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foundry workshop manualDifferent browsers and fonts will cause the text to move, but the text will remain roughly where it is in the original manual. In addition to errors we have attempted to preserve from the original this text was captured by optical character recognition. This process creates errors that are compounded while encoding for the Web.The revised manual contains information for persons who operate or are employed in a foundry.The recommended practices are based on procedures proved workable under Navy conditions and are supplemented by information from industrial sources.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 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.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.It is during this stage of cooling that warpage and casting stresses occur.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.http://coyada.com/up_images/up_images/foundry-manual.xml
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All metals behave in a similar manner. However, the time required to reach a given thickness of skin varies among the different metals.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.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.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.http://www.ecuadoratualcance.com/images/foundry-manual-part-2.xmlIn 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 slightlyThis 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.If risers are not provided at selected spots on the casting, shrinkage voids will occur in the casting. These voids can occur in different ways, depending on the shape of the casting and on the type of the metal. Piping, the type of shrinkage illustrated in figure 3a, occurs in pure metals and in alloys having narrow ranges of solidification temperature. Piping in a riser is usually a good indication that it is functioning properly. Gross shrinkage, illustrated in figure 3b, occurs at a heavy section of a casting which has been improperly fed. Centerline shrinkage, illustrated in figure 3c, occurs in the center of a section where the gradually thickening walls of solidified metal from two surfaces meet.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 crystals or grains of metal. This type of shrinkage, which is illustrated in figure 3d, is most often found in metals having a long solidification temperature range. Microporosity may also be caused by gas being trapped between the arms of the crystals.This cooling is accompanied by contraction, which is allowed for by the patternmaker in making the pattern for the casting. Contraction in cast metals after solidification is resisted by the mold. Often, different cooling rates of thin and heavy sections result in uneven contraction. This uneven contraction canSharp internal corners are natural points for these stresses.https://directori.p2pvalue.eu/explore/cbpp-communities/community/datasheet/eheim-automatic-fish-feeder-manualIn 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.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.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. 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. ActualThese 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). 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.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.On the other hand, there is no orderly arrangement of atoms in molten metal. Solidification, therefore, is the formation and growth of crystals, layer by layer, from the melt. The size of the crystals is controlled by the time required for the metal to solidify and by its cooling rate in the mold. Obviously, the heavy sections take more time to freeze than the light sections. As a result, the crystalline structure of a heavy section is usually coarser than that of the lighter members. This may be seen in figure 6.As one example, coarse grains lower the strength of steel.Once a crystal starts to form, it grows progressively larger until its growth is stopped by other crystals around it or until there is no more molten metal to feed it. The growth of metal crystals is similar to the growth of frost crystals on a pane of glass.A three-dimensional sketch of crystal growth is shown in figure 8. Part (a) shows the crystal shortly after it has formed and has started to grow. In part (b), the crystal has become elongated and growth has started in two other directions. Still further growth is shown by part (c). The original body of the crystal has grown still longer and has become thicker in cross section. Two other sets of arms have started growing near the ends of the longest arms of the crystal. A still further stage of growth is shown in part (d). Crystals grow in this manner with continued branching and thickening of the arms. Because of its branching nature, the type of crystal shown in figure 8 is called a dendrite. When the metal is completely solidified, the arms will have grown and thickened until they have formed a continuous solid mass. A photograph of dendrites in a shrink area of an aluminum casting is shown in figure 9. The branching of the dendrite arms at right angles can be seen in this photograph. Close examination will also show where the growth of crystals was stopped by the growth of neighboring dendrites.Such grains are called randomly oriented, equiaxed grains. The crystals of zinc on the surface of galvanized steel are a familiar example. Another example of crystal structure is shown in figure 10. The faces of the individual crystals can be seen easily and growth would have continued if it had not been dumped to reveal the crystals.The growth of the metal crystals in the skin will take place by the building up of metal on some of the crystals of the surface layer