and constructional alloy steels.
- Exothermic grade Fe-Mn – Several grades of Fe-Mn briquettes are sold in the market which contains constituents that cause an exothermic reaction when added to a steel bath, e.g. barium (Ba) salts. This special material is used for ladle additions of Mn to prevent chilling of the ladle contents.
To cover the need for Mn and Si, the steelmaker has the choice of a blend of HC Fe-Mn, ferro-silicon (Fe-Si) and Si-Mn governed of by specifications on C, Si, and Mn. Normally earlier a mixture of HC Fe-Mn and Fe-Si was 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.
Properties of Fe-Mn
The chemical name of the ferro-alloy is FeMn. It is a master alloy of Fe and Mn with a minimum Mn content of 65 %, and maximum Mn content of 95 %. It has the CAS no. is 12604-53-4. The three main groups of this metal alloy which are normally used are HC Fe-Mn, MC Fe-Mn, and LC Fe-Mn.
Fe-Mn is a solid lumpy material practically odorless when dry and usually with a silvery metallic surface. The surface becomes generally covered with a dark layer of oxides during storage. It is generally free from extraneous contaminations such as slag and non-metallic inclusions etc. It is normally supplied as crushed and screened material. The normal standard sizes for HC Fe-Mn consist of (i) 20 mm to 80 mm with undersize 10 % maximum, (ii) 10 mm to 50 mm with undersize 10 % maximum, and (iii) 3 mm to 25 mm with undersize 5 % maximum. The oversize limit in all three cases is 10 % maximum. Fe-Mn is usually delivered as bulk normally packed in big bags.
Fe-Mn has a density in a range of 7.3 to 7.4 grams per cubic centimeter. It has a bulk density of around 4 tons per cubic meter. The angle of repose varies in the range of 40 degree to 60 degree depending on size of the material. The melting range of the Fe-Mn ranges from 1050 deg C to 1260 deg C depending upon its chemical specification. Fe-Mn is inflammable and is insoluble in water. It is non-reactive under normal conditions of use, storage and transport.
Iron-manganese phase diagram
Iron-manganese phase diagram is at Fig 1. Iron-manganese phase diagram shows the phases which are to be expected at equilibrium for different combinations of Mn content and temperature.
In pure iron, the temperature A4 (1394 deg C) and A3 (912 deg C) transformations take place at constant temperatures. If an element enters into solid solution in iron — forming in that way a binary alloy — each of these transformations are required by the phase rule to occur over a range of temperature.Mn raises the A4 temperature and lowers the A3 transformation temperatures, increasing, in effect, the extent of the gamma field in the Fe-C phase diagram.
The Fe-Mn phase diagram has been calculated with public binary alloy thermodynamic database. The melting point of Fe and Mn at the 1 atmospheric pressure has been taken as 1538 deg C and 1246 deg C respectively.
Fig 1 Iron – manganese phase diagram
Uses of Fe-Mn
Fe-Mn alloys are mostly used in steelmaking and foundry activities. Some 30 % of the Mn used today in steelmaking is still being used for its properties as a deoxidizing agent and a sulphide former. As a sulphide former it combines with S avoiding the formation of iron sulphides, which are the low melting point phases that become liquid at hot rolling temperatures and which, consequently, generate surface cracking.
The other 70 % of the Mn is used purely as an alloying element. Steels usually contain from 0.2 % to 2 % Mn depending on grades as Mn is the cheapest alloying element among those which enhance some key mechanical properties like strength and toughness. In the specific case of stainless steel it can substitute expensive Ni in some austenitic grades called 200 series.
Mn has an important influence on the structure and properties of steel, depending on the amount used and the combined effect with other alloying elements. Among all other alloying elements, Mn has the strongest effect on the hardenability of steels. Mn also improves the tensile strength, workability, toughness, hardness and resistance to abrasion. It also reacts with the remaining S in the steel, thus preventing hot shortness.
Boron (B), Titanium (Ti), N2, and P are elements which can be controlled in case of restricted specification of Fe-Mn alloy. A very specific application of these refined Fe-Mn alloys is a constituent in the coating of welding electrodes.