Casting (1998) Part 11 ppt

MALLEABLE IRON is a cast ferrous metal that is initially produced as white cast iron and is then heat treated to convert the carbon-containing phase from iron carbide to a nodular form of graphite called temper carbon. There are two types of ferritic malleable iron: blackheart and whiteheart. Only the blackheart type is produced in the United States. This material has a matrix of ferrite with interspersed nodules of temper carbon. Cupola malleable iron is a blackheart malleable iron that is produced by cupola melting and is used for pipe fittings and similar thin-section castings. Because of its low strength and ductility, cupola malleable iron is usually not specified for structural applications. Pearlitic malleable iron is designed to have combined carbon in the matrix, resulting in higher strength and hardness than ferritic malleable iron. Martensitic malleable iron is produced by quenching and tempering pearlitic malleable iron. Malleable iron, like ductile iron, possesses considerable ductility and toughness because of its combination of nodular graphite and low-carbon metallic matrix. Because of the way in which graphite is formed in malleable iron, however, the nodules are not truly spherical as they are in ductile iron but are irregularly shaped aggregates. Malleable iron and ductile iron are used for some of the same applications in which ductility and toughness are important. In many cases, the choice between malleable and ductile iron is based on economy or availability rather than on properties. In certain applications, however, malleable iron has a distinct advantage. It is preferred for thin-section castings; for parts that are to be pierced, coined, or cold formed; for parts requiring maximum machinability; for parts that must retain good impact resistance at low temperatures; and for some parts requiring wear resistance (martensitic malleable iron only). Ductile iron has a clear advantage where low solidification shrinkage is needed to avoid hot tears or where the section is too thick to permit solidification as white iron. (Solidification as white iron throughout a section is essential to the production of malleable iron.) Malleable iron castings are produced in section thicknesses ranging from about 1.5 to 100 mm ( 1 16 to 4 in.) and in weights from less than 0.03 to 180 kg ( 1 16 to 400 lb) or more. Composition. The chemical composition of malleable iron generally conforms to the ranges given in Table 1. Small amounts of chromium (0.01 to 0.03%), boron (0.0020%), copper (~1.0%), nickel (0.5 to 0.8%), and molybdenum (0.35 to 0.5%) are also sometimes present. Table 1 Chemical composition of malleable iron Element Composition,% Carbon 2.16-2.90 Silicon 0.90-1.90 Manganese 0.15-1.25 Sulfur 0.02-0.20 Phosphorus 0.02-0.15 Five-digit designations are assigned in ASTM A 47 (Ref 1), which covers two grades of ferritic malleable iron, and in A 220 (Ref 2), which deals with pearlitic malleable iron. The first three digits indicate the minimum yield strength (in kips per square inch), and the last two indicate the minimum percentage of elongation in 50 mm (2 in.). Another standard, ASTM A 602 (Ref 3), covers both types of malleable iron; it assigns an "M" and four digits. The first two digits are typical yield strength in kips per square inch, and the last two are typical percentage of elongation in 50 mm (2 in.) test specimens cut from actual cast parts. References 1. "Standard Specification for Malleable Iron Castings," A 47, Annual Book of ASTM Standards, American Society for Testing and Materials 2. "Standard Specification for Pearlitic Malleable Iron Castings," A 220, Annual Book of ASTM Standards, American Society for Testing and Materials 3. "Standard Specification for Automotive Malleable Iron Castings," A 602, Annual Book of ASTM Standards, American Society for Testing and Materials Malleable Iron Melting Practices Melting can be accomplished by batch cold melting or by duplexing. Cold melting is done in coreless or channel-type induction furnaces, electric arc furnaces, or cupola furnaces. In duplexing, the iron is melted in a cupola or electric arc furnace, and the molten metal is transferred to a coreless or channel-type induction furnace for holding and pouring. Charge materials (foundry returns, steel scrap, ferroalloys, and, except in cupola melting, carbon) are carefully selected, and the melting operation is well controlled to produce metal having the desired composition and properties. Minor corrections in composition and pouring temperature are made in the second stage of duplex melting, but most of the process control is done in the primary melting furnace. Detailed information on induction furnaces, electric arc furnaces, and cupolas is available in the articles "Melting Furnaces: Electric Arc Furnaces," "Melting Furnaces: Induction Furnaces," and "Melting Furnaces: Cupolas" in this Volume. Molds are produced in green sand, silicate CO 2 bonded sand, or resin bonded sand (shell molds) on equipment ranging from highly mechanized or automated machines to that required for floor or hand molding methods, depending on the size and number