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Foote Fo.Undry Facts • FOOTE FO.UNDRY FACTS PUBLISHED BY FOOTE MINERAL COMPANY, EXTON, PENNA. 19341 NUMBER 2 "LEARN TO CONTROL GRAPHITE TYPE AND PARTICLE SIZE IN GRAY AND DUCTILE CAST IRONS," SAYS PATTERSON K eeping carbon under control is so important that we're Regardless of the matrix structure, however, any carbon going to devote the next several issues of FOOTE FOUNDRY in the form of graphite will tend to lower strength. Depend· FACTS to a discussion of how carbon behaves, and what we ing upon the type of graphite and size of the particles, impor· can do to improve its behavior. This will obviously lead us tant mechanical and physical properties may vary as much to the subject of inoculants, and their use in gray and as 50-60%. The need for control is therefore obvious. ductile iron. Below we have shown typical exa.mples of the principal Although our primary concern will be with the "free types of graphite structure which characterize the main carbon" which precipitates in the form of graphite, rather classifications of cast iron. Our emphasis will be upon the tha.n the carbon combined in the matrix of the cast iron, we first two types, gray and ductile cast iron, since only with realize that the latter has a sub· these two types can we control graphite formation through stantial dfect on the properties of the use of inoculants. the cast iron, depending on the rate of cooling after solidification. GRAPIDTE STRUCTURES OF mE FOUR Alloying elements, such as chro· GENERAL TYPES OF CAST ffiON mium, molybdenum, nickel, copper, tin and vanadium, also influence GRAY CAST IRON: the matrix structure. I ' , l Graphite u in fl4ke form, _which u detri­ ....,. I mental to ductility. Depending upon VERNON H. PATTERSON Briefly, there are normally six compOsition and solidification rate, OM types of matrix structures which or more of five general types of flakes mar be formed. Size of the fl4kes also can be present in cast iron. These include ferrite, pearlite affect physical and mechanical properties. (coarse and fine), acicular, tempered martensite, and a combination of free cementite and pearlite. DUCTILE CAST IRON: ..... Ferrite is the weakest and softest of the matrix struc· Graphile is in nodular or spheroidal • .. form which favors hish strensth and ~ .. tures and can be present in gray, ductile and malleable iron. 1 .. ductility. Notch effect produced by flake . • Generally, slow cooling after solidification favors the forma· graphite in gray cast iron i$ absent. •• • tion of ferrite. A faster rate of cooling in the solid state, Nodules are produced by treatment of • • molten base .iron. with ma1f11esium, cer· • • or the addition of alloys favor the formation of coarse, or ium, or other elements, foUowed by ade· fine pearlite, depending on the rate of cooling. The faster quate post inoculation. • •' the cooling, the finer the pea~lite. A still faster rate of cooling promotes the formation of martensite, a hard, brittle WHITE CAST IRON: transition constituent, which can be toughened· by tempering AU of the carbon is in the combined at temperalures ranging from 500 to 750°F. Certain alloy· form known as cementite (Fe~). No free ferTile is present. These conditions ing elements when present in the iron assist in the formation uplain the high wear rewtance of this of martensite. The acicular, or bainitic, structure can be material. produced in gray, ductile and malleable iron by special heat treatment. However, it is generally produced by the addition MALLEABLE IRON: of suitable combinations of nickel and molybdenum. The T hroush suitable heat treatment, graph· acicular structure is hard and tough. Unlike a martensitic ite in the form of temper carbon i$ structure, it can be machined with proper cutting tools, but precipitated from the matrix of white cast iron. TAU is generally an asgre8fJU ) with difficulty. The combination of cementite and pearlite, or sraphite of irre,uiar shape, more favor· martensite, is the typical matrix structure of white cast iron. able to strength than the flake form, but slishtlr less desirable than the true It is hard, brittle, and! resistant to wear. spheroidal form. PAGE 2 FOOTE FOUNDRY FACTS BASIC PRINCIPLES When the solidification rate is slow a.nd equilibrium condi­ In general, the higher the carbon content, the greater tions exist, the eutectic graphite will precipitate and grow as the number of graphite particles, and the larger the average Type A flakes. The size of the flakes will depend upon the ' size. Rap!d cooling, but not fast enough to cause chill, tends section size and the number of nucleating cehters. The to produce many, small-sized particles; slow cooling encour· thinner the section (hence, the faster the cooling rate) and ages larger and fewer