铸造专业英语文选.doc

Lesson one CastingThe word “casting” is used to describe both the process and the component or part that results when molten metal is poured into a mold. Casting, then is the process by which molten metal is transformed in one step into a part a component. The part or component produced is called a casting. The casting process is basically simple. First, a cavity is formed in a mold. The shape of the cavity determines the shape of the casting. Liquid (or molten) metal is poured into the mold, then is allowed to cool and become solid. After the metal has solidified, the casting can be removed from the mold. The procedure can be repeated for production of duplicate parts. A given shape may be produced in quantities in the millions. An important characteristic of casting is that even an intricate part can often be produced in one piece. A similar part prepared by welding or bolting would have to be assembled for many pieces. There are a variety of specific techniques for making a casting, but they all require a mold. The molds may be of two kinds: sand molds and metal molds. Of all cast metals, the most common is cast iron. Most cast iron is melted in a cupola. It will melt at a proper temperature and will be fluid when it is molten. Cast iron can be made very fluid, so it can be cast into complicated shapes by using a mold. Lesson Two Patterns for Castingsthe first step in making a casting is to prepare a pattern. Most patterns are made of wood because of its cheapness and ability to be worked easily. When high production of sand castings is required, more durable and stronger materials, such as ferrous and nonferrous metals, are employed to make patterns. Brass, aluminum alloy, and magnesium alloy patters are recommended for medium and light castings. Ferrous alloys are frequently used for large patterns. If metal patterns are decides upon, the first or master patterns are usually made from suitable wood and, thereafter, the metal patterns are cast from these wood patterns. In making patterns, additional pattern allowances must be provided in order to obtain sand castings of the desired dimensions. The quality of the final products depends upon intelligent planning of the pattern by the pattern maker. Before a pattern is made, the pattern maker must visualize from the blueprint what the casting will look like when completed and how it can be best molded. This preliminary estimate is important, since the molding expense in the foundry depends to a great extent on proper pattern construction. Lesson 3 molding sandsSand is the principal basic molding material used by the foundry man, whether it is for iron, steel, non-ferrous or light-alloy castings. For every ton of iron castings produced it is estimated that one ton of sand is used. Molding sands must possess many useful properties. They are fineness, plasticity, strength or bond, permeability, refractoriness and durability. All natural molding sands have these various qualities in different degrees. The chief constituents in sands are silica grains, which resist the heat, and aluminum silicate, or clay, which serves as a binder and makes molding possible. Other binders are frequently present in natural sands, and artificial binders are often added. Fineness is required, particularly for green sand work, to ensure a smooth surface in the casting. Plasticity is necessary to obtain a clean impression of the pattern or strikle. In green sand molding, this demands correct proportioning with water. Strength or bond is the ability of the sand to hold or bind together. There are two kinds of bond, i.e. green bond and dried bond. Permeability is a measure of the ease with which air or gas can pass through rammed sand. Refractoriness is required to avoid the fusing of the sand at the mould face on to the casting, so as to leave a smooth clean skin on the casting. Durability is an important property of the sand, it relates to the ability of the sand to withstand periods of service. Lesson 4 Green sand moldingOf all the methods in the production of castings, the most common is the green sand molding process. The green sand molds can be made by hand or machinery. The sand is called green because it depends on moisture for bond. Molding sands have three principal ingredients: silica sand of specified grain size, shape, and uniformity; green bond of desired plasticity; and moisture. The sand is not suitable for thick cross-sections, being readily washed off by the metal stream, but the expense of drying is avoided, greater collapsibility ensured, and the molding boxes are usable several times a day. Medium-sized green sand molds are enclosed in flasks that consist of two parts, the upper half or cope, and the lower half or drag. The plane separating the cope and drag is called parting line. The shape of the desired casting is simulated by a pattern around which the molding sand id formed. When the pattern is removed, the shape of the mold cavity is identical with the shape of the pattern. Sufficient taper, called draft, is placed on the sides of the pattern to facilitate withdrawal of the pattern without damage to the mold cavity. The vertical passageway through which molten metal flows down to the parting plane is called the sprue. The horizontal connection in the parting plane between the mold cavity and the sprue is the gate. For maximum venting, the sand should be highly permeable, with open grains, and provided with plentiful vents. Lesson 5 dry sand moldsDry sand molds are over-dried to a depth of 1/2 inch, or more. (the dried layer is usually deeper than 1/2 inch, depending on section thickness, and may extend clear through the section.) the molds are baked at 300 to 700 for 8 to 48 hours, depending on the binders used in the sand mixture and the amount of sand surface to be dried, and on the requirements of the production cycles. Dry sand molds generally used are used in preference to green sand molds for making medium-size to large castings, such as large rolls(滚筒机，碾压机), housings（壳体，机架）, gears and machinery components. Advantages of dry sand molds are:1. They are stronger than green sand molds, and thus are less susceptible to damage in handling. 