US Navy Foundry Manual 1958

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<p>AVSHIPS 250-0334 FOUNDRY MANUAL</p> <p>Department of the Navy, Bureau of Ships JANUARY 1958 BUREAU OF SHIPS NAVY DEPARTMENT WASHINGTON 25, D. C.</p> <p>Superintendent of Documents, U.S. Government Printing Office Washington, D.C. - Price $3</p> <p>NAVY DEPARTMENT, Bureau of Ships, 15 April 1958 The Foundry Manual of 1944 has been revised to reflect the advancement in foundry technology and to indicate current foundry practice. The revised manual contains information for persons who operate or are employed in a foundry.</p> <p>J. B. Duval, Jr. Captain, USN Assistant to the Assistant Chief of Bureau for Shipbuilding and Fleet Maintenance</p> <p>PREFACE This Manual is intended primarily for use by foundry personnel aboard repair ships and tenders. The recommended practices are based on procedures proved workable under Navy conditions and are supplemented by information from industrial sources. The Manual is divided into two general sections. The first section, chapters 1 through 13, contains information of a general nature, such as "How Metals Solidify," "Designing a Casting," "Sands for Molds and Cores," "Gates, Risers, and Chills," and "Description and Operation of Melting Furnaces." Subjects covered in these chapters are generally applicable to all of the metals that may be cast aboard ship. The second section, chapters 14 through 21, contains information on specific types of alloys, such as "Copper-Base Alloys," "Aluminum-Base Alloys," "Cast Iron," and "Steel." Specific melting practices, suggestions for sand mixes, molding practices, gating, and risering are covered in these chapters. This manual has been written with the "how-to-do-it" idea as the principal aim. Discussions as to the "why" of certain procedures have been kept to a minimum. This manual contains information that should result in the production of consistently better castings by repair ship personnel.</p> <p>1 Chapter I HOW METALS SOLIDIFY Making a casting involves three basic steps: (1) heating metal until it melts, (2) pouring the liquid metal into a mold cavity, and (3) allowing the metal to cool and solidify in the shape of the mold cavity. Much of the art and science of making castings is concerned with control of the things that happen to metal as it solidifies. An understanding of how metals solidify, therefore, is necessary to the work of the foundry-man. The control of the solidification of metal to produce better castings is described in later chapters on casting design, gating, risering, and pouring. The change from hot molten metal to cool solid casting takes place in three main steps. The first step is the cooling of the metal from the pouring temperature to the solidification temperature. The difference between the pouring temperature and the solidification temperature is called the amount of superheat. The amount of superheat determines the amount of time the foundryman has available to work with the molten metal before it starts to solidify. The second step is the cooling of the metal through the range of temperature at which it solidifies. During this step, the quality of the final casting is established. Shrink holes, blow holes, hot cracks, and many other defects form in a casting while it solidifies. The third step is the cooling of the solid metal to room temperature. It is during this stage of cooling that warpage and casting stresses occur. THE START OF SOLIDIFICATION Solidification of a casting is brought about by the cooling effect of the mold. Within a few seconds after pouring, a thin layer of metal next to the mold wall is cool enough for solidification to begin. At this time, a thin skin or shell of solid metal forms. The shell gradually thickens as more and more metal is cooled, until all the metal has solidified. Solidification always starts at the surface and finishes in the center of a section. In other words, solidification follows the direction that the metal is cooled. The way in which metal solidifies from mold walls is illustrated by the series of steel castings shown in figure 1. The metal that was still molten after various intervals of time was dumped out to show the progress of solidification. All metals behave in a similar manner. However, the time required to reach a given thickness of skin varies among the different metals. The speed of solidification depends on how fast the necessary heat can be removed by the mold. The rate of heat removal depends on the relation between the volume and the surface area of the metal. Other things being equal, the thin sections will solidify before the thick ones. Outside corners of a casting solidify faster than other sections because more mold surface is available to conduct heat away from the casting. Inside corners are the slowest sections of the casting to solidify. The sand, in this case, is exposed to metal on two sides and becomes heated to high temperatures. Therefore, it cannot carry heat away so fast. Changes in design