Tài liệu ASM Metals HandBook P2 pdf

°C °F °C °F 2.52 1295 2360 160 290 3.04 1245 2270 210 380 3.60 1175 2150 280 500 Microstructure The usual microstructure of gray iron is a matrix of pearlite with graphite flakes dispersed throughout. Foundry practice can be varied so that nucleation and growth of graphite flakes occur in a pattern that enhances the desired properties. The amount, size, and distribution of graphite are important. Cooling that is too rapid may produce so-called chilled iron, in which the excess carbon is found in the form of massive carbides. Cooling at intermediate rates can produce mottled iron, in which carbon is present in the form of both primary cementite (iron carbide) and graphite. Very slow cooling of irons that contain large percentages of silicon and carbon is likely to produce considerable ferrite and pearlite throughout the matrix, together with coarse graphite flakes. Flake graphite is one of seven types (shapes or forms) of graphite established in ASTM A 247. Flake graphite is subdivided into five types (patterns), which are designated by the letters A through E (see Fig. 2). Graphite size is established by comparison with an ASTM size chart, which shows the typical appearances of flakes of eight different sizes at 100× magnification. Fig. 2 Types of graphite flakes in gray iron (AFS- ASTM). In the recommended practice (ASTM A 247), these charts are shown at a magnification of 100×. They have been reduced to one-third size for reproduction here. Type A flake graphite (random orientation) is preferred for most applications. In the intermediate flake sizes, type A flake graphite is superior to other types in certain wear applications such as the cylinders of internal combustion engines. Type B flake graphite (rosette pattern) is typical of fairly rapid cooling, such as is common with moderately thin sections (about 10 mm, or 3 8 in.) and along the surfaces of thicker sections, and sometimes results from poor inoculation. The large flakes of type C flake graphite are typical of kish graphite that is formed in hypereutectic irons. These large flakes enhance resistance to thermal shock by increasing thermal conductivity and decreasing elastic modulus. On the other hand, large flakes are not conducive to good surface finishes on machined parts or to high strength or good impact resistance. The small, randomly oriented interdendritic flakes in type D flake graphite promote a fine machined finish by minimizing surface pitting, but it is difficult to obtain a pearlitic matrix with this type of graphite. Type D flake graphite may be formed near rapidly cooled surfaces or in thin sections . Frequently, such graphite is surrounded by a ferrite matrix, resulting in soft spots in the casting. Type E flake graphite is an interdendritic form, which has a preferred rather than a random orientation. Unlike type D graphite, type E graphite can be associated with a pearlitic matrix and thus can produce a casting whose wear properties are as good as those of a casting containing only type A graphite in a pearlitic matrix. There are, of course, many applications in which flake type has no significance as long as the mechanical property requirements are met. Solidification of Gray Iron. In a hypereutectic gray iron, solidification begins with the precipitation of kish graphite in the melt. Kish grows as large, straight, undistorted flakes or as very thick, lumpy flakes that tend to rise to the surface of the melt because of their low relative density. When the temperature has been lowered sufficiently, the remaining liquid solidifies as a eutectic structure of austenite and graphite. Generally, eutectic graphite is finer than kish graphite. In hypoeutectic iron, solidification begins with the formation of proeutectic austenite dendrites. As the temperature falls, the dendrites grow, and the carbon content of the remaining liquid increases. When the increasing carbon content and decreasing temperature reach eutectic values, eutectic solidification begins. Eutectic growth from many different nuclei proceeds along crystallization fronts that are approximately spherical. Ultimately, the eutectic cells meet and consume the liquid remaining in the spaces between them. During eutectic solidification, the austenite in the eutectic becomes continuous with the dendritic proeutectic austenite, and the structure can be described as a dispersion of graphite flakes in austenite. After solidification, the eutectic cell structure and the proeutectic austenite dendrites