of castings to be produced (see the article "Sand Molding" in this Volume). In general, the technology of molding and pouring malleable iron is similar to that used to produce gray iron (see the article "Gray Iron" in this Volume). Heat treating is done in high-production controlled-atmosphere continuous furnaces or batch-type furnaces, again depending on production requirements. After it solidifies and cools, the metal is in a white iron state, and gates, sprues, and feeders can be easily removed from the castings by impact. This operation, called spruing, is generally performed manually with a hammer because the diversity of castings produced in the foundry makes the mechanization or automation of spruing very difficult. After spruing, the castings proceed to heat treatment, while gates and risers are returned to the melting department for reprocessing. Malleable iron castings are produced from the white iron by a two-stage annealing process. First- and second-stage annealing processes are described in detail in the section "Control of Annealing" in this article. After heat treatment, ferritic or pearlitic malleable castings are cleaned by shotblasting, gates are removed by shearing or grinding, and, where necessary, the castings are coined or punched. Close dimensional tolerances can be maintained in ferritic malleable iron and in the lower-hardness types of pearlitic malleable iron, both of which can be easily straightened in dies. The harder pearlitic malleable irons are more difficult to press because of higher yield strength and a greater tendency toward springback after die pressing. However, even the highest-strength pearlitic malleable can be straightened to achieve good dimensional tolerances. Control of Melting. Metallurgical control of the melting operation is designed to ensure that the molten iron will have a certain composition and will: • Solidify white in the castings to be produced • Anneal on an established time-temperature cycle set to minimum values in the interest of economy • Produce the desired graphite distribution (nodule count) upon annealing Changes in melting practice or composition that would satisfy the first requirement listed above are generally opposed to satisfaction of the second and third, while attempts to improve annealability beyond a certain point may result in an unacceptable tendency for the as-cast iron to be mottled instead of white. The common elements in malleable iron are generally controlled within about ±0.05 to ±0.15%. A limiting minimum carbon content is required in the interest of mechanical quality and annealability because decreasing carbon content reduces the fluidity of the molten iron, increases shrinkage during solidification, and reduces annealability. A limiting maximum carbon content is imposed by the requirement that the casting be white as-cast. The range in silicon content is limited to ensure proper annealing during a short-cycle high-production annealing process and to avoid the formation of primary graphite during solidification. Manganese and sulfur contents are balanced to ensure that all sulfur is combined with manganese and that only a safe, minimum quantity of excess manganese is present in the iron. An excess of either sulfur or manganese will retard annealing in the second stage and therefore increase annealing costs. The chromium content is kept low because of the carbide-stabilizing effect of this element and because it retards both the first-stage and second-stage annealing reactions. A mixture of gray iron and white iron in variable proportions that produces a mottled (speckled) appearance is particularly damaging to the mechanical properties of the annealed casting, whether ferritic or pearlitic malleable iron. Primary control of mottle is achieved by maintaining a balance of carbon and silicon contents. Because economy and castability are enhanced when the carbon and silicon contents of the base iron are in the higher portions of their respective ranges, some malleable iron foundries produce iron with carbon and silicon contents at levels that might produce mottle and then add a balanced, mild carbide stabilizer to prevent mottle during casting. Bismuth and boron in balanced amounts accomplish this control. A typical addition is 0.01% Bi (as metal) and 0.001% B (as ferroboron). Bismuth retards graphitization during solidification; small amounts of boron have little effect on graphitizing tendency during solidification, but accelerate carbide decomposition during annealing. The balanced addition of bismuth and boron permits the production of heavier sections for a given base iron or the utilization of a higher-carbon higher- silicon base iron for a given section thickness. Tellurium can be added in amounts of 0.0005 to 0.001% to suppress mottle. Tellurium is a much stronger carbide stabilizer than bismuth during solidification, but also strongly retards annealing if the residual exceeds 0.003%. Less than 0.003% residual tellurium has little effect on annealing, but has a significant influence on mottle control. Tellurium is more effective if added together with copper or bismuth. Residual boron should not