mud pump series steel castings free sample

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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 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.

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

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.

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.

Although the physical properties of coarse-grained metals differ from those of fine-grained metals of the same chemical composition, this difference will not be considered in detail. As one example, coarse grains lower the strength of steel.

arranged in any particular pattern and that grow about the same length in each direction. 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.

Many defects in castings are caused by gases which dissolve in the metal and then are given off during solidification. 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. As an example, at room temperature and atmospheric pressure, 0.001 percent by weight of hydrogen in a metal occupies a volume equal to that of the metal, and at 2,000°F., the same amount of hydrogen would occupy a volume equal to four times that of metal.

A gas frequently absorbed by metals is the hydrogen produced from water vapor. 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.

Gas defects in castings are not always caused by gas that is dissolved in the molten metal. In some cases, these defects are caused by gases driven into the metal from the mold.

The gases are trapped as the metal solidifies. 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.

A good example of the formation of a casting defect due to gas in pinhole formation in steel. 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 which is formed in this reaction passes through the thin layer of solid steel and enters the still molten steel. 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.

Design influences the soundness, freedom from dirt, shrinkage, porosity, hot tears, and cracks found in a casting, and thus affects its serviceability. 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.

One of the major factors that cause the untimely failure of castings is the concentration of stresses that results from improper design. Stresses, of course, are the forces and loads that cause a casting to crack, tear, or break.

Sharp corners and notches should be avoided in castings because they are points of high stress. 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.

There are some castings in which the design must allow for the absorption of casting stresses in order to produce a good casting. 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.

7. A casting should be made as simple as possible. 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.

Loose Patterns. The majority of molds made aboard repair ships are made with loose patterns, since castings required are usually few in number and not too often repeated. 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.

The main advantage of the mounted pattern over the loose pattern is that it is easier to use and store. For these reasons, a mounted pattern is generally warranted when several of the castings (say, five or more) are to be made during one "run" or when the casting is made at frequent intervals.

Another advantage of the mounted pattern is that a pattern of the gating system also can be mounted on the match plate. This 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.

Core Boxes. Core boxes are actually negative patterns. When looking at a pattern, one sees the casting in its actual shape. A core box on the other hand shows the cavity which will be created by the core. Core boxes are used not only to make cores for holes in castings but also to make parts of a mold. In some cases, a pattern cannot be made so that it can be drawn. In such a case, the part of the casting which would hinder drawing is made as a core that can be placed in the mold after the pattern proper has been withdrawn. The making and proper use of cores is described in Chapter 6, "Making Cores."

Metal patterns are usually used as mounted patterns, with the gating included in the pattern. Their use is warranted only when a large number of castings must be made. Mounted metal

A material which maybe used for an emergency pattern, when only a small number of castings are required and there is not sufficient time to make a wooden pattern, is plaster or gypsum cement. Gypsum cement is made from gypsum rock, finely ground and heated to high temperatures. When mixed with water, it forms a plastic mass which can be molded, shaped, or cast. Plaster patterns have the disadvantage of being very fragile and require careful handling, therefore, it is recommended for use only in an emergency.

A good practice for constructing core prints is shown in figure 35. It results in castings with fewer fins at the parting line. Fins tend to produce cracks, and require extra time to clean off the casting. Larger core prints provide better core location and support in the mold. In addition, they reduce the tendency for cracks to form in the cored openings from core fins.

Chaplets. When the design of the core is such that additional support over and above that given by the core prints is needed, it is necessary to use chaplets. These chaplets are pieces of metal especially designed to support the core. Detailed description of chaplets and their use will be found in Chapter 5, "Making Molds." Their use is to be avoided wherever possible, particularly on pressure castings. If chaplets are necessary, their location and size should be indicated on the pattern and core box by raised sections such as shown in figure 36. This additional metal in the mold cavity serves two main purposes; first, it accurately locates the best chaplet position and insures that the location will be consistently used; second, it provides an additional mass of metal to aid in the fusion of the chaplet, which is necessary to obtain pressure tightness.