cannot be distinguished metallographically except by special etching or in strongly hypoeutectic iron. With eutectic compositions, obviously, solidification takes place as the molten alloy is cooled through the normal eutectic temperature range, but without the prior formation of a proeutectic constituent. During the solidification process, the controlling factor remains the rate at which the solidification is proceeding. The rapid solidification favored by thin section sizes or highly conductive molding media can result in undercooling. Undercooling can cause the solidification to start at a temperature lower than the expected eutectic temperature for a given composition (Fig. 3). This can result in a modification of the carbon form from A to E type or can completely suppress its formation and form primary carbides instead. Room-Temperature Structure. Upon cooling from the eutectic temperature, the austenite will decompose, first by precipitating some of the dissolved carbon and then, at the eutectoid temperature, by undergoing complete transformation. The actual products of the eutectoid transformation depend on rate of cooling as well as on composition of the austenite, but under normal conditions the austenite will transform either to pearlite or to ferrite plus graphite. Transformation to ferrite plus graphite is most likely to occur with slow cooling rates, which allow more time for carbon migration within the austenite; high silicon contents, which favor the formation of graphite rather than cementite; high values of carbon equivalent; and the presence of fine undercooled (type D) flake graphite. Graphite formed during decomposition is deposited on the existing graphite flakes. When carbon equivalent values are relatively low or when cooling rates are relatively fast, the transformation to pearlite is favored. In some instances, the microstructure will contain all three constituents: ferrite, pearlite, and graphite. With certain compositions, especially alloy gray irons, it is possible to produce a martensitic matrix by oil quenching through the eutectoid transformation range or an austempered matrix by appropriate isothermal treatment (Ref 1). These treatments are often done deliberately in a secondary heat treatment where high strength or hardness is especially desired, such as in certain wear applications. The secondary heat treatment of gray iron castings is of great value in producing components that must be hard when machining requirements prohibit the use of components that are cast to final shape in white iron. Reference cited in this section 1. M.D. VanMaldegiam and K.B. Rundman, On The Structure and Properties of Austempered Gray Cast Iron, Trans. AFS, 1986, p 249 Section Sensitivity Fig. 3 Undercooling from rapid cooling of a eutectic composition In practice, the minimum thickness of section in which any given class of gray iron may be poured is more likely to depend on the cooling rate of the section than on the fluidity of the metal. For example, although a plate 300 mm (12 in.) square by 6 mm (0.24 in.) thick can be poured in class 50 as well as in class 25 iron, the former casting would not be gray iron because the cooling rate would be so rapid that massive carbides would be formed. Yet it is entirely feasible to use class 50 iron for a diesel engine cylinder head that has predominantly 6 mm (0.24 in.) wall sections in the water jackets above the firing deck. This is simply because the cooling rate of the cylinder head is reduced by the "mass effect" resulting from enclosed cores and the proximity (often less than 12 mm, or 0.47 in.) of one 6 mm (0.24 in.) wall to the other. Thus the shape of the casting has an important bearing on the choice of metal specification. It should be recognized that the smallest section that can be cast gray, without massive carbides, depends not only on metal composition, but also on foundry practices. For example, by adjusting silicon content or by using graphitizing additions called inoculants in the ladle, the foundryman can decrease the minimum section size for freedom from carbides for a given basic composition of gray iron. The mass effect associated with increasing section thickness or decreasing cooling rate is much more pronounced in gray iron than in cast steel. The mass effect in cast steel results in increased grain size in heavy sections. This also applies to gray iron, but the most important effects are on graphite size and distribution, and on amount of combined carbon. For any given gray iron