文献翻译-金属的砂型铸造，锻造与热处理

英文原文MENTALS SANDCASTING.FORGING AND HEAT TREATMENTFirst SAND CASTINGMost metal castings are made by pouring molten metal into a prepared cavity and allowing it to solidify. The process dates from antiquity. The largest bronze statue in existence today is the great Sun Buddha in Nara Japan. Cast in the eighth century, it weighs 500 metric tons and is more than 21 m high .Artisans of the Shang Dynasty in China created art works of bronze with delicate filigree as sophisticated as anything that is designed and produced today.There are many casting processes available today , and selecting the best one to produce a particular part depends on several basic factors,such as cost , size , production rate , finish , tolerance , section thickness , physical-mechanical properties , intricacy of design , machinability, and weldability . Sand casting, the oldest and still the most widely used casting process, will be presented in more detail than the other processes since many of the concepts carry over into those processes as well. Green Sand Green sand generally consists of silica sand and additives coated by rubbing the sand grains together with clay uniformly wetted with water. More stable and refractory sands have been developed, such as fused silica, zircon, and mullite, which replace lower-cost silica sand and have only 2%linear expansion at ferrous metal temperatures. Also, relatively unstable water and clay bonds are being replaced with synthetic resins, which are much more stable at elevated temperatures. Green sand molding is used to produce a wide variety of castings in sizes of less than a pound to as large as several tons. This versatile process is applicable to both ferrous and nonferrous materials . Green sand can be used to produce intricate molds since it provides for rapid collapsibility; that is, the mold is much less resistant to the contraction of the casting as it solidifies than are other molding processes. This results in less stress and strain in the casting. The sand is rammed or compacted around the pattern by a variety of methods, including hand or pneumatic-tool ramming, jolting, squeezing, and driving the sand into the mold at high velocities. Sand slingers are usually reserved for use in making very large castings where great volumes of sand are handled.For smaller castings, a two-part metal box or flask referred to as a cope and drag is used. First the pattern is positioned on a mold board, and the drag or lower half of the flask is positioned over it. Parting powder is sprinkled on the pattern and the box is filled with sand. A jolt squeeze machine quickly compacts the sand. The flask is then turned over and again parting powder is dusted on it. The cope is then positioned on the top half of the flask and is filled with sand, and the two-part mold with the pattern board sandwiched in between is squeezed. Patterns Patterns for sand casting have traditionally been made of wood or metal. However, it has been found that wood patterns change as much as 3% due to heat and moisture. This factor alone would put many castings out of acceptable tolerance for more exacting specifications. Now, patterns are often made from epoxies and from cold-setting rubber with stabilizing insets. patterns of simple design with one or more flat surface , can be molded in one piece , provided that they can be withdrawn without disturbing the compacted sand . Other patterns may be split into two or more parts to facilitate their removal from the sand when using two-partflasks. The pattern must be tapered to permit easy removal from the sand, The taper is referred to as draft. When a part does not have some natural draft, it must be added. A more recent innovation in patterns for sand casting has been to make them out of foamed polystyrene that is vaporized by the molten metal. This type of casting, known as the full-mold process, does mot require pattern draft. Sprues, Runners and GatesAccess to the mold cavity for entry of the molten metal is provided by sprues, runners, and gates, as shown in Fig.7-1. A pouring basin can be carved in the sand at the top of the sprue, or a pour box, which provides a large opening, may be laid over the sprue to facilitate pouring. After the metal is poured, it cools most rapidly in the sand mold. Thus the outer surface forms a shell that permits the still molten metal near the center to flow toward it. As a result, the last portion of the casting to freezw will be deficient in metal and, in the absence of a supplemental metal-freeze will be deficient in metal and, in the absence of a supplemental metal-feed source, will result in some form of shrinkage. This shrinkage may take the form of gross shrinkage or the more subtle microshrinkage. These porous spots can be avoided by the use of risers, as shown in Fig.7-1, which provide molten metal to make up for shrinkage losses. Cores Cores are placed in molds wherever it is necessary to preserve the space it occupies in the mold as a void in the resulting castings. As shown in Fig.7-1, the core will be put in place after the pattern is removed. To ensure its proper location, the pattern has extensions known as core rints that leave cavities in the mold into which the core is seated. Sometimes the core may be molded integrally with the green sand and is then referred to as a green-sand core. Generally, the core is made of sand bonded with core oil, some organic bonding materials, and water. These materials are thoroughly blended and placed in a mold or core box. After forming, they are removed and baked at . Cores that consist of two or more parts are pasted F00453together after baking. 