to control solidification rate sometimes can be made by the designer. If, however, a change in solidification rate is required for the production of a good casting, the foundryman is usually limited to methods that result in little or no change in the shape of the casting. The rate of solidification can be influenced in three other ways: (1) by changing the rate of heat removal from some parts of the mold with chills; (2) by proper gating and risering, mold manipulation, and control of pouring speed, and (3) by padding the section with extra metal that can be machined off later. CONTRACTION Metals, like most other materials, expand when they are heated. When cooled, they must contract or shrink. During the cooling of molten metal from its pouring temperature to room temperature, contraction occurs in three definite steps corresponding to the three steps of cooling. The first step, known as liquid contraction, takes place while the molten metal is cooling from its pouring temperature to its freezing temperature. The second, called solidification contraction, takes place when the metal solidifies. The third contraction takes place when the solidified casting cools from its freezing temperature to room temperature. This is called solid contraction. Of the three steps in contraction, the first liquid contraction causes least trouble to the foundryman because it is so small in amount. Figure 2, which shows the change in volume of a steel alloy as it cools from the pouring temperature to room temperature, illustrates these contractions. In a similar way, most of the metals considered in this manual contract in volume when cooling and when solidifying. The amount of shrinkage in several metals and alloys is given in table 1. Notice that some compositions of gray cast iron expand slightly</p> <p>2 TABLE 1. THE AMOUNT OF SHRINKAGE FROM POURING TEMPERATURE TO ROOM TEMPERATURE FOR SEVERAL METALS AND ALLOYS Name Composition Decrease in Total Volume Decrease During in Volume, Solidification, percent percent Copper Red brass Yellow brass Bearing bronze Manganese bronze Aluminum bronze Aluminum Nickel Monel Nickel silver Carbon steel Gray cast iron Deoxidized 85 Cu, 5 Zn, 5 Pb, 5 Sn 70 Cu, 27 Zn, 2 Pb, 1 Sn 80 Cu, 10 Sn, 10 Pb 3.8 6.3 6.4 7.3 10.7 10.6 12.4 11.2 11.5 11.2 12.2 14.2 13.9 12.1 11.4 7.8</p> <p>56-3/4 Cu, 40 Zn, 1-1/4 Fe, 1/2 Sn, 1 Al, 4.6 1/2 Mn 90 Cu, 10 Al Commercial 98 Ni, 1-1/2 Si, 0.1 C 67 Ni, 32 Cu 20 Ni, 15 Zn, 65 Cu 0.25 C, 0.2 Si, 0.6 Mn 2.18 C, 1.24 Si, 0.35 Mn 3.08 C, 1.68 Si, 0.44 Mn 3.69 C, 2.87 Si, 0.59 Mn 4.1 6.5 6.1 6.3 5.5 3.8 1.6 4.85 1.94 -1.65 (expands)</p> <p>Nickel cast iron 13 Ni, 7 Cu, 2 Cr, 3 C</p> <p>during solidification. This results from the formation of graphite, which is less dense than iron. The formation of graphite compensates for a part of the shrinkage of the iron.</p> <p>Centerline shrinkage occurs most frequently in alloys having a short solidification range and low thermal conductivity. Microshrinkage, which is also known as microporosity, occurs as tiny voids scattered through an area of metal. It is caused by inability to feed metal into the spaces between the arms of the individual Reservoirs of molten metal, known as risers, are crystals or grains of metal. This type of shrinkage, required to make up for the contraction that occurs which is illustrated in figure 3d, is most often found in during solidification. If risers are not provided at metals having a long solidification temperature range. selected spots on the casting, shrinkage voids will occur in the casting. These voids can occur in different Microporosity may also be caused by gas being ways, depending on the shape of the casting and on the trapped between the arms of the crystals. type of the metal. Piping, the type of shrinkage illustrated in figure 3a, occurs in pure metals and in After solidification, cast metal becomes more rigid as alloys having narrow ranges of solidification it cools to normal room temperature. This cooling is temperature. Piping in a riser is usually a good accompanied by contraction, which is allowed for by indication that it is functioning properly. Gross the patternmaker in making the pattern for the casting. shrinkage, illustrated in figure 3b, occurs at a heavy Contraction in cast metals after solidification is section of a casting which has been improperly fed. resisted by the mold. Often, different cooling rates of Centerline shrinkage, illustrated in figure 3c, occurs in thin and heavy sections result in uneven contraction. the center of a section where the gradually thickening This uneven contraction can severely stress the walls of solidified metal from two surfaces meet. partially solidified, and still weak, heavier sections.