Gray iron inoculation revisited.
In that article it was noted that a metallurgist might expand on the (AFS Metalcasting Dictionary) definition of inoculation as a process for improving graphite morphology, eliminating iron carbides, increasing eutectic cell count and reducing section sensitivity, "but," said the author, "for the foundryman whose metallurgical training takes place on the melt floor and not in the classroom or laboratory, that definition does little to increase his understanding of the inoculation process."
"A basic undestanding of the science (of inoculation) is more of a necessity now and less of a nicety, increasingly so as the foundry industry enters a new age of sophistication and technological awareness, "the author wrote, somewhat prophetically, over a decade ago.
Producing better castings consistently is a matter of shop floor control, especially in today's fiercely competitive foundry industry. Control is the key to effective inoculation, and understanding is the key to effective control. It is well for every foundryman to understand the consequences of his part in the melting and casting succession.
What Is Inoculation?
Inoculation is the addition made to a melt which alters the solidification structure of gray iron. It is the use of certain materials that, when added to molten iron prior to casting, make higher quality, more predictable, gray and ductile iron castings.
Its primary purposes are to improve the mechanical properties and the machinability of iron castings. It does this by modifying the solidification process sufficient to allow the formation of Type A graphite in gray iron and nodular graphite in ductile iron.
Inoculation is a familiar foundry practice that facilitates within a molten iron bath the formation of nucleation sites and the precipitation (formation) of graphite as flakes or nodules. Together with controlled cooling, it helps to create a balance, or equilibrium state, between cooling time and temperature. By creating this favorable balance for solidification, inoculation serves several secondary purposes such as:
* control of graphite structure (morphology);
* elimination or reduction of iron carbide formation (chill);
* reduction of casting section sensitivity;
* prevention of undercooling.
Countless ladles are inoculated every day in foundries throughout the world. That it is an established metalcasting process is well known. An understanding of the why and how of inoculation is important at a time when gray and ductile iron casting quality and consistency could be an added key to a foundry's success.
Inoculation is an important final step in controlling the structure and properties of gray iron. In gray iron, it promotes the formation of small, uniformly dispersed Type A graphite flakes by providing nucleation sites to affect the formation of eutectic cells.
The metallurgical principles resulting from inoculation are complex, but a basic understanding of the science involved can provide the foundryman with information linking that knowledge to improved melting practices. A proven foundry iron quality control tool, inoculation provides the means to regulate the growth of desirable types and size of graphite during solidification. It is graphite that gives gray and ductile irons their unique mechanical and machinability qualities.
Inoculants most commonly are 70-90% ferrosilicon blended with calcium (0.50% minimum) and aluminum (1.0-1.4%). Barium, manganese, magnesium and the rare earths also may be present in the proprietary inoculants. These materials affect and control the cast iron's microstructure and resulting physical properties.
Inoculant particle size affects inoculant effectiveness. Generally, coarser inoculants (1 in. or smaller) are used for ladle inoculation and finer materials (20 mesh and smaller) are used for mold inoculation. In either case, it is important that the inoculant be distributed evenly to affect all parts of the iron bath. It is also important that the highest concentration of inoculant be developed in the melt before the temperatures at which the graphite begins to precipitate and that this temperature level be maintained until nucleation is completed.
As a molten metal is cooled to its freezing temperature, solidification begins in the region of lowest temperature. Submicroscopic metal crystallites, called nuclei, first form then grow, generally in pine tree, or dendritic, fashion. The dentrites from each nucleus grow until the mass is completely solid. Each nucleus, then, leads to a grain or crystal.
During solidification, the metal atoms attach themselves to the growing dendrites in evenly-spaced columns and rows so that the final structure may be pictured as many unit cells stacked one upon the other. Under equilibrium (ideal) conditions, pure metals melt and freeze at a single temperature; above that temperature they are completely liquid, below it, they are completely solid. The cooling curve of a pure metal would show the metal slowly cooling to its freezing point then leveling off at a constant temperature as the metal loses its heat of fusion. Only after the metal is completely solid can further cooling occur.
When a pure metal is cooled quickly, crystal formation is sometimes delayed, and the freezing temperature drops below the freezing point. At a point below the liquid/solid temperature, nucleation of solid suddenly begins and the release of heat of fusion may raise the temperature to the equilibrium temperature. Such cooling below the normal solidification temperatures is called undercooling or supercooling. In commercial foundries producing a wide variety of alloys, undercooling is much more common, and the problems it can cause producers of gray and ductile irons have a direct bearing on the metallurgical structures of alloys. These problems make the inoculation process essential to most gray iron foundry operations.
Cast irons are alloys made of three primary elements -- iron, carbon and silicon -- of which carbon has the greatest overall effect on the iron's physical properties. These elements in turn are composed of atoms. The geometrical, three-dimensional arrangement of the iron atom is called a cubic space lattice. As the iron atoms build up, they form a crystal; as more crystals form, the result is a casting.