exceed 0.0035% in order to avoid nodule alignment and carbide formation. Also, the addition of 0.005% Al to the pouring ladle significantly improves annealability without promoting mottle. Malleable Iron Microstructure Malleable iron is characterized by microstructures consisting of uniformly dispersed fine particles of free carbon in a matrix of ferrite or tempered martensite. These microstructures can be produced in base metal of essentially the same composition. Structural differences between ferritic malleable iron and the various grades of pearlitic or martensitic malleable iron are achieved through variations in heat treatment. The microstructure of a casting of any type of malleable iron is derived by controlled annealing of white iron of suitable composition. During the annealing cycle, carbon that exists in combined form, either as massive carbides or as a microconstituent in pearlite, is converted to a form of free graphite known as temper carbon. Ferritic malleable iron requires a two-stage annealing cycle. The first stage converts primary carbides to temper carbon, and the second stage converts the carbon dissolved in austenite at the first-stage annealing temperature to temper carbon and ferrite. The microstructure of ferritic malleable iron is shown in Fig. 1. A satisfactory structure consists of temper carbon in a matrix of ferrite. There should be no flake graphite and essentially no combined carbon in ferritic malleable iron. Pearlitic and martensitic malleable irons contain a controlled quantity of combined carbon, which, depending on heat treatment, may appear in the metallic matrix as lamellar pearlite, tempered martensite, or spheroidite. Fig. 1 Structure of annealed ferritic malleable iron showing temper carbon in ferrite. 100× Molten iron produced under properly controlled melting conditions solidifies with all carbon in the combined form, producing the white iron structure fundamental to the manufacture of either ferritic or pearlitic malleable iron (Fig. 2). The base iron must contain balanced quantities of carbon and silicon to simultaneously provide castability, white iron in even the thickest sections of the castings, and annealability; therefore, precise metallurgical control is necessary for quality production. Thick metal sections cool slowly during solidification and tend to graphitize, producing mottled or gray iron. This is undesirable, because the graphite formed in mottled iron or rapidly cooled gray iron is generally of the type D configuration, a flake form in a dense, lacy structure, which is particularly damaging to the strength, ductility, and stiffness characteristics of both ferritic and pearlitic malleable iron. Fig. 2 Structure of as-cast malleable white iron showing a mixture of pearlite and eutectic carbides. 400× Control of Nodule Count. Proper annealing in short-term cycles and the attainment of high levels of casting quality require that controlled distribution of graphite particles be obtained during first-stage heat treatment. With low nodule count (few graphite particles per unit area or volume), mechanical properties are reduced from optimum, and second- stage annealing time is unnecessarily long because of long diffusion distances. Excessive nodule count is also undesirable, because graphite particles may become aligned in a configuration corresponding to the boundaries of the original primary cementite. In martensitic malleable iron, very high nodule counts are sometimes associated with low hardenability and nonuniform tempering. Generally, a nodule count of 80 to 150 discrete graphite particles per square millimeter (80 to 150 in 15.5 in. 2 of a photomicrograph at 100×) appears to be optimum. This produces random particle distribution, with short distances between particles. Temper carbon is formed predominantly at the interface between primary carbide and saturated austenite at the first-stage annealing temperature, with growth around the nuclei taking place by a reaction involving diffusion and carbide decomposition, Although new nuclei undoubtedly form at the interfaces during holding at the first-stage annealing temperature, nucleation and graphitization are accelerated by the presence of nuclei that are created by appropriate melting practice. High silicon and carbon contents promote nucleation and graphitization, but these elements must be restricted to certain maximum levels because of the necessity that the iron solidify white. Control of Annealing. The rate of annealing of a hard iron casting depends on chemical composition, nucleation tendency (as discussed above), and annealing temperature. With the proper balance of boron content and graphitic materials in the charge, optimum number and distribution of graphite nuclei are developed in the early part of first-stage annealing, and growth of the temper carbon particles proceeds rapidly at any annealing temperature. An optimum iron will anneal completely through the first-stage reaction in approximately 3 1 2 h at 940 °C (1720 °F). Irons with lower silicon contents or less-than-optimum nodule counts may require as much as 20 h for completion of first-stage annealing. The