particles. By suitable treatment the gteater the number of nucleating centers, the smaller in the molt~n state, followed by controlled cooling, we can will be the flake size. The stronger irons are usually aaso· produce a large number of small graphite particles which ciated with small, Type A graphite flakes, having sizes 4 are well dispersed in the casting. This type of structure is through 6 shown in above table. One of the functions of an most desirable from the standpoint of strength. The amount inoculant is to create sufficient centers of nucleation to insure of carbon in solution in the liquid phase prior to cooling the smallest Type A graphite flake possible in a given section determines how much is available for graphite formation. thickness. It is generally believed that carbon precipitates from If the optimum number of nucleation centers is not the molten phase, or nuclei that are present in the liquid. As available, and the rate of solidification is a little too great, the temperature decreases, the tendency is to build upon Type B, or rosette type graphite will result. Irons with existing nuclei rather than form new ones. One of the func­ Type B graphite will generally not be so strong as the same tions of inoculation is to increase, through additions to composition iron in the same section size but having small the molten phase, more nuclei upon which the precipitating Type A graphite flakes. This is particularly true in gray carbon can build. The result is more graphite particles, hut irons of hypoeutectic composition (carbon plus one-third of a smaller average size. silicon is less than 4.3%.) If this same composition iron in the same section size The natural tendency of graphite is to form from the should solidify at a slightly faster rate than that which molten phase in the shape of flakes-typical of the structure produced Type B graphite flakes, more undercooling would of gray iron, shown on page 1. To form the nodules which be involved, and Type D graphite would result. This type are characteristic of the ductile iron structure we must of graphite is known as interdendritic and is usually asso­ resort to the addition of certain elements which promote this ciated with considerable ferrite in the matrix, and thus type of graphite structure: magnesium and cerium, for undesirable from a strength standpoint. example. However, the relationship between average size As an example of the e1Ject of the graphite flake type and average number of graphite particles will apply to both on the tensile strength of cast iron, let us consider a typical gray and ductile iron. cast iron having an analysis of 3.00 C and 2.40 Si. GRAPHITE FLAKES IN GRAY CAST IRON Table I shows the comparative tensile strengths in a 1.2'' For micrographic study, it is convenient to consider test bar. graphite flakes as occurring in five general configurations TABLE I (see figure at bottom of page). The size of flakes may he divided into 8 groups, according to their measured length at Test Bar Graphite Tensile Matrix 100 magnittcations: No. Type Strength Structure Size 1 (4''+) Size 5 (~"·Y2") 1 A 38,000 psi medium to coarse pearlite Size 2 (2"-4") Size6 (Ys"·~") 2 B 33,000psi coarse pearlite Size 3 (1"-2'') Size 7 (1/16"-1/8") and ferrite Size 4 (Y2"·1") Size 8 (-1/16") 3 D 25,000 psi mostly ferrite FIVE TYPES OF GRAPHITE FLAKE TYPE A TYPE B TYPE C TYPED TYPE E ) Microstructure photographs reprinted by permission of Precisicn Scientific Co., Chicago, Illinois. FOOTE FOUNDRY FACTS PAGE 3 •) There is no question that the matrix structure markedly predominantly fine pearlitic matrix, but having some of affects the tensile strength of cast iron. However, in this the Type C graphite replaced with Type A graphite, as example, it is because of the graphite flake types in test bars shown in Figure 2, will still be termed a low strength gray 2 and 3 that ferrite is present in the matrix. Proper inocu· iron. However, its tensile strength will be considerably lation in test bars 2 and 3 would have caused small Type A graphite flakes to precipitate during solidification, with the FIGURE 2 accompanying pearlitic, or higher strength matrix. Low Strensth Gray Iron In the case of test bar 3, the tensile strength would Types A, C and D have been considerably higher if the matrix structure was Fine Pearlite and Ferrite pearlite, rather than ferrite. This would have required 20 ,IXXJ to 30 ,IXXJ psi TS either a heat treatment, or a fast cooling rate after solidifi. cation in order to retain sufficient carbon in solution in the higher on the average than the iron in Figure 1 exhibiting iron to form the pearlite. more Type C graphite, even though there is more pearlite Type C graphite is generally associated with gray cast in the matrix.
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