In like manner, when the atoms of carbon arrange themselves into a lattice and these lattices combine to form a crystal, the result is crystalline graphite, the type that forms during cast iron solidification. Atoms arrange themselves to form a lattice and then join together in an atomic cluster. However, in any atomic lattice, not all the space between the atoms is occupied. Empty spaces, called interstices, are part of each lattice. Thus, the interstices, or spaces, within the iron atom for instance, are available to accommodate other atoms, such as the smaller, lighter carbon atoms.
When these carbon atoms settle into the latticework of the iron atom, they are said to be in solid solution, but spaces in the iron lattices are limited. Only a certain amount of carbon atoms can be held in the crystal lattice of the iron atom. As the iron temperature rises, however, the percentage of dissolvable carbon the iron lattice will accept also increases.
During the first phase of solidification of an iron with less than 4.3% carbon equivalent, the first iron crystals formed are called austenite. In this phase, up to 2.0% carbon can be held (or dissolved) in the austenite solution at the end of solidifaction (compared with 0.025% dissolved in the room temperature crystal form called ferrite).
An example of this kind of saturation can be illustrated by adding sugar to a glass of water. At room temperature, the water will dissolve sugar until the water becomes saturated and there is no more space between the molecules (atoms of oxygen combined with atoms of hydrogen) for additional sugar molecules. The excess sugar will then settle out (precipitate) to the bottom of the glass.
Heat the water, and it is possible to dissolve a still greater quantity of sugar. This is because the added heat (energy) makes the water molecules move faster, weakening the bonds that hold them together and creating more spaces for sugar molecules. As the water cools, however, the water molecules slow down and begin to assume their original bonding lattice. This restricts the number of sugar molecules able to be accomodated in the cooling water molecules.
As a result, the excess sugar is forced out of the solution and settles to the bottom of the glass once again. The analogy holds true for carbon in solution with molten iron of greater than 4.3% carbon equivalent. As the molten iron cools, excess carbon participates out in the form of graphite.
Pure metals melt and freeze at a single temperature. Alloys, usually, do not. For a given alloy, there is a particular temperature at which it is liquid (the liquids temperature) and another, lower, temperature at which it is solid (the solidus temperature). In between, there is a zone where they coexist. There is the solid material with little or no strength and the liquid interspersed with the solid in what is called the mushy zone.
Alloying elements are often added to pure metals to improve the metal's foundry characteristics, i.e., lower the melting point or alter its solidification mode. Inoculation is a method of controlling the chemical characteristics of cast iron alloys.
Gray iron is composed primarily of iron, carbon and silicon. Free or uncombined carbon in cast iron is called graphite. The degree to which graphite growth is controlled during cast iron solidification determines the predictable end qualities of the castings. Solidification, time, temperature and inoculation chemistry determine the graphite shape and distribution in the gray and ductile iron castings.
Inoculation stimulates the formation of small, uniformly dispersed Type A graphite flakes that are the signature of gray iron. It does this by providing more microscopic nucleation sites for eutectic cells to form in the molten iron. As the eutectic freezing temperature is neared, there sites become precipitation points for the formation of graphite (free, or uncombined, carbon), and minimize any tendency for the molten iron to form undesirable carbines or graphite types.
Each time a eutectic cell is formed, it releases a small amount of heat to the surrounding liquid metal. This added heat slows the melt's heat loss and the melt's subsequent rate of solidification. The more eutectic cells formed, the greater the amount of heat generation and the slower the iron's solidification rate. The retarded freezing rate helps the iron to solidify at its equilibrium temperature. It also assures that Type A graphite will precipitate out of the molten iron.
Inoculation also reduces the tendency of the iron to solidify as white iron (chill) in rapidly cooled casting sections and influences the shrinkage characteristics of the iron by eliminating, or significantly lowering, carbide formation. Carbides usually result from too rapid a solidification rate caused by undercooling and nonequilibrium (faster) solidification. By providing more nucleation sites, and, therefore, greater heat of crystallization during solidification, undercooling is minimized and equilibrium solidification is aided.
Effects on Iron
The properties of all metals are influenced by the rate at which they solidify, but gray iron is especially sensitive to cooling rate. The slower solidification and cooling of heavier castings results in the formation of coarser graphite flakes and a softer matrix structure that combine to lower iron strength. Without inoculation, thin sections can solidify so rapidly that hard white iron (iron carbide) is formed instead of graphitic iron.
The graphite structures of gray and ductile cast irons significantly affect the physical properties of these metals. Their mechanical properties also depend on their microstructures. Basically, the matrix metal provides the strength: the harder and stronger the matrix, the harder and stronger the iron.
