DUCTILE IRON CASTING MANUFACTURING FROM INDUCTION FURNACE (300 TO 400 MT/MONTH)

Ductile iron casting refers to a process in which magnesium/Cerium (as an alloy of magnesium / Cerium) is added to cast iron. It was first manufactured by K.D. Mills in 1943. While most varieties of cast iron are comparatively brittle, ductile iron castings are much more ductile due to the inclusion of nodular graphite. Solidified castings of ductile iron contain nearly perfect spheres of graphite. Ductile iron possesses the processing advantages of grey iron, such as low melting point, good fluidity, castability and machinability, and engineering advantages of steel, including high strength, ductility and wear resistance. Achieving the desired quality consistently at low cost in a production foundry is, however, still a challenge. Addition of small amount of cerium or magnesium to molten cast iron changes the shape of graphite from laminar to spheroidal, giving rise to spheroidal graphite iron or ductile iron. The rapid growth in industrial applications of ductile iron (DI) is driven by its versatility and high performance at low cost. It offers a good combination of tensile strength and ductility. This allows designers to select ductile iron for a wide range of applications. Ductile iron also offers cost savings compared to steel and malleable iron castings through higher yield and thereby lower melting energy. Formation of graphite during solidification leads to lower volumetric shrinkage in ductile iron (compared to steel), necessitating smaller and fewer feeders to prevent the formation of shrinkage defects. Further cost advances can be achieved by eliminating heat treatment of as-cast DI parts. The near-spherical shape of the graphite nodules distributed evenly in the matrix phase of ductile iron enhances its ductility and impact resistance along with tensile and yield strength equivalent to a low carbon steel. While ferritic ductile iron can be used as ‘as-cast’, it may also be annealed to increase its ductility and low-temperature toughness. The pearlitic ductile iron has graphite spheroids in a matrix of pearlite, resulting in high strength, good wear resistance, moderate ductility and impact resistance. The most commonly used ferritic-pearlitic ductile iron containing both ferrite and pearlite in matrix offers a good combination of tensile strength and ductility with good machinability and low production costs. The ductile iron castings are produced in a wide range of weight, from a few grams to a hundred tons or more, greatly varying in shape and size depending on the applications (Figure 1). Many forged and fabricated steel components are getting replaced by ductile iron castings, owing to their good combination of mechanical properties such as strength, wear resistance, fatigue strength, toughness and ductility coupled with economic production. The specifications of ductile irons with ferrite/pearlitic matrix with different grades are shown in Table 1. They are used in safety parts in automobiles, armatures, pumps and machine tools. They are also used in parts subjected to high pressure, such as pressure containers and hydraulics. Many welded assemblies and forgings used in governor housings, armatures and car parts (like brake calipers and gear housings, hydraulic parts, crankcases and blower buckets) are being replaced by ferritic ductile iron castings. Cast or grey iron is an alloy characterized by its relatively high content of carbon flakes (2% to 4%). In contrast, the carbon in ductile iron is in the form of spherical nodules. The formation of such nodules is achieved by the addition of ‘nodulizers’ like magnesium or cerium into the castings melt. Due to its inherent properties, such nodules resist the creation of cracks and augment its ductility. That is why this process is called as ductile iron casting. In the as-cast condition, the matrix will consist of varying proportions of pearlite and ferrite, and as the amount of pearlite increases, the strength and hardness of the iron also increase. Ductility and impact properties are principally determined by the proportions of ferrite and pearlite in the matrix. The mechanical properties of ductile iron are controlled by the presence of graphite nodules. The Different grades of Ductile Iron Castings are produced by obtaining different matrix microstructures in the Iron. Alloying elements may be added to enhance as cast properties of Ductile Iron. In some special cases Heat Treatment can be employed to achieve the higher properties. The Grades of Ductile Iron Castings are based on the Mechanical Properties of the casting. Ductile iron with high strength and toughness has been available as an engineering material for many years, replacing forging steel, cast steel, and malleable cast-iron. It has undergone a phenomenal development and has become the only ferrous casting material with positive growth rate. The ductile iron will still be an important construction material in the 21st century. Ductile iron is a ternary Fe-C-Si alloy, in which the concentrations of carbon and silicon vary typically from 3.5 to 3.9% and from 1.8 to 2.8%, respectively. The selection of the composition is dictated by the casting section size and by the targeted mechanical properties. The output characteristics depend on the matrix structure and the shape, size, and distribution of the graphite spheroids. Matrix and spheroids, in their turn, depend on the chemical composition of the melt, on the desulphurizing and spheroidizing methods applied in the treatment ladle, onthe inoculation method and finally, on the time elapsing between these events and the casting in the mould . Moreover, the mechanics of spheroid formation itself has not yet been completely understood and many models are still in competition. Production of ductile iron is influenced by a large number of metallurgical, technological, heat transfer, and designing parameters. The first step of the production of ductile iron castings is careful selection of the charge materials. Manganese and chromium have the strongest influence on mechanical properties of the ductile iron. For this reason, their concentration in the metal is of particular importance. These elements arise in the charge from the steel scrap, pig iron, and returns. It is a recommended practice to surchage steel scrap so that the average Cr content remains below 0.1 percent. Ideally, the same advice would be given for Mn but, unfortunately, all steel scraps contain Mn, mostly about 0.5 percent. The amount of steel scrap in the charge must ensure the production of castings that are as free of carbides as possible. It is necessary not to mix grey iron return scrap with the ductile iron one, because grey iron castings have an increased manganese and chromium content. Ductile iron return scrap has large silicon and small sulphur content. However, if spheroidizing elements are present in excessive concentration they act as despheroidizers. Charge materials result in the average size of graphite spheroids. For instance, if the amount of the steel scrap in the charge is more than 50 percent then an average spheroid diameter is 33 µm, if it is 30 percent then the average diameter is 57 µm . The amount of the steel scrap affects the metallic matrix structure as well, increasing the pearlite formation. However, fully pearlitic castings are produced more easily by adding copper. The graphite structure is also affected by the carbon content. If the initial metal does not contain enough carbon then graphite particles have a compact form . The metallic matrix structure is affected not only by carbon equivalent but also by the C/Si ratio. Increasing this ratio in ductile iron decreases the proportion of ferrite and increases the proportion of pearlite. The formation of graphite spheroids is obtained through a special treatment, during which spheroidizing elements are added to the melt. Both Mg and various Mg alloys are most commonly used for ductile iron spheroidization. The choice of a treatment method (open ladle, sandwich, tundish cover, in-mould, plunger, converter, injection, and others for an individual foundry must be based on the circumstances present in the foundry. Inoculation, which may take place at different phases of the process, is a necessary step in the production of ductile iron castings. Most inoculants are ferrosilicons. An inoculant grade FeSi always contains elements in relatively low concentration, which are active inoculants, such as Ca, Al, Zr, Ba, Sr, and Ti. These elements are used to increase the solubility of the alloys. There are three ways to inoculate the metal, which are used individually or in combination: in the ladle, in the stream while pouring or reladling, and in the mould.