temperature of first-stage annealing exercises considerable influence on the rate of annealing and the number of graphite particles produced. Increasing the annealing temperature accelerates the rate of decomposition of primary carbide and produces more graphite particles per unit volume. However, high first-stage annealing temperatures can result in excessive distortion of castings during annealing, which leads to straightening of the casting after heat treatment. Annealing temperatures are adjusted to provide maximum practical annealing rates and minimum distortion and are therefore controlled within the range of 900 to 970 °C (1650 to 1780 °F). Lower temperatures result in excessively long annealing times, while higher temperatures produce excessive distortion. After first-stage annealing, the castings are cooled as rapidly as practical to 740 to 760 °C (1360 to 1400 °F) in preparation for second-stage annealing. The fast cooling step requires 1 to 6 h, depending on the equipment being employed. Castings are then cooled slowly at a rate of about 3 to 11 °C (5 to 20 °F) per hour. During cooling, the carbon dissolved in the austenite is converted to graphite and deposited on the existing particles of temper carbon. This results in a fully ferritic matrix. In the production of pearlitic malleable iron, the first-stage heat treatment is identical to that used for ferritic malleable iron. However, some foundries then slowly cool the castings to about 870 °C (1600 °F). During cooling, the combined carbon content of the austenite is reduced to about 0.75%, and the castings are then air cooled. Air cooling is accelerated by an air blast to avoid the formation of ferrite envelopes around the temper carbon particles (bull's-eye structure) and to produce a fine pearlitic matrix (Fig. 3). The castings are then tempered to specification, or they are reheated to reaustenitize at about 870 °C (1600 °F), oil quenched, and tempered to specification. Large foundries usually eliminate the reaustenitizing step and quench the castings in oil directly from the first-stage annealing furnace after stabilizing the temperature at 845 to 870 °C (1550 to 1600 °F). Fig. 3 Structure of air-cooled pearlitic malleable iron. (a) Slowly air cooled. 400×. (b) Cooled in an air blast. 400× The furnace atmosphere for producing malleable iron in continuous furnaces is controlled so that the ratio of CO to CO 2 is between 1:1 and 20:1. In addition, any sources of water vapor or hydrogen are eliminated; the presence of hydrogen is thought to retard annealing, and it produces excessive decarburization of casting surfaces. Proper control of the gas atmosphere is important for avoiding an undesirable surface structure. A high ratio of CO to CO 2 retains a high level of combined carbon on the surface of the casting and produces a pearlitic rim, or picture frame, on a ferritic malleable iron part. A low ratio of CO to CO 2 permits excessive decarburization, which forms a ferritic skin on the casting with an underlying rim of pearlite. The latter condition is produced when a significant portion of the subsurface metal is decarburized to the degree that no temper carbon nodules can be developed during first-stage annealing. When this occurs, the dissolved carbon cannot precipitate from the austenite, except as the cementite plates in pearlite. The rate of cooling after first-stage annealing is important in the formation of a uniform pearlitic matrix in the air-cooled casting, because slow rates permit partial decomposition of carbon in the immediate vicinity of the temper carbon nodules, which results in the formation of films of ferrite around the temper carbon (bull's-eye structure). When the extent of these films becomes excessive, a carbon gradient is developed in the matrix. Air cooling is usually done at a rate not less than about 80 °C (150 °F) per minute. Air-quenched malleable iron castings have hardnesses ranging from 269 to 321 HB, depending on casting size and cooling rate. Such castings can be tempered immediately after air cooling to obtain pearlitic malleable iron with a hardness of 241 HB or less. High-strength malleable iron castings of uniformly high quality are usually produced by liquid quenching and tempering, using any of the three procedures. The most economical procedure is direct quenching after first-stage annealing. In this procedure, castings are cooled in the furnace to the quenching temperature of 845 to 870 °C (1550 to 1600 °F) and held for 15 to 30 min to homogenize the matrix. The castings are then quenched in agitated oil to develop a matrix microstructure of martensite having a hardness of 415 to 601 HB. Finally, the castings are tempered at an appropriate temperature between 590 and 725 °C (1100 and 1340 °F) to develop the specified mechanical properties. The final microstructure consists of tempered martensite plus temper carbon, as shown in Fig. 4. In heavy sections, higher- temperature transformation products such as fine pearlite are usually present. Fig. 4 Structure of all-quenched and tempered martensitic malleable iron. (a) 163 HB. 500×. (b) 179 HB. 500×. (c) 207 HB. 500×. (d) 229 HB. 