Graphite flakes have a weakening effect on iron stregth. The more and coarser the graphite flakes present, the more the strength of the iron will be reduced. The size, shape and distribution of graphite flakes and nodules develop during solidification. A random distribution of flakes in gray iron, such as Type A in small No. 5 or 6 sizes, is considered desirable. Graphite classifications are illustrated in Fig. 1.
The small, Type A graphite flakes will precipitate uniformly if the solidification rate is slow enough. Too rapid a solidification rate will result in chill or the formation of iron carbides and/or less desirable graphite (B, D and E types). Inoculation prevents the formation of Types B and D graphites (usually associated with areas of ferrite) and promotes a pearlitic casting matrix that has superior machinability.
The graphite, up to this precipitation point, has been dissolved in the molten gray iron as elemental carbon, but the introduction of the inoculant facilitates the carbon coming out of solution as graphite when the eutectic freezing point is reached as the metal cools. Coming out of solution, the graphite flakes build at the nucleation sites as solidification proceeds.
Proper inovulation assures that enough nucleation sites are formed to precipitate graphite flakes sized and configured to satisfy the metallurgical requirements of the iron alloy being cast. Both gray and ductile irons require inoculation to control the graphite structure of ferrous metal castings. Typically, the graphite in gray iron forms as flakes distributed evenly through the metal matrix. In ductile iron, the graphite forms as individual spheres, or nodules, dispered throughout the matrix.
Even after solidification, graphite can continue to grow slightly in volume as the solubility of carbon in austenite is reduced and the rejected carbon migrates to existing flakes or nodules. The number of nucleation sites formed before freezing determines the physical characteristics of the subsequent gray or ductible iron casting.
The lower the temperature of the molten iron at the time of pouring into the mold, whether inoculated or not, the greater will be the chilling characteristics of the iron. As the iron freezes, some undercooling takes place allowing fewer nucleation sites to form.
Inoculation decreases section sensitivity, a condition resulting from structural differences between thick and think sections within the same casting. This sensitivity is caused by different rates of solidification in these sections. Inoculation compnesates for this by providing graphite flakes of more uniform shape and size throughout the casting.
The type of iron being made is largely established as the iron solidifies. Because the properties of gray iron are greatly influenced by the thermal and chemical changes occurring from its molten stage to cooled casting, more nucleation sites and more heat of crystalization limit undercooling and promote equilibrium, or slower, solidification.
Gray iron is highly sensitive to the rate at which the metal cools through the eutectic freezing range. Slow cooling permits the alloy to be in the eutectic range for a longer time than does rapid cooling. The longer period encourages graphite nucleation and graphite flake growth. In contrast, rapid cooling dors not allow sufficient time for the iron in the eutectic range and undercooling occurs, thus, limiting graphite formation.
Because the nucleation effect of the inoculant peaks quickly then fades rapidly (estimated at a 50% fade rate for every five minutes after inoculation), the inoculant preferably is added immediately preceding or during the mold pouring operation. To further delay the introduction of inoculant to the metal stram as long as possible, another option is to inoculate in the mold itself.
The carefully-timed infusion of inoculant elements assures that predetermined metallographic characteristics are met because graphite form and structure are a function of metal chemistry, temperature, cooling rate and the inoculant effectiveness. The degree to which graphite formation is controlled during solidification determines predictable castings. The graphite that forms as gray iron solidifies can assume a number of shapes depending on freezing time and temperature.
The metallurgist seeks graphite in a gray iron matrix to be arranged so that resultant castings achieve the levels of tensile strength, hardness and machinability prescribed by the casting buyer's specifications. The melter's task is to cause graphite in the desired form and shape to crystallinze during solidification at inoculant-induced nucleation sites.
In a definitive test  to gauge the connection between pouring time and temperature and the effect of inoculation, a heat of inoculated gray iron was tapped into a receiving ladle and transferred to three pouring ladle without benefit of further inoculation.
The first ladle, poured immediately at 2750F showed primarily size 4-5 Type A graphite in a medium spaced pearlitic matrix having an average tensile strength of 34,000 psi and an average Brinell hardness number of 190.
The second pouring ladle was filled five minutes later, but at 2680F. It showed some Type A graphite, considerable amounts of B graphite and near the surface of the casting some Type D graphite. The matrix structure was coarse pearlite and considerable amounts of ferrite, a matrix usually associated with Types B and D graphite. Tensile strength had dropped to 28,000 psi and Brinell hardness numbers averaged 170.
The differences in the structure and mechanical properties between the first and second set of castings poured involved temperature loss and inoculant fade. The lower temperature caused the castings to solidify faster, resulting in some undercooling. Inoculant fade reduced formation of graphite nucleation sites in the cooling metal. The addition of more inoculant, even at the reduce temperature, would have reduced the degree of undercooling and subsequent loss of nucleation sites.