The shrinkage rule to be used in constructing a pattern must be selected for the metal which will be used in the casting. It must be remembered that the shrinkage rule will also vary with the casting design. For example, light and medium steel castings of simple design and no cores require a 1/4-inch rule,

whereas for pipes and valves where there is a considerable resistance offered to the contraction of the steel by the mold and cores, a 3/16- inch rule will be adequate. Shrinkage allowances for various metals and mold construction are listed in table 4.

that are to be machined. Some castings do not require finish since they are used in the rough state just as they come from the final cleaning operation. Most castings are finished only on certain surfaces, and no set rule can be given as to the amount of finish to be allowed. The finish is determined by the machine shop practice and by the size and shape of the casting. A casting may become distorted from stresses during the casting process or surfaces, because of the lower strength of isolated volumes of sand. The draft is dependent on the shape and size of the casting and should at all times be ample. The actual draft to be used is usually determined by consultation between the patternmaker and the molder. Proper and improper drafts are shown in figure 39.

Distortion Allowance. Many times a casting is of a design which results in cooling stresses that cause distortion in the finished casting. The design may also be such that it cannot be corrected in the design. In such a case, the experience of the molder and pattern maker must be relied upon to produce a good casting. Distortion allowances must be made in a pattern and are usually determined by experience. Recorded information on castings of this type is very useful in determining distortion allowances on future work.

For applications where the quantity of castings required is small and the designs are quite simple, gypsum cement can be used as pattern material with success. See Pattern Materials.

The patterns normally made aboard repair ships are used for a few castings and then they must be stored. It is important that storage space be provided which is as free of moisture as possible. This precaution will maintain the patterns in good condition and prevent warping and cracking. The storage of patterns should be in properly constructed racks wherever possible. This will keep pattern damage down to a minimum.

In table 6 are areas and volumes for calculating weights of castings. This table shows the various shapes and formulas which are useful in calculating casting weights.

Synthetic sands consist of a naturally occurring sand with a very low clay content, or a washed sand (all of the natural clay removed), and an added binder, such as bentonite. Synthetic sands have the following advantages over naturally bonded sands: (1) more uniform grain size, (2) higher refractoriness, (3) mold with less moisture, (4) require less binder, (5) the various properties are more easily controlled, and (6) less storage space is required, since the sand can be used for many different types of castings.

Sands that are used for a variety of casting sizes and types of metals are called "all-purpose" sands. In commercial practice, different sands are used to cast different metals and different sizes of castings of the same metal, but in a shipboard foundry, the limitation of storage space makes the practice of maintaining many special sands impossible. A synthetic sand used as a base for an all-purpose sand has the requirements for a molding sand for shipboard use. Naturally, some advantages will have to be sacrificed in using one sand for making all types of castings. The major factor that will be sacrificed in this respect is that of surface finish. However, the principal purpose of a shipboard foundry is to produce serviceable castings. Surface finish is often not a major requirement. As an example, a coarse-grained sand suitable for steel castings will produce rough surface finishes on lighter nonferrous castings made in the same sand. This is a minor disadvantage for an all-purpose sand when compared to its advantages for repair- ship use.

The general effect of grain size on permeability is shown by figure 45. Data for this curve were obtained by screening a given sand through a series of test screens and then making a permeability test on the sand retained on each screen. The permeability of the coarse sand is very high. As the sand grains become smaller, the permeability decreases rapidly. This decrease is due to the smaller voids or openings between the individual sand grains for the fine sand. Coarse sand grains have the same general size relation to fine sand grains as basketballs have to marbles.

Moisture Content. The effect of moisture content on permeability was shown in figures 46 and 47. Low permeability at very low moisture content is caused by the dry clay particles filling the spaces between the sand grains. Figures 46 and 47 both show an increase in permeability to a maximum value, and then a decrease with further additions of water. The increase in permeability is produced when the moisture causes the clay particles to agglomerate or stick together. This action is similar to the addition of water to dust to form a firm piece of soil. When water is added in excess of the amount to produce this sticking together, the excess water begins to fill in the holes between the sand grains and as a result, the permeability goes down. This action is similar to the addition of water to a firm soil to produce mud.