2. Over-all dimensional accuracy of the mold is better than for green sand molds. 3. Surface finish of castings is better, mainly because dry sand molds are coated with a wash. Disadvantages of dry sand molds are: 1. Castings are more susceptible to hot tears. (热纹)2. Distortion is greater than for green sand molds, because the baking. 3. More flask equipment is needed to produce the same number of finished pieces, because processing cycles are longer than for green sand molds. 4. Production is slower than for green sand molds. Lesson 6 coreIf the casting is to be hollow or have a hole through it, a core must be used. The core will be placed in the mold, positioned by core prints（芯头）, and the molten metal allowed to solidify around it. The external shape of the core thus becomes the internal shape of the casting. Core may be made of metal, plaster, and investment and ceramic materials as well as sand. Sand cores, along with molding, are the most frequently used. Most cores are made of core-sand mixture consisting of sand grains and organic binders which provide green strength, baked strength and collapsibility. Green strength is required so that the core sand may be molded to shape, i.e. for core-making. The core obtains its real strength and hardness when it is baked in a core oven. However, because the strength comes from core oil, the strength is lost and the core becomes collapsible when hot metal is poured around the core. Core-making is done manually and with machines. Making small cores, which are made by hand-filling core boxes with the core sand, usually is done at core benches (制芯台). In this case, only a core plate is required as equipment. The core is filled with core sand, rammed, and sruck off. Then it is transferred to a core plate for baking. For quantity production, a coreblowing machine (吹芯机) can be used. A two-part core box（芯盒）, designed to form the core in one piece is arranged so that sand can be mobbed into it under air pressure. Lesson 7 molding machineMolding machines have been designed to do part of the work that a molder previously has done by hand. No satisfactory automatic molding machine has been developed up to the present time. The chief operations performed by molding machines consist of ramming the sand into the mold, rolling the mold over, and drawing the pattern. Ramming the sand into the mold by hand is a slow process. The pneumatic (气动的，气体的) rammer increases the amount of ramming a man can do. In molding machines the whole flask may be rammed at one time, or the filling and ramming may be accomplished at one time. If no consideration were taken of the large increase in production possible by their use, the improvement in quality of castings alone would oftentimes warrant their installation, as the decrease in cost of machining castings produced by this method pays good dividends on the investment. The use of unskilled workmen on these machines is no small item in their favor. Several types of molding machines have been developed to ram the sand into the mold. The three types of ramming machines that have survived are the squeezer(压实造型机), the jar or jolt ram machine（震实造型机）, and the sand slinger（抛砂机）. The squeezer is limited to small, shallow flasks that are handled by hand. The jar or jolt ram machine is best adapted to molds that are larger and too deep to squeeze. The sand slinger is a machine that is used for medium and heavy work. Lesson 8 Sand-slingerUniform packing of the sand in molds is an important operation in the production so castings. To accomplish this operation in a satisfactory manner, particularly for large molds, a mechanical device known as the sand-slinger had been developed. The supply of sand is carried in a large tank of about 300cubic feet capacity, which may be refilled at intervals by overhead handling equipment. A delivery belt, feeding out of a hopper on the frame at the fixed end, conveys the sand to the rotating impeller head. The impeller head, which is enclosed, contains a single, rotating, cup-shaped part which slings sand into the mold. This part, rotating at high speed, slings over a thousand small buckets of sand a minute. The ramming capacity of this machine is 7 to 10 cubic feet, or 1000 pounds of sand per minute. The density of the packing can be controlled by the speed of the impeller head. For high production, machines of this type are available, having a capacity of 4000 pounds of sand per minute. Sand-slingers can be obtained either with a tractor mounting or as a stationary unit. Tractor-type sand-slingers travel along the sand piled on the floor and are used in foundries having no auxiliary sand-handling equipment. In addition to the ramming operation, these machines cut, riddle, and magnetically separate the sand from the scrap. The stationary machine is adapted to the production work and must be served by sand preparation and conditioning equipment, as well as conveyors for removing the molds. Sand-slinger machines greatly increase foundry production and insure the uniform ramming of molds. Lesson 9 CupolaIn most cases cupola is the lowest-cost unit for melting gray and ductile iron. The greater amount of gray iron is melted in the cupola and a constantly increasing amount in electric arc and induction furnaces. It is also widely used to melt malleable iron, which is usually further processed through an air furnace or electric arc furnace. The cupola is very simple in construction. It is divided into five zonesthe hearth, the tuyere, melting zone, charging zone and the stack. The hearth is sometimes called the well because the melted iron drops down into it before