composition, the rate of cooling from the freezing temperature to below about 650 °C (1200 °F) determines the ratio of combined to graphitic carbon, which controls the hardness and strength of the iron. For this reason the effect of section size in gray iron is considerably greater than in the more homogeneous ferrous metals in which cooling rate does not affect the form and distribution of carbon throughout the metal structure. Typical Effects of Section Size. When a wedge-shape bar with about a 10° taper is cast in a sand mold and sectioned near the center of the length, and Rockwell hardness determinations are made on the cut surface from the point of the wedge progressively into the thicker sections, the curves so determined show to what extent continually increasing section size affects hardness (Fig. 4). Fig. 4 Effect of section thickness on hardness and structure. Hardness readings were taken at increasing distance from the tip of a cast wedge section, as shown by inset. Composition of iron: 3.52% C, 2.55% Si, 1.01% Mn, 0.215% P, and 0.086% S. Source: Ref 2 Progressing along the curve from the left in Fig. 4, the following metallographic constituents occur. The tip of the wedge is white iron (a mixture of carbide and pearlite) with a hardness greater than 50 HRC. As the iron becomes mottled (a mixture of white iron and gray iron), the hardness decreases sharply. A minimum is reached because of the occurrence of fine type D flake graphite, which usually has associated ferrite in large amounts. With a slightly lower cooling rate, the structure becomes fine type A flake graphite in a pearlite matrix with the hardness rising to another maximum on the curve. This structure is usually the most desirable for wear resistance and strength. With increasing section thickness beyond this point, the graphite flakes become coarser, and the pearlite lamellae become more widely spaced, resulting in slightly lower hardness. With further increase in wedge thickness and decrease in cooling rate, pearlite decomposes progressively to a mixture of ferrite and graphite, resulting in softer and weaker iron. The structures of most commercial gray iron castings are represented by the right-hand downward-sloping portion of the curve in Fig. 4, beyond 5 mm (0.2 in.) wedge thickness, and increasing section size is normally reflected by the gradual lowering of hardness and strength. However, thin sections may be represented by the left-hand downward-sloping portion. Figure 5 shows the average tensile strength (up to ten tests per point) of two irons, for each of which six sizes of cylindrical round bars were cast and appropriate tensile specimens machined. With the class 20 iron, strength increases as the as-cast section decreases down to the 6 mm (0.24 in.) cast bar. However, for the class 30 iron, a section 6 mm (0.24 in.) in diameter is so small that the strength falls off sharply, because of the occurrence of type D flake graphite or mottled iron, or both. The other graph in Fig. 5 shows similar data for the same two classes of iron and for three higher classes. Fig. 5 Effect of section diameter on tensile strength at center of cast specimen for five classes of gray iron Section sensitivity effects are used in the form of a wedge test in production control to judge the suitability of an iron for pouring a particular casting. In this test, a wedge-shape casting is poured and upon solidification is evaluated. The standard W2 wedge block specified in ASTM A 367 is shown in Fig. 6. The evaluation consists of measuring the length of the "chilled zone." The measurement, usually made in 0.8 mm ( 1 32 in.) increments, is related to empirically determined data obtained from a "good" casting. If the evaluation indicates an excessive sensitivity for a part, corrections are made to the molten metal prior to pouring. Fig. 6 Standard W2 wedge block used for measuring depth of chill (ASTM A 367). Dimensions given in inches Volume/Area Ratios. It is extremely difficult to predict with accuracy the cooling rate for castings other than fairly simple shapes. However, because minimum limitations are involved here, the problem can be resolved through comparisons of the casting design with ratios of volume to surface area or with minimum plate sections. The volume/area (V/A) ratios for round, square, and plate sections provide a fairly accurate indication of the minimum casting sections possible in simple geometrical shapes (Table 2). The V/A ratios can be reported in either English or metric units and can be