2COCores cores are made by ramming up moist sand in a core box . Sodium silicate is used as a binder, which is quickly hardened by blowing gas over it. The system has the 2CO2COadvantage of making the cores immediately available. Pouring the MetalSeveral types of containers are used to move the molten metal from the furnace to the pouring area. Large castings of the floor-and-pit type are poured with a ladle that has a plug in the bottom, or, as it is called, a bottom-pouring ladle. It is also employed in mechanized operations where the molds are moved along a line and each is poured as it is momentarily stopped beneath the large bottom-pour ladle.Ladles used for pouring ferrous metals are lined with a high alumina-content refractory. After long use and oxidation, it can be broken out and replaced. Ladles used in handling ferrous metals must be preheated with gas flames to approximately 3600 to 2700 F (1427 to 1482C) before filling. Once the ladle is filled, it is used constantly until it has been emptied.For nonferrous metals, simple clay-graphite crucibles are used. While they are quite susceptible to breakage, they are very resistant to the metal and will hold up a long time under normal conditions. They usually do not require preheating, although care must be taken to avoid moisture pickup. For this reason they are sometimes baked out to assure dryness. The pouring process must be carefully controlled, since the temperature of the melt greatly affects the degree of liquid contraction before solidification, the rate of solidification, which in turn affects the amount of columnar growth present at the mold wall, the extent and nature of the dendritic growth, the degree of alloy burnout, and the feeding characteristics of the risering system.Finishing OperationsAfter the castings have solidified and cooled somewhat, they are placed on a shakeout table or grating on which the sand mold is broken up, leaving the casting free to be picked out. The casting is then taken to the finishing room where the gates and risers are removed. Small gates and risers may be broken off with a hammer if the material is brittle. Larger ones require sawing, cutting with a torch, or shearing. Unwanted metal protrusions such as fins, bosses, and small portions of gates and risers need to be smoothed off to blend with the surface. Most of this work is done with a heavy-duty grinder and the process is known as snagging or snag grinding. On large castings it is easier to move the grinder than the work, so swing-type grinders are used. Smaller castings are brought to stand-or bench-type grinders. Hand and pneumatic chisels are also used to trim castings. A more recent method of removing excess metal from ferrous castings is with a carbon-air torch. This consists of a carbon rod and high-amperage current with a stream of compressed air blowing at the base of it. This oxidizes and removes the metal as soon as it is molten. In many foundries this method has replaced nearly all chipping and grinding operations.SECOND FORGING Bulk deformation of metals refers to various processes, such as forging, rolling, or extruding, where there is a controlled plastic flow or working of metals into useful shapes. The most well known of these processes is forging, where deformation is accomplished by means of pressure, impact blows, or a combination of both .Hammer ForgingHammer forging consists of striking the hot metal with a large semiautomatic hammer. If no dies are involved, the forging will be dependent mainly on the skill of the operator. If closed or impression dies are used, one blow is struck for each of several die cavities. Again, productivity and quality depend to a large degree on the skill of the hammer operator and the tooling.Press ForgingPress forging is characterized by a slow squeezing action. Again, open or closed dies may be used. The open dies are used chiefly for large, simple-geometry parts that are later machined to shape. Closed-die forging relies less on operator skill and more on the design of the perform and forging dies. As an example of the versatility of the process, newer developments have made it possible to produce bevel gears with straight or helical teeth. Rotation of the die during penetration will press bevel gears with spiral teeth.Open-die ForgingOpen-die forging is distinguished by the fact that the metal is never completely confined as it is shaped by various dies. Most open-die forgings are produced on flat, V, or swaging dies . Round swaging dies and V dies are used in pairs or with a flat die. The top die is attached to the ram of the press, and the bottom die is attached to the hammer anvil or, in the case of press open-die forging, to the press bed.As the work piece is hammered or pressed, it is repeatedly manipulated between the dies until hot working forces the metal to the final dimensions, as-shown in Fig.8-1. After forging, the part is rough-and finished machined. As an example of the amount of material allowed for machining, a 6.5 in. diameter shaft would have to be forged to 7.4 in. diameter.In open-die forging of steel, a rule of thumb says that 50lb of falling weight is required for each square inch of cross section.Impression-die Forging In the simplest example of impression-die forging, two dies are brought togethe, and the work piece undergoes plastic deformation until its enlarged sides touch the side walls of the die. A small amount of material is forced outside the die impression, forming flash cools rapidly and presents increased resistance to deformation, effectively becoming a part of the tool, and helps build up pressure inside the bulk of the work piece that aids material flow into unfilled impressions. Closed-die forgings, a special form of impression-die forging, does not depend on the formation of flash to achieve complete filling of the die. Thus closed-die forging is considerably more demanding on die design. Since pressing is often completed in one stroke, careful control of the work piece volume is necessary to achieve complete filling without generating extreme pressures in the dies from overfilling. Extrusion ForgingAs with upsetting, extrusion forging is often accomplished by cold working. Three principal types of metal displacement by plastic flow