CAST IRON INOCULATION - Elkem

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CAST IRON INOCULATION THE TECHNOLOGY OF GRAPHITE SHAPE CONTROL Elkem manufactures and markets a series of high quality inoculants to treat cast iron and ensure the production of an ideal graphite shape, distribution and freedom from chill (cementite). All inoculants are available in sizes suitable for ladle or in-stream additions. This brochure describes some of the conditions in the production of cast iron that call for the addition of an inoculant to ensure the reliable production of a sound, strong, tough, machinable casting. The mechanism of inoculation and graphite nucleation in cast iron during solidification is also described. What is Inoculation of Cast Iron Inoculation is the means of controlling structures and properties of cast iron by minimizing undercooling and increasing the number of nucleation sites during solidification. An inoculant is a material added to the liquid iron just prior to casting that will provide suitable sites for nucleation of graphite during the subsequent cooling. Traditionally, inoculants have been based on graphite, ferrosilicon or calcium silicide. Almost exclusively, inoculants today are ferrosilicon based containing small quantities of active elements such as Al, Ba, Ca, Sr, Zr and RE (Rare Earth metals). The purpose of inoculation is to assist in providing sufficient nucleation sites for dissolved carbon to precipitate as graphite rather than iron carbide (cementite, Fe 3 C). This is done by preventing undercooling below the metastable eutectic temperature where carbidic (white) structures are formed. The iron solidification mechanism is prone to form chilled iron structures when the inoculation is inadequate. There are several reasons why chilled structures are normally undesirable. They are hard and brittle and interfere with machining, necessitate additional heat treatment operations, resulting in nonconformance with specifications and, in general, increase the total cost of production. Inoculation changes the structure of cast iron by altering the solidification process. A look at the solidification process for hypoeutectic grey iron (iron with a carbon equivalent less than 4.3) helps in understanding the effect of inoculation. The first metal to solidify in hypoeutectic grey iron is primary austenite. As cooling continues, the remaining iron grows richer in dissolved carbon. Eventually, the liquid reaches the eutectic composition of 4.3% carbon equivalent, at which final or eutectic solidification would start under equilibrium conditions. However, equilibrium solidification does not occur under practical foundry conditions. Due to variations in chemistry, pouring temperature, solidification rate, section thickness and other conditions, the metal will cool below the eutectic temperature before the start of final solidification. If the undercooling is slight, random graphite flakes form uniformly in the iron matrix, see Figure 1. This is known as Type A graphite. As the undercooling increases, the graphite will branch, forming abnormal patterns. This is known as Types B, D and E graphite. A further increase in undercooling will suppress the formation of graphite and results in a hard white iron carbide structure. The role of the inoculant is to produce nuclei in the liquid iron melt which enhance the graphite nucleation with a low degree of undercooling. This will in turn, promote the formation of Type A graphite structures in grey iron, and a high number of small graphite nodules in ductile iron. Figure 1: Graphite type versus undercooling. 1. Structure and Phases in Cast Iron The structure of cast iron has a domi nant influence on strength and machinability, and in order to obtain a machinable grey iron structure for thin sections, the addition of an inoculant to molten iron is widely practiced and often absolutely necessary. For convenience, potential difficulties with machinability can be determined by carrying out a hardness test (Brinell hardness) on iron castings and, in general, machinability improves with decreasing hardness. The cast iron structure can be influenced at two distinct stages in the production route: • during solidification • during heat treatment However, for economic reasons, the desired structure should be achieved during solidification without the necessity for heat treatment.
The microstructure of an iron casting consists of several phases, each having varying levels of carbon, iron and other elements present. Table 1 shows the analysis and specific densities of the solid and liquid phases which take part in the solidification process. When solidification is complete, the following combination of phases may be found: 1) Austenite + Graphite = GREY structure 2) Austenite + Graphite + Cementite = MOTTLED structure 3) Austenite + Cementite = WHITE structure This review demonstrates that solidifica tion results in a minimum of two solid phases; and austenite is present in all the phase combinations. As the casting cools, the austenite subsequently transforms to pearlite and/or ferrite in solid state (eutectoid transformation). Of all the solid phases listed above, cementite has the highest hardness (~660 HB), whilst graphite is a relatively soft material of low density, which can act as a lubricant. Hardness and machinability of the as-cast structure are, therefore, influenced by the relative amounts of cementite and graphite, with austenite playing only a minor role. Table 1: Approximate analysis and specific densities of phases in the solidification range of cast iron with 2.4% Si. 2. Structure Stability A metastable white or mottled structure can be transformed into a stable grey structure by annealing, but the reverse transformation is not possible as the stable structure represents the lowest possible energy level (at a given temperature and composition). The graphite produced by annealing will have a different structure to that formed during solidification. Cementite, austenite and liquid iron have similar densities and all contain carbon in solution, see Table 1. No major redistribution of the atom species is required for a white structure to be produced during solidification. However, the formation of a stable grey structure containing graphite is quite different. Graphite precipitated from molten iron is virtually pure carbon, and since it has a lower specific density than the alternative phases; a major redistribution of atoms is required to develop a stable structure. A slow rate of solidification is therefore more likely to produce a grey iron structure. The precipitation of cementite, re quir ing less atom redistribution than graphite, will be more likely during rapid solidification. This can be demonstrated by examining a typical wedge test specimen. The narrow tip of the wedge solidifies at a faster rate than the thicker section at the base of the wedge, and will show a white structure whilst the area of slow cooling at the base will display a grey structure, see Figure 2. Consequently, a slow rate of solidification (slow cooling rate) and a small value of undercooling encourages the formation of a grey structure with good machinability and discourages a hard white structure. Figure 2: Chill Wedge with fast solidifying ‘white’ tip and slowly cooled ‘grey’ base. 3. Influence of Elements on As-Cast Structure Within the composition of cast iron, graphitizing elements will promote the carbon-carbon bond to produce graphite in the as-cast structure, whereas carbide stabilizing elements promote the carboniron bond and cementite will appear in the structure. Table 2 lists a number of such stabilizing elements. As an example, in malleable cast irons the need for the as-cast structure to solidify white determines that the silicon level is much lower than in grey irons. Also, since chromium is a carbide promoting element, it has to be kept at a low level to allow transformation to a graphitic structure during subsequent heat treatment. In normal furnace charge materials, steel and external cast iron scrap may be heterogeneous materials, especially on different deliveries, with contents of Cr, Cu, Sn, Sb, V, Mo, Ti, etc., depending on the original source and ultimately on the ability of the scrap dealer. Pig iron produced from steel scrap can also display a similar variable response to inoculation due to fluctuating trace element contents. A more consistent response to inoculation is attainable by adopting a charge containing a reasonable proportion of ore-based pig iron due to its low level of trace elements of the carbide stabilising type. Controlling the concentration of trace elements allows the foundryman a means of promoting grey as-cast structures and, also, helps in avoiding other undesirable effects of trace elements on microstructure and properties. Table 2: Graphitizing and carbide promoting elements.
Page 1: CAST IRON INOCULAtion THE TECHNOLOG
Page 5 and 6: 5.2 Constituents of an inoculant Mo
Page 7 and 8: 5.5 Specification of Inoculants The
Page 9 and 10: Table 5: Eutectic cell count (30 mm
Page 11 and 12: 9. Inoculation and Shrinkage The so

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