converted simply by treating them as length measurements. Table 2 Volume/area (V/A) ratios for round bars, square bars, and plates V/A ratio Cast form and size mm in. Bar, 13 mm ( 1 2 in.) diam × 533 mm (21 in.) 3.1 0.12 Bar, 13 mm ( 1 2 in.) square × 533 mm (21 in.) 3.1 0.12 Plate, 6.4 × 305 × 305 mm ( 1 4 × 12 × 12 in.) 3.0 0.12 Bar, 30 mm (1.2 in.) diam × 533 mm (21 in.) (a) 7.4 0.29 Bar, 30 mm (1.2 in.) square × 533 mm (21 in.) 7.4 0.29 Plate, 16 × 305 × 305 mm ( 5 8 × 12 × 12 in.) 7.1 0.28 Bar, 50 mm (2 in.) diam × 560 mm (22 in.) 12.2 0.48 Bar, 50 mm (2 in.) square × 560 mm (22 in.) 12.2 0.48 Plate, 28.5 × 305 × 305 mm (1 1 8 × 12 × 12 in.) 12.0 0.47 Bar, 100 mm (4 in.) diam × 460 mm (18 in.) 22.9 0.90 Bar, 100 mm (4 in.) square × 460 mm (18 in.) 22.9 0.90 Plate, 65 × 305 × 305 mm (2 9 16 × 12 × 12 in.) 22.8 0.90 Bar, 150 mm (6 in.) diam × 460 mm (18 in.) 32.7 1.29 Bar, 150 mm (6 in.) square × 460 mm (18 in.) 32.7 1.29 Plate, 114 × 305 × 305 mm (4 1 2 × 12 × 12 in.) 32.7 1.29 Source: Ref 3 (a) ASTM size B test bar Comparison of the ratios of volume to surface area for different shapes gives good agreement with the actual cooling rates of castings made in the same mold material. For long round bars and infinite flat plates, V/A is diameter/4 for bars and thickness/2 for plates; that is, a large plate casting would have the same cooling rate as a round bar with a diameter twice the plate thickness. Most castings, however, freeze somewhat faster than an infinite flat plate, and rather than establishing a 2-to-1 ratio of bar to plate, a smaller ratio will often give a better correlation with the cooling rate. The bar and plate sizes shown in Table 3 are nearly equivalent in cooling rate. Table 3 Bar and plate sizes of equivalent cooling rate For 305 mm (12 in.) square plates, as recorded in Table 2 Bar diameter, in. Plate thickness, in. Ratio of bar diameter to plate thickness 1 2 1 4 2.0 1.2 5 8 1.92 2 1 1 8 1.78 4 2 9 16 1.56 6 4 1 2 1.33 Similar comparisons have been made for production castings. In one study, the properties of a flat section from a 0.6 m (24 in.) cross pipe fitting having a nominal thickness of 29.5 mm (1.16 in.) were compared with the properties of a 50 mm (2 in.) diam cylindrical test bar cast from the same heat. The tensile strengths of the test bars were within about 16 MPa (2.3 ksi) of the tensile strengths of the cross pipe fittings for eight heats ranging in strength from about 205 to 310 MPa (30 to 45 ksi), an average variation of less than 8%. These results from production castings correlate well with the calculated equivalence given in Table 3. Other examples of this type of correlation are given in Ref 3. Relationships developed for various specific castings are valid when an iron of controlled composition, and therefore of similar section sensitivity, is used consistently. For instance, with a copper-molybdenum iron of well-controlled composition, a tensile strength of 450 MPa (65 ksi) in the ASTM B test specimen has been found to ensure 345 MPa (50 ksi) tensile strength in a cast crankshaft 2.13 m (7 ft) long with sections thicker than 30.5 mm (1.2 in.). Such translation of properties of a small test bar to properties expected in a larger section cannot be done indiscriminately, because different irons may vary widely in section sensitivity. References cited in this section 2. R. Schneidewind and R.G. McElwee, Composition and Properties of Gray Iron, Parts I and II, Trans. AFS, Vol 58, 1950, p 312-330 3. H.C. Winte, Gray Iron Castings Section Sensitivity, Trans. AFS, Vol 54, 1946, p 436-443 Prevailing Sections Although the ASTM size B test bar (30.5 mm, or 1.2 in., diam) is the bar most commonly used for all gray irons from class 20 to class 60, ASTM specification A 48 provides a series of bar sizes from which one that approximates the cooling rate in the critical section of the casting can be selected. In practice, it is customary to be somewhat more definite regarding the prevailing values of minimum casting section considered feasible for the various ASTM classes of cast iron. As summarized in Table 4, these minimum, prevailing sections include the requirement of freedom from carbidic areas. In a platelike section, occasional thinner walls (such as ribs) are of no importance unless they are very thin or are appended to the outer edges of the casting. Table 4 Minimum prevailing casting sections recommended for gray irons Minimum thickness V/A ratio (a) ASTM A48 class in. mm in. mm 20 1 8 3.2 0.06 1.5 25 1 4 6.4 0.12 3.0 30 3 8 9.5 