which are favorably positioned for further growth. Figure 11 shows the small grains at the mold surface, with some of them positioned for further growth. The position for favorable growth is perpendicular to the mold wall and parallel to the direction of heat transfer from the casting. Properly oriented crystals will grow in toward the center because side growth will stop as soon as adjacent crystals meet. This type of crystal growth toward the center of the casting is known as columnar grain growth. Depending on the pouring temperature and the type of metal, growth of elongated grains may extend to the center of the casting. If the characteristics of the metal are such that it is impossible to feed properly the last parts of the dendrites, the casting defect known as centerline shrinkage is formed. This is shown in figure 12a. A point may be reached during solidification when the solidification temperature is reached by the entire remaining liquid metal. Nucleation and growth of crystals will then start throughout the melt and result in an equiaxed crystal structure in that part of the casting. Solidification which started as dendritic growth and finished as an equiaxed structure is shown in figure 12b.This process of heat extraction is called heat transfer.As the casting cools and solidifies, the transfer of heat is carried on at a reduced rate. The rapid heat transfer in the early period of solidification is due to the ability of the sand to store a large amount of heat. As the maximum capacity of the sand to store heat is reached, the sand becomes saturated with heat,Because this is a much slower process than the absorption of heat by the sand, the transfer of heat away from the casting takes place at a lower rate. Many times, the rate of transfer is further slowed by an air gap which is formed when the solidified casting starts to contract and draw away from the mold. The presence of this air gap causes a further decrease in the rate of heat transfer. Chills produce an increased rate of solidification because of their increased heat-storage capacity, as compared to an equal volume of sand, and their ability to conduct heat at a rate much more rapid than that at which sand can conduct it.These defects may range in size and form from microscopic porosity to large blow holes. Because of the large volume that a small weight of gas occupies, very little gas by weight can cause the foundryman a lot of trouble.Here, we are primarily concerned with the gas absorption during melting. The gases in any melting process often come from water vapor in the air, or from water which is introduced into the melt by careless foundry practice.The solubility of hydrogen in nickel and steel at various temperatures is shown in figure 13. Notice that it is possible to dissolve more hydrogen in molten metal than in solid metal. Therefore, gas that is absorbed during melting may escape when the molten metal cools and solidifies. If the gas cannot escape from the metal freely, bubbles are trapped in the casting causing defects. The treatment of metals to reduce their gas content before they are poured into the mold is discussed in later chapters dealing with the specific metals.In some cases, these defects are caused by gases driven into the metal from the mold.In some cases, gas is generated by chemical reactions within the metal, such as may sometimes occur between carbon and oxygen in steel to form carbon monoxide.This takes place as shown in figure 14. When the molten steel comes in contact with moist sand in the mold, a thin skin of steel is formed almost immediately. At the same time, the water in the sand is changed to steam with an increase in volume of approximately 5,000 times. The steam is highly oxidizing to the steel and reacts with it. As a result, iron oxide and hydrogen are formed. The iron oxide produces the scale which is seen on steel castings when they are shaken out of the mold.The hydrogen in the molten steel can then react with iron oxide, which is also dissolved in the steel. This reaction produces water vapor. As the steel cools, it must reject some of this water vapor and hydrogen, just as an ice cube must reject gas as it freezes. A bubble is formed and gradually grows as more steel solidifies. The bubbles become trapped between the rapidly growing crystals of steel and cause the familiar pinhole defect.These grains grow inwardly from the surface until they meet other grains growing from other surfaces. When these growing surfaces meet, the casting is solid.Casting defects which can occur if the freezing characteristics of metals are not taken into account are as follows: (1) microshrinkage, (2) centerline shrinkage, (3) shrink holes, (4) certain types of gas holes, (5) piping, and (6) hot tears.He is usually called upon to make a casting from a loose pattern or from the broken parts of an existing casting. Very rarely is he consulted as to what is good casting design from the foundryman's point of view. Nevertheless, an understanding of what constitutes good casting design will help the molder to make a consistently better product.A capable foundryman may produce satisfactory castings that violate some of the principles of good design, but he will never produce them with any degree of consistency. Superior craftsmanship of the foundry-man should not be relied upon to overcome poor design.The first thing to consider is the intended use of the casting, and the second is which alloy should be used. The intended use of the casting (that is, whether it is a supporting structure, moving part, pressure casting, or bearing) will be the major factor in determining the general shape of the casting. The amount of corrosion resistance, wear resistance, machinability, and strength that are needed will determine which alloy should be used. More often than