are involved. Backward and forward, tube, and impact extrusion are shown in Fig.8-3. . The metal is placed in a container and compressed by a ram movement until pressure inside the metal reaches flow-stress levels. The work piece completely fills the container, and additional pressure causes it to leave through an orifice an form the extruded product.Extruded products may be either solid or hollow shapes. Tube extrusion in used to produce hollow shapes such as containers and pipe. Reverse-impact extrusion is used for mass production of aluminum cans. The ram hits a slug of metal in the die at high impact, usually 15times the yield strength of the metal, which causes it to flow instantaneously up the walls of the die. Other common hollow extrusion products are aerosol cans, lipstick cases, flashlight cases, and vacuum bottles. Secondary operations, such as beading, thread rolling, dimpling, and machining, are often needed to complete the item. Roll Forging Roll forging in its simplest form consists of a heated billet passing between a pair of rolls that deform it along its length. Compared to conventional rolling processes, the rolls are relatively small in diameter and serve as an arbor into which the forging tools are secured. The active surface of the tool occupies only a portion of the roll circumference to accommodate the full cross section of the stock. The reduction of the cross section obtainable in one pass is limited by the tendency of the material to spread and form an undesirable flash that may be forged into the surface as a defect in the subsequent operations. The work piece is int reduced repeatedly with rotation 09between passes. Ring RollingRing rolling offers a homogeneous circumferential grain flow, ease of fabrication and machining, and versatility of material size. Manufacture of a rolled ring starts with a sheared blank, which is forged to a pancake, punched, and pierced.There is no limit to the size of the rolled rings, ranging from roller-bearing sleeves to rings 25ft in diameter with face heights of 80in. Various profiles may be rolled by suitably shaping the driven, idling rolls. CAD/CAM in Forging CAD/CAM is being increasingly applied to forging. Using the three-dimensional description of a machined part, which may have been computer designed, it is possible to generate the geometry of the associated forging. Thus the forging sections can be obtained from a common data base. Using well-known techniques, forging loads and stresses can be obtained and flash dimensions can be selected for each section where metal flow is approximated as two dimensional. In some relatively simple section geometries , computer simulation can be conducted to evaluate initial guesses on perform sections .Once the perform has been developed to the designers satisfaction , this geometric data base can utilized to write NC part programs to obtain the NC tapes or disks for machining .THIRD HEAT TREATMENT OF METAL AnnealingThe word anneal has been used before to describe heat-treating processes for softening and regaining ductility in connection with cold working of material. It has a similar meaning when used in connection with the heat treating of allotropic materials. The purpose of full annealing is to decrease hardness, increase ductility, and sometimes improve machinability of high carbon steels that might otherwise be difficult to cut. The treatment is also used to relieve stresses, refine grain size, and promote uniformity of structure throughout the material.Machinability is not always improved by annealing. The word machinability is used to describe several interrelated factor, including the ability of a material to be cut with a good surface finish. Plain low carbon steels , when fully annealed , are soft and relatively weak , offering little resistance to cutting , but usually having sufficient ductility and toughness that a cut chip tends to pull and tear the surface from which it is removed , leaving a comparatively poor quality surface , which results in a poor machinability rating . For such steels annealing may not be the most suitable treatment. The machinability of many of the higher plain carbon and most of the alloy steels can usually be greatly improved by annealing, as they are often too hard and strong to be easily cut at any but their softest condition.The procedure for annealing hypoeutectoid steel is to heat slowly to approximately C06above the line , to soak for a long enough period that the temperature equalizes throughout 3CAthe material and homogeneous austenite is formed , and then to allow the steel to cool very slowly by cooling it in the furnace or burying it in lime or some other insulating material . The slow cooling is essential to the precipitation of the maximum ferrite and the coarsest pearlite to place the steel in its softest, most ductile, and least strained condition.NormalizingThe purpose of normalizing is somewhat similar to that of annealing with the exceptions that the steel is not reduced to its softest condition and the pearlite is left rather fine instead of coarse. Refinement of grain size, relief of internal stresses, and improvement of structural uniformity together with recovery of some ductility provide high toughness qualities in normalized steel. The process is frequently used for improvement of machinability and for stress relief to reduce distortion that might occur with partial machining or aging.The procedure for normalizing is to austenitize by slowly heating to approximately C08above the or temperature for hypoeutectoid or hypereutectoid steels , respectively ; 3CA3cmproviding soaking time for the formation of austenite ; and cooling slowly in still air . Note that the steels with more carbon than the eutectoid composition are heated above the instead of cmAthe used for annealing. The purpose of normalizing is to attempt to dissolve all the 31ccementite during austenitization to eliminate, as far as possible, the settl