500× Some foundries produce high-strength malleable iron by an alternative procedure in which the castings are forced-air cooled after first-stage annealing, retaining about 0.75% C as pearlite, and then reheated to 840 to 870 °C (1545 to 1600 °F) for 15 to 30 min, followed by quenching and tempering as described above for the direct-quench process. Rehardened-and-tempered malleable iron can also be produced from fully annealed ferritic malleable iron with a slight variation from the heat treatment used for arrested-annealed (air-quenched) malleable. The matrix of fully annealed ferritic malleable iron is essentially carbon free, but can be recarburized by heating at 840 to 870 °C (1545 to 1600 °F) for 1 h. In general, the combined carbon content of the matrix produced by this procedure is slightly lower than that of arrested-annealed pearlitic malleable iron, and the final tempering temperatures required for the development of specific hardnesses are lower. Rehardened malleable iron made from ferritic malleable may not be capable of meeting certain specifications. Tempering times of 2 h or more are needed for uniformity. In general, control of final hardness of the castings is precise, with process limitations approximately the same as those encountered in the heat treatment of medium- or high-carbon steels. This is particularly true when specifications require hardnesses of 241 to 321 HB where control limits of ±0.2 mm Brinell diameter can be maintained with ease. At lower hardnesses, a wider process control limit is required because of certain unique characteristics of the pearlitic malleable iron microstructure. The relationships between tempering conditions and properties (yield strength and hardness) are illustrated in Fig. 5. Fig. 5 Hardness and minimum yield strength of pearlitic malleable iron. Relationships of tempering time and temperature to hardness and minimum yield strength are given. Malleable Iron Applications The requirement that any iron produced for conversion to malleable iron must solidify white places definite section thickness limitations on the malleable iron industry. Thick metal sections can be produced by melting a base iron of low carbon and silicon contents or by alloying the molten iron with a carbide stabilizer. However, when carbon and silicon are maintained at low levels, difficulty is invariably encountered in annealing, and the time required to convert primary and pearlitic carbides to temper carbon becomes excessively long. High-production foundries are usually reluctant to produce castings more than about 40 mm (1 1 2 in.) thick. Some foundries, however, routinely produce castings as thick as 100 mm (4 in.). Automotive and associated applications of ferritic and pearlitic malleable irons include many essential parts in vehicle power trains, frames, suspensions, and wheels. A partial list includes differential carriers, differential cases, bearing caps, steering-gear housings, spring hangers, universal-joint yokes, automatic-transmission parts, rocker arms, disc brake calipers, wheel hubs, and many other miscellaneous castings. Examples are shown in Fig. 6. Ferritic and pearlitic malleable irons are also used in the railroad industry and in agricultural equipment, chain links, ordnance material, electrical pole line hardware, hand tools, and other parts requiring section thicknesses and properties obtainable in these materials. Fig. 6 Examples of malleable iron automotive applications. (a) Driveline yokes. (b) Connecting rods. (c) Diesel pistons. (d) Steering gear housing. Courtesy of Central Foundry Division, General Motors Corporation Malleable iron castings are often selected because the material has excellent machinability in addition to significant ductility. In other applications, malleable iron is chosen because it combines castability with good toughness and machinability. Malleable iron is often chosen because of shock resistance alone. Table 2 lists some of the typical applications of malleable iron castings. Table 2 Applications of malleable iron castings Mechanical properties are given in Table 3. Specification No. Class or grade Microstructure Typical applications Ferritic ASTM A 47, ANSI G48.1, FED QQ-1- 666c 32510, 35018 Temper carbon and ferrite General engineering service at normal and elevated temperatures for good machinability and excellent shock resistance ASTM A 338 32510, 35018 Temper carbon and ferrite Flanges, pipe fittings, and valve parts for railroad, marine, and other heavy-duty service to 345 °C (650 °F) ASTM A 197, ANSI G49.1 . . . Free of primary graphite Pipe fittings and valve parts for pressure service Pearlitic and martensitic ASTM A 220, ANSI G48.2, MIL-I-11444B 40010, 45008, 45006, 50005, 60004, 70003, 80002, 90001 Temper carbon in necessary matrix without primary cementite or graphite General engineering service at normal and elevated temperatures. Dimensional tolerance range for castings is stipulated Automotive M3210 Ferritic For low-stress parts requiring good machinability: steering-gear housings, carriers, and mounting brackets M4504 Ferrite and tempered pearlite (a) Compressor crankshafts and hubs M5003 Ferrite and tempered pearlite (a) For selective hardening: planet carriers, transmission gears, and differential cases M5503 Tempered martensite For machinability and improved response to induction hardening M7002 Tempered martensite For high-strength parts: connecting rods and universal-joint yokes ASTM A 602, SAE J158 M8501 Tempered martensite For high strength plus good wear resistance: certain gears (a) May be all tempered martensite for some applications [...]