The third ladle of iron, poured at 2500F and 20 minutes after the original transfer, showed graphite deterioration caused by temperature loss in the molten metal. Some Type B graphite was present but most was Type D, and evidence showed the presence of carbides, or chill, at the outer edges of test castings. No Type A graphite was present. The mechanical properties of the castings were poor compared to the previous two pours: 24,000 psi, 140 Bhn mixed with a surface hardness of 330 Bhn due to chill.
Poor microstructures and physical properties of castings from the third laddle were due primarily to cold metal, in oculant fade and oxidation of the iron, the latter caused by exposure to the air prior to pouring.
There are two types of inoculants for cast iron: one for graphitizing and the other for stabilizing. Both are used in gray iron metallurgy, but only the graphitizers are used in ductile cast irons to achieve a desired nodule count and shape.
* Graphitizing inoculants promote the precipitation of dissolved carbon as graphite during solidification. They also minimize the formation of iron carbides (by preventing undercooling) and limit edge chill in castings.
* Stabilizing inoculants promote the formation of graphites during solidification, too, but they also promote the formation of fine pearlite during solid-state cooling to produce high strength castings with a minimum amount of chill. The lower the carbon equivalent of the iron, the less the amount of inoculant necessary. Fading is not so critical as with graphitizing inoculants. So long as the metal is hot enough to dissolve the inoculant and able to prevent misruns when poured, inoculation should be effective.
There is no precise answer as to why nucleation and subsequent graphite formation occurs as the result of inoculation, but it is sufficient to know that adding the right materials, properly sized and at the right time and temperature causes graphite to crystallize at established nucleation sites. The more sites, the finer the graphite structure and, in the case of ductile iron, the higher the nodule count.
How the metal reacts to the inoculation additives determines the graphite forms in the solidified iron matrix. The types of graphite formed enable the control of physical properties of gray and ductile irons that make these irons so important to industry.
It is important that inoculant be dry prior to use and that the correct amount by weight or volume be used. Too much inoculant addition could cause casting porosity. It is equally important to thoroughly mix inoculatnt into the metal and to avoid inoculating metal at too low a temperature. Regular chill testing should be used to check the efficiency of the inoculant.
Several current foundry terms regarding gray iron production include the following.
Equilibrium--This is a term used to describe the solidification condition of gray iron. It represents a balance between temperature and time during which molten iron goes through its various freezing phases from molten to fully cooled iron in the shortest practical period. Equilibrium exists when the solidification rate is slow enough so that the carbon in solution precipitates out in the form of Type A graphite.
The lower the temperature of the molten metal when it is poured, the faster the solidification rate. Under-cooling can result in gray iron castings that will contain either carbides that cause machining problems or Types B and D graphite which are generally undesirable because they are surrounded by a ferritic matrix and are normally softer and weaker than those containing Type A graphite.
* Undercooling--A condition that results when a molten metal solidifies below its normal freezing temperature. Inoculation helps prevent this. Undercooling is caused by the iron cooling too rapidly for sufficient numbers of nucleation sites to form. Nucleation occurs in the first stage in solidification of liquids. Without sufficient nuclei present at the equilibrium temperature of solidification, the molten iron will undercool before it starts to freeze.
Undercooling has a marked effect on the microstructure and mechanical properties of cast iron. Too rapid a cooling rate hinders the excess carbon from coming out of solution and reaching nucleation sites. Both iron and carbon will stay in solution past the eutectic point. Eventually the iron will freeze, squeezing out the free carbon which will then attach itself to iron atoms to form undesirable [Fe.sub.3.C] (iron carbide).
If eutectic or hypoeutectic iron solidifies under equilibrium conditions, Type A graphite will result. If the freezing rate falls too rapidly, the graphite types will progressively change from A to less desirable B and D. A further increase in the solidification rate will result in the formation of carbides. If the rate of solidification is fast enough, white iron will be formed.
* Inoculant fade -- The effects of inoculation are at peak potency immediately after the inoculant is added to the molten metal and then quickly begin to fade. The rate of fade depends upon the composition of the inoculant and the iron to which it is added.
Inoculation effects may last only for a matter of minutes, and, for that reason, many foundries impose a limit on the time that iron can be held in the ladle after inoculation. Systems exist that add inoculants at the last possible moment before pouring, and in-mold inoculation is an alternative.
* Matrix structure--The metal that surrounds the free carbon in a cast iron establishes the properties of the iron. This metal is called the matrix, and its form depends upon the analysis of the metal, the structure that develops during solidification and the cooling rate of the casting after the metal has solidified.
 D.P. Kanicki, "Cast Iron Inoculation: Understanding the Basics," modern casting, pp 44-48 (Aug 1979).
 Cupola Handbook, 5th Ed., American Foundrymen's Socity, Inc., Des Plaines, Illinois (1984).
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|Date:||Nov 1, 1991|
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