not, a casting must meet a combination of requirements.Therefore, the first step in the production of a casting should be a careful study of its design in the light of the information given in this chapter. This applies equally to a new design and to the replacement of a casting of an old design. In the replacement of a casting, the defective part should be thoroughly studied to determine if failure was in any way due to design faults; whether faulty design contributed to casting unsoundness, or whether it adversely affected the service strength of the solid part.A casting should be designed so that the strength requirements are met withCare should be taken not to overdesign a casting. Many times when a casting fails, certain regions in the vicinity of the failure will be made larger with the idea that additional strength will be gained with an increase in thickness. In reality, this overdesign frequently produces casting defects which offset the desired increase in strength.As a general rule, a metal has lower strength per square inch of cross section when cast in thick sections than it does in thin sections. The effect of increasing section size on the strength and elongation of four different copper-base alloys is shown in figure 15. It is evident that the tin bronze and red brass are very sensitive to section thickness, while aluminum bronze and manganese bronze are less affected by section size. From this, it can be seen that the effect of section size on the properties of a casting must be considered if the casting is to make the best use of the metal poured into it.Stresses, of course, are the forces and loads that cause a casting to crack, tear, or break.The liberal use of fillets and rounded corners of proper size is the easiest way to reduce the concentration of stresses in corners. A sharp corner will also produce a plane of weakness in a casting where crystal growth from two sides meet. This is shown in figure 16a. The combination of high stresses and the plane of weakness result in early failure of the casting. The partial removal of this plane of weakness by rounding the corners is shown in figure 16b, and its complete elimination, in figure 16c.The stresses in this case result from the rapid solidification and contraction of the thin section. This contraction will set up very high stresses at the junction with the hotter, weaker, heavy section and may produce hot tearing. Where sections of different thicknesses are necessary, they should be blended together to reduce theRecommended practices for the blending of junctions are shown in figure 17. Although shown for aluminum, the same practices should be followed for all metals.A spoked wheel is an example. Correct and incorrect designs for wheels are shown in figure 18. The original design (with straight spokes) caused hot cracks at the junction of the spokes with the rim and hub. The modified design (with a curved spoke) produced a casting without hot tears. The modified design permits the spokes to stretch and distort slightly without tearing under the stresses set up by contraction. Two other patterns made to prevent tearing in a wheel casting are shown in figure 19.When distortion cannot be solved directly by design, as with the wheel casting, it must be allowed for by the patternmaker after consultation with the molder.The normal minimum sections that can be cast from several metals are listed in table 2.The use of adequate but not excessive section thickness in a casting cannot be stressed too strongly, because it is a major factor in good design.A sudden change in section thickness should be avoided wherever possible. Where a change in section thickness must be made, it should be gradual. A blending, or gradual change in section thickness reduces stresses at the junctions. Figure 20 shows various methods for changing from one section thickness to another.An effort is always made by the foundry-man to get solidification to progress toward the riser from the point furthermost from the riser. Casting design is a determining factor in the control of the direction of solidification, and every effort should be made to apply the principles of good design to reach this objective.The metal is poured through the riser, and as it flows over the mold surface, it gives up some of its heat to the mold. Such a condition will mean that when the mold is filled, the metal at the right end will not be as hot as the metal near the riser. The first metal to solidify will then be the metal at the right, as shown in figure 21a. The mold to the left of the casting will also have been heated by the molten metal flowing over it and its ability to conduct heat away from the casting will be reduced so that the cooling of the casting in that area will be retarded. Figure 21b shows the casting with solidification in a more advanced stage. Because of controlled solidification, this will probably be a sound casting. However, the reduction of area at the corner is undesirable from the structural design standpoint.In such cases, the desired directional solidification of the casting must be obtained by other methods. In designing a casting to control directional solidification, tapering sections can be used. The sections are tapered with the larger dimensions toward the direction of feeding. When a flat casting is poured, solidification will begin at about the same rate from both sides and centerline shrinkage will be found because of the lack of directional solidification. Solidification of this type is known as progressive solidification and is shown in figure 22a. If this casting did not have Figure 22b shows the taper employed to obtain directional solidification. It will be noted that although solidification has taken place at the same rate from the opposing walls, the taper permits molten metal to feed