... eutectic 14.3% Si cast iron is approximately 118 0 °C (2160 °F), and the alloy is generally poured at approximately 1345 °C (2450 °F) Because of the very brittle nature of high-silicon cast iron, castings are usually shaken out after cooling to ambient temperature However, some casting geometries demand hot shakeout while the castings are still red hot, so that the castings can be immediately stress relieved... surface hardening The maximum hardness obtainable in the matrix of a properly hardened pearlitic malleable part is 67 HRC However, conventional hardness measurements made on castings show less than 67 HRC because of the presence of the graphite particles, which are averaged into the hardness Generally, a casting with a matrix microhardness of 67 HRC will have about 62 HRC average hardness, as measured with... "Standard Specification for Pearlitic Malleable Iron Castings," A 220, Annual Book of ASTM Standards, American Society for Testing and Materials 3 "Standard Specification for Automotive Malleable Iron Castings," A 602, Annual Book of ASTM Standards, American Society for Testing and Materials 4 C.F Walton and T.J Opar, Ed., Iron Castings Handbook, Iron Castings Society, 1981, p 297-321 5 W.L Bradley, Fracture... Consequently, care should be taken in shaking out too hot, and mold cooling is recommended for intricately shaped castings and castings of widely varying section thickness Heat treatment of nickel-alloyed ductile irons serves to strengthen the casting and to stabilize the microstructure of the casting for increased durability Additional information on the heat treatment of ductile iron is available in... properties of ferritic malleable castings depend on the quantity, size, shape, and distribution of the temper carbon particles and on the composition of the ferrite Fully annealed ferritic malleable iron castings contain 2.00 to 2.70% graphitic carbon by weight, which is equivalent to about 6 to 8% by volume Because the graphitic carbon contributes nothing to the strength of the castings, those with the lesser... "Welding of Cast Irons and Steels" in this Volume References cited in this section 1 "Standard Specification for Malleable Iron Castings," A 47, Annual Book of ASTM Standards, American Society for Testing and Materials 4 C.F Walton and T.J Opar, Ed., Iron Castings Handbook, Iron Castings Society, 1981, p 297-321 Malleable Iron Pearlitic and Martensitic Malleable Iron Pearlitic malleable iron is produced... "Ductile Iron" in this Volume Stress relief heat treatments are typically conducted at temperatures between 620 and 675 °C (115 0 and 1250 °F) to remove residual casting stresses Mold cooling to 315 °C (600 °F) is a satisfactory alternative to furnace stress relief Annealing of some castings may be necessary to reduce hardness Annealing is performed at 955 to 1040 °C (1750 to 1900 °F) for 30 min to 5 h,... (low carbon-high phosphorus) Group E-2 2.29 1.01 0.75 0.086 0 .11 Group C-2 2.65 1.35 0.41 0.15 0.018 0.0020 B Group W-1 2.45 1.38 0.41 0.12 0.04 0.032 Group E-3 2.21 1.13 0.88 0 .110 0.122 0.021 0.47Mo,1.03Cu Group L-1 2.16 1.18 0.72 0.120 0.128 0.34Mo,0.83 Ni Group L-2 2.16 1.18 0.80 0.123 0.128 0.40Mo,0.62 Ni Group L-3 2.32 1.14 0.82 0 .117 0.128 0.38Mo,0.65 Ni Group G-2 Pearlitic (high carbon-low... cooled to prevent cracking Molding and Casting Design These alloys are routinely cast in sand molds, investment molds, and permanent steel molds Cores must have good collapsibility to prevent fracture during solidification Flash must be minimized because it will be chilled iron and can readily become an ideal nucleation site for cracks that propagate into the casting Casting design for these high-silicon... Specification for Corrosion-Resistant High-Silicon Iron Castings," A 518M, Annual Book of ASTM Standards, American Society for Testing and Materials References 1 "Standard Specification for Austenitic Gray Iron Castings," A 436, Annual Book of ASTM Standards, American Society for Testing and Materials 2 "Standard Specification for Austenitic Ductile Iron Castings," A 439, Annual Book of ASTM Standards, American . advantage. It is preferred for thin-section castings; for parts that are to be pierced, coined, or cold formed; for parts requiring maximum machinability; for parts that must retain good impact resistance. difficult. After spruing, the castings proceed to heat treatment, while gates and risers are returned to the melting department for reprocessing. Malleable iron castings are produced from the. high levels of casting quality require that controlled distribution of graphite particles be obtained during first-stage heat treatment. With low nodule count (few graphite particles per unit