0.17 4.3 35 3 8 9.5 0.17 4.3 40 5 8 15.9 0.28 7.1 50 3 4 19.0 0.33 8.4 60 1 25.4 0.42 10.7 (a) V/A ratios are for square plates. Mechanical properties of class 30 and class 50 gray irons in various sections are shown in Fig. 7. For class 30 iron, the combined carbon content and hardness are still at a safe level in sections equivalent to a 10 mm (0.4 in.) plate, which has a V/A ratio of about 5 mm (0.20 in.). For class 50 iron, however, both combined carbon and Brinell hardness show marked increases when the thickness of the equivalent plate section is decreased to about 15 mm (0.6 in.), with V/A ratio around 7 mm (0.27 in.). These results are consistent with the minimum prevailing casting sections recommended in Table 4. Fig. 7 Mechanical properties of class 30 and class 50 gray iron as a function of section size. Composition of the class 30 iron: 3.40% C, 2.38% Si, 0.71% Mn, 0.423% P, and 0.152% S; for the class 50 iron: 2.96% C, 1.63% Si, 1.05% Mn, 0.67% Mo, 0.114% P, and 0.072% S. Source: Ref 4 The hazards involved in pouring a given class of gray iron in a plate section thinner than recommended are discovered when the casting is machined. Typical losses as a result of specifying too high a strength for a prevailing section of 9.5 mm ( 3 8 in.) are given below (rejections were for "hard spots" that made it impossible to machine the castings by normal methods): Class Rejections, % 35 Negligible 45 25 55 80-100 In marginal applications, a higher class of iron may sometimes be used if the casting is cooled slowly (in effect, increasing the section thickness) by judicious placement of flow-offs and risers. An example is the successful production of a 25 mm (1 in.) diam single-throw crankshaft for an air compressor. This shaft was hard at the extreme ends when poured in class 50 iron. The difficulty was corrected by flowing metal through each end into flow-off risers that adequately balanced the cooling rate at the ends with the cooling rate at the center. In sum, the selection of a suitable grade of gray iron for a specific casting necessarily requires an evaluation of the size and shape of the casting as related to its cooling rate, or volume/area ratio. For a majority of parts, this evaluation need be no more than a determination of whether or not the V/A ratio of the casting exceeds the minimum V/A ratio indicated for the grade considered. Reference cited in this section 4. R.A. Flinn and R.W. Kraft, Improved Test Bars for Standard and Ductile Grades of Cast Iron, Trans. AFS, Vol 58, 1950, p 153-167 Test Bar Properties Mechanical property values obtained from test bars are sometimes the only available guides to the mechanical properties of the metal in production castings. When test bars and castings are poured from metal of the same chemical history, correlations can be drawn between the thermal history of the casting and that of the test bar. The strength of the test bar gives a relative strength of the casting, corrected for the cooling rate of the various section thickness. Through careful analysis of the critical sections of a casting, accurate predictions of mechanical behavior can be achieved. Usual Tests. Tension and transverse tests on bars that are cast specifically for such tests are the most common methods used for evaluating the strength of gray iron. Yield strength, elongation, and reduction of area are seldom determined for gray iron in standard tension tests. The transverse test measures strength in bending and has the additional advantage that a deflection value may be obtained readily. Minimum specification values are given in Table 5. Data can usually be obtained faster from the transverse test than from the tension test because machining of the specimen is unnecessary. The surface condition of the bar will affect the transverse test but not the tension test made on a machined specimen. Conversely, the presence of coarse graphite in the center of the bar, which can occur in an iron that is very section sensitive, will affect the tension test but not the transverse test. Table 5 Transverse breaking loads of gray irons tested per ASTM A 438 Corrected transverse breaking load (a) Approximate tensile strength A bar (b) B bar (c) C bar (d) ASTM class (a) MPa ksi kg lb kg lb kg lb 20 138 20 408 900 816 1800 2720 6,000 25 172 25 465 1025 907 2000 3080 6,800 30 207 30 522 1150 998 2200 3450 7,600 . size in gray iron is considerably greater than in the more homogeneous ferrous metals in which cooling rate does not affect the form and distribution of carbon