the casting properly.The most effective and most easily used is the chill. Chills are used to start or speed up solidification in a desired section of a casting.The method of inscribed circles, illustrated in figure 23, can be used to predict the location of hot spots, which are locations of final solidification and possible shrinkage. In the L section, the largest circle which can be drawn in the junction is larger than the largest circles that can be drawn in the walls. The same is true of the T section, where the circle at the junction is even larger than the one for the L section. The larger circles in both of the junctions predict the location of a hot spot, which will be unsound unless special precautions are taken. Figure 23b shows similar junctions with the progress of solidification indicated by the shaded areas. These sketches were made from actual laboratory studies of the solidification of the junctions. The location of the small white area in each case indicates the location of the hot spot. These small spots are within the large circles inscribed at the junctions as shown in figure 23a.If small fillets and rounded corners are used in the L or V-type junction, a heavy section will be formed. Radii should be used so that the thickness in the junction will be the same as that in the adjoining walls. This is shown in figure 24. The area within the dashed line shows the amount of metal which should be eliminated to avoid hot spots. The wall thickness at the junction can be reduced further by using radii which will produce a junction thinner than the adjoining sections. Such junctions would be used only if they wereThe first method is most commonly used. Chills may also be used to produce a sound junction.The only way to reduce the wall section in this type of junction is to use a core, as shown in figure 25b, to produce a hole in the junction. A method which is preferred, especially when the junction is a result of ribbed construction, is that of staggering the sections so as to produce T junctions which can be more easily controlled with chills. Figure 25c shows the staggered design. Various treatments for a T section are shown in figure 26. A cored hole can be used, as in figure 26a; the section thickness can be used, as in figure 26b; the external chills can be used, as in figure 26c; or internal chills can be used, as in figure 26d. Internal chills should not be used without authorization from the foundry supervisor.The use of ribs produces a hot spot at the junction because it is thicker. The heavy section may also be reduced by using a core to make a hole at the junction of the rib with the casting section, as shown in figure 27.Everything should be done, however, to give the casting a section having a gradual taper, so that the best possible conditions for solidification can be obtained. A detailed discussion of a good casting design cannot be given here, but a few examples are given of design features which can be of help to the molder and patternmaker in making a better casting.As originally designed, the tubular section had a heavier wall than the plate. Redesigning eliminated the heavy section in the casting. A hub casting is shown in figure 29. The inscribed circle shows the heavy section which would be difficult to feed and would probably cause a shrinkage defect. A cross section of the same casting is shown in figure 30 as it was redesigned to eliminate the heavy section and make the casting more adaptable to directional solidification.The bracket shown in figure 31 is such a casting. The original design did not haveThis not only made the making of the mold difficult, but also resulted in heavy sections in the casting with the possibilities of shrinkage defects. By padding the area as shown by the shaded portions, the pattern was easier to draw and feeding of the lugs was simplified.Note that the thin sections are connected to the heavy sections which are located so that they may be easily fed.It must be remembered that in many cases, these rules cannot be followed to the letter. There also may be a conflict between rules. In such a case, a compromise must be made which will best suit the casting desired.Sections should never be tapered so that thick sections are far from the risers.Ribbed construction can often be used to replace a heavier section.The use of cores should be kept to a minimum. If a casting is complicated, consider the use of several simpler castings which can be welded together.As such, it is a tool in the hands of the foundryman. A great deal of success in producing a good casting depends on the quality and design of the pattern. For example, a pattern that does not have the proper draft is difficult to draw from the sand without breaking the mold.The casting design should be as simple as possible, since it will determine the ease with which a pattern can be drawn from the mold, the number of loose pieces required in the pattern, and the number of cores needed.A loose pattern is the wood counterpart of the casting, with the proper allowance in dimensions for contraction and machining. A typical loose pattern is shown in figure 33. A loose pattern may be made in one piece or it may be split into the cope and drag pieces to make molding easier. A split pattern is shown in figure 34.In such a case, the part to be used as a pattern must be built up to allow for the contraction of the cast metal and prevent the new casting from being too small. When Celastic dries, it will adhere firmly and form a hard surface which may be sandpapered or sawed like wood. For directions on the use of Celastic, see theThis practice of molding the gating system eliminates the loose sand that often results when gates are hand cut. As a result, the castings produced usually are better than those produced with the loose patterns.