being tapped out. The melting zone is where the melting takes place and is the hottest party of the cupola. In principle, the cupola is little more than a stack in which is built a coke fire. Pig iron and scrap are charged into the furnace in layers to be melted. A large blower provides a controlled blast of air to produce the necessary heat. Limestone is added to the charge to promote fluxing and melting of the metal. In practice, cupola operation is quite complex. Careful attention is given to the volume of air and pressure of the blast and to coke size, amount, and composition. Charges of pig iron, scrap, coke and limestone are carefully calculated to produce the desired chemistry, temperature, etc., and are painstakingly weighed. Frequent test specimens are poured and tested to be sure that specifications are being met. Temperature readings are taken often to be sure that pouring temperatures are adequate. Good cupola operating practices, adequate controls, and frequent tests are vital to the quality of iron produced. There are great differences among foundries in the quality of cupola practices employed. Lesson 10 Cast IronCast iron is a general term applied to a wide range of iron-carbon-silicon alloys in combination with smaller percentages of several other elements. It is an iron containing so much carbon, or its equivalent, that it is not malleable. Quite obviously, cast iron has a wide range of properties, since small percentage variations of its elements may cause considerable change. Cast iron should not be thought of as a metal containing a single element, but rather, as one having in its composition at least six elements. All cast irons contain iron, carbon silicon, manganese, phosphorus, and sulfur. Alloy cast iron has still other elements which should not be thought of as impurities, for they all have important effects on the physical properties. Pure iron, known as ferrite, is very soft and has few uses in industrial work. All desirable properties, such a strength, hardness, and machinability, are controlled by regulating the elements other than ferrite in the cast iron. The cast iron family includes gray, ductile (also called nodular), white, malleable, and high alloy irons. Gray iron is itself a family of casting alloys, and is the most widely used, with an annual production several times the total for all other metal cast/ A relatively new type of cast iron is ductilealso called nodulariron, as its name implies, ductile iron offers more ductility than gray iron, plus higher strength. Lesson 11 cast steelsMost steel castings are plain carbon steel alloys. These have various amounts of carbon without substantial amounts of other allying elements. These are classified into three general categories: low, medium and high carbon steel. Each class contains carbon as following:Low-carbon has up to 0.20%. Medium-carbon has between 0.20% and 0.50%. High-carbon has more than 0.50%.Low-carbon steels are the softest and most ductile of the three. Their properties are not greatly influenced by heat treatment. Medium-carbon steels, the most common, are harder than low-carbon steels; heat treatment increases their ductility and impact resistance. Because of the higher carbon content, high-carbon steels offer the highest strength and hardness. They are used where wear and abrasion-resistance are necessary. When other alloying elementsin addition to carbonare added in substantial amounts, the steels are called alloy steels. The two main categories are low alloy and high alloy steels. Low alloy cast steels have a carbon content 0.45% or less, and total alloying elements of less than 8%. Because of the favorable effects of these elements, low-alloy steels develop higher strength through heat treatment. If low-alloy steels have less than 8% alloying elements, the high-alloy steels must have more than 8%. Types of high-alloy steels:Chromium steelchromium from 12%-30%Chromium-nickel steelchromium from 18%-32% and nickel from 8$-20%Nickel-chromium steel-nickel from 33%-41% and chromium from 13%-21%High-nickel-chromium steelnickel from 58%-68% and chromium from 10%-19%Lesson 12 aluminum alloyMany aluminum alloys are castable and used widely where light weight and corrosion-resistance offset their higher cost. The principal alloying elements with aluminum are as follows:1) Silicon, which improves the casting qualities, making the metal flow better into intricate molds and thin cross sections. 2) Copper, which reduces shrinkage and improves casting qualities but also reduces corrosion resistance. 3) Magnesium, which makes the alloy heat-treatable when more than 6 percent is used or where it is combined with other elements. Aluminum alloys containing magnesium as a definite alloying constituent require special attention because they are particularly prone to metal/mold reaction. Magnesium is liable to considerable oxidation whilst melting, and it is desirable to use a cover flux which will not only provide a protection but cleanse the alloy of oxides. 4) Zinc, which improves mechanical properties, and, when it is used to the extent of 5 percent to 7 percent, is the basis for the “self-aging” alloys. After an aging period of a few weeks, these develop characteristics almost equivalent to those of the heat-treated alloys. Parts can be made from these alloys that would not be practical to heat-treat because of strains and distortion. 5) Nickel, which adds dimensional stability and heat resistance. 6) Titanium, which refines the grain size and improves mechanical properties. Most aluminum casting alloys contain several or all of these elements in varying proportions, according to the qualities desired and the method of casting. Lesson 13 FluidityFluidity as used in the cast metal industry is quite different from the term defined in physical chemistry. In metal casting, it is the ability to fill a mold. To provide a measure of fluidity, a