</p> <p>Resistance to contraction of the casting results in severe "contraction stresses" which may tear the casting or which may remain in the casting until removed by suitable heat treatment. Sharp internal corners are natural points for these stresses. Some metals, such as steel, undergo other dimensional changes as they pass through certain temperature ranges in the solid state. In the case of castings with extreme variations in section thickness, it is possible for contraction to take place in some parts at the same time that expansion occurs in others. If the design of the junctions of these parts is not carefully considered, serious difficulties will occur in the foundry and in service. FREEZING TEMPERATURE OF METALS Molten metal has the ability to dissolve many substances, just as water dissolves salt. The most important elements that are soluble in molten iron are other metals and five nonmetals--sulfur, phosphorus, carbon, nitrogen, and hydrogen. When substances are dissolved in a metal, they change many of its properties. For example, pure iron is relatively soft. A small amount of carbon dissolved in the iron makes it tough and hard. Iron containing a small amount of carbon is called steel. More carbon dissolved in the iron makes further changes in its properties. When enough carbon is dissolved in the molten iron, the excess carbon will form flakes of graphite during solidification. This metal is known as cast iron. The graphite flakes lower the effective cross section of the metal, lower the apparent hardness, and have a notch effect. These factors cause cast irons to have lower strengths and lower toughness than steels. One of the most important changes in a metal as it dissolves other substances is a change in the freezing characteristics. Pure metals and certain specific mixtures of metals, called eutectic mixtures, solidify without a change in temperature. It is necessary, however, to extract heat for solidification to occur. The solidification of pure metals and eutectic mixtures is very similar to the freezing of water. Water does not begin to freeze until the temperature is lowered to 32F. The temperature of the ice and water does not change from 32F. until all of the water is converted to ice. After this, the ice can be cooled to the temperature of its surroundings, whether they are zero or many degrees below zero. This type of temperature change during cooling, shown in figure 4a, is typical of pure metals, eutectic mixtures, and water. Actual</p> <p>solidification temperatures are different for each material. Most of the metals used by foundrymen are impure and are not eutectic mixtures. These metals solidify over a range of temperature known as the solidification range. Mixtures of metals have many of the solidification characteristics of mixtures of salt and water. Just as the addition of salt to water changes the temperature at which water starts to freeze, so does the addition of one metal to another change the freezing point of the second metal. An example of such a mixture of metals is the copper-nickel system shown in figure 4b (right). A given mixture of copper and nickel will be liquid until it reaches the temperature that crosses the line marking the upper boundary of Area A + L. In the Area A + L, the mixture will be partly liquid, and in the Area A, it will be entirely solid. It will be noted that the addition of copper to nickel lowers the freezing temperature. On the other hand, the addition of nickel to copper raises the freezing temperature. A metal system which has the same general shape as the copper-nickel system is said to have complete solid solubility. Like the mixture of water and salt, metal mixtures of this type must be cooled well below the temperature at which freezing begins before they are completely solidified. In its simplest form, the cooling curve looks like that in figure 4b (left). The range of temperature between the upper and lower line is the solidification range. Most of the metal mixtures used in the foundry do not have cooling curves as simple as those shown in figures 4a and 4b. As an example, the addition of tin to lead lowers the freezing temperature of the mixture (see figure 4c, right). The addition of lead to tin also lowers the freezing temperature of the mixture. However, there is one specific mixture which has a lower freezing temperature than either lead, tin, or any other mixture of the two. The mixture that has the lowest freezing temperature is the eutectic mixture. A typical set of alloys that has an eutectic mixture is that of the lead-tin system shown in figure 4c (right). A cooling curve for one lead-tin alloy is also shown in figure 4c (left). In such mixtures, the mechanism of solidification is quite complicated. The melting temperatures of important metals are shown in figure 5. The melting temperatures of many metals are so high that they create real problems in selecting materials for handling the molten metal and for making the mold. CRYSTALLIZATION A casting is made up of many closely packed and joined grains or crystals of metal. Within severely stress the partially solidified, and still weak, heavier sections.</p> <p>On the other hand, there is no orderly arrangement of atoms in molten metal. Solidification, therefore, is the formation and growth of...</p>