The dry strength of sand mixtures is generally affected in the same way as green strength by grain fineness, grain shape, and moisture content. Different binders, however, can affect dry strength and green strength differently. For example, in comparison with western bentonite, southern bentonite produces a high green strength and a low dry strength. Southern bentonite is widely used for its low dry strength and the resulting easy shakeout of castings.

Mulling Sand. To obtain the maximum properties from a molding sand, a muller should be used for the mixing of all foundry sands. It is especially important that core sands and facing sands be mixed in a muller, but the mixing of all sands in a muller provides a more uniform day-to-day operation. The use of a muller to mix and rebond sands is essential to good sand control, and shows up in the production of better castings.

A mixing time longer than those listed in table 7 does not increase the green strength. This is shown in figure 57. It is good practice to make a series of tests for green strength after various mulling times to determine the time needed to attain the maximum green strength.

It is recommended that a test series (such as that required to produce the information for figures 58 and 59) be made on each new shipment of sand before it is used in the foundry. Conducting such a series of tests and putting the information in graphical form would be a useful and informative way of conducting shipboard instruction periods. The information is developed by making a series of sand mixtures having different bentonite (or other binder) contents and different moisture contents. As an example, a series of 2 percent bentonite sand mixes with 1/2, 1, 2, 3, 4, 5, and 6 percent moisture can be tested for green strength, permeability, and dry strength. A second series of sand mixes containing 3 percent of bentonite and the same moisture contents can be tested to obtain the same properties. This procedure is then repeated for the remaining bentonite contents. The final information is then plotted to produce graphs similar to those shown in figures 58 and 59.

When a new shipment of sand is received aboard ship, a few spot tests can be made to determine how the new lot of sand compares with the previous lot. If the properties are reasonably close, the charts developed for the previous sand may be used for the new sand. However, if there is a significant difference in the physical properties, a complete series of tests should be conducted on the new lot of sand to develop a complete picture of the properties of the new sand.

Baking Time and Temperature. The best combination of baking time and temperature varies with: (1) the type of binder used, (2) the ratio of oil to sand, and (3) the type of core ovens used. Figure 61 shows the dependence of the baked strength on baking time and temperature. It will be noted that the same strength was attained in one hour when baking at 450°F., as was attained in six hours at a baking temperature of 300°F. It is always good practice to make a series of tests on the effect of baking time and temperature on the baked strength of cores before using a new core mix. Such information will provide the shortest baking time to obtain a given strength for that mix. This type of investigation will also provide information on the baking characteristics of a core oven.

To establish a full appreciation of the problems of drying cores, a series of 3-inch, 5-inch, and 8-inch cube cores should be made without rods and baked at temperatures of 400°F., 425°F., 450°F., 475°F., and 500°F. for varying predetermined times. After being taken out of the oven and cooled, they should be cut open with a saw to determine the extent to which they are baked. This simple test will aid in determining the proper times and temperatures to use for various cores in a given oven and under given atmospheric conditions.

In brass castings, where erosion and penetration are problems, a core wash made from a silica base is satisfactory. A plumbago wash is useful for bronze castings. A core wash for use with high-lead alloys and phosphor bronzes, may be made from a paste of plumbago and molasses water. Such a treatment should be followed by a thin coating of the regular core wash.

The testing of foundry sands should not be a series of tests for obtaining a great deal of meaningless information. Regular sand testing along with records of the results is the one way of establishing the cause of casting defects due to sand. Regular sand testing results in a day-to-day record of sand properties, and indicates to the molder how the sand properties behave. Proper interpretation of the results of sand tests permits the molder to make corrections to the sand before it is rammed up in molds, thereby not only saving time but also preventing casting losses.

The sample for determining the grain fineness number should be washed of all clay as described under "Clay Content," and thoroughly dried. A 50-gram sample of the sand is then screened through a series of standard sieves. The sand remaining on each screen should be

The need for proper sand control through the use of sand-test equipment cannot be stressed too strongly. There is only one way to determine the properties of molding sands and core sands, and that is to make tests. Day-by-day testing of foundry sands provides the molder with information which enables him to keep the molding sand in proper condition. The recording of these test results, along with appropriate comments as to the type of castings made and any defects which may occur, can help the molder to determine the causes of casting defects, and point the way toward corrective measures.

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