Chemical Or Extractive Metallurgy, Mechanical WorkingPhysical metallurgy, Metallic coatings
Metallurgy is the science and technology of metals. As indicated in Table 1, the recorded history of metal working goes back over 6,000 years.
Chemical or extractive metallurgy is concerned with the extraction of metals from ores and with the refining of metals. Physical metallurgy is concerned with the physical and mechanical properties of metals as affected by composition, mechanical working, and heat treatment.
INGOT CASTING. Steel and nonferrous-metal ingots that will be further worked are usually cast into ingot molds made of cast iron. In 1875, Sir Henry Bessemer patented a method of continuous casting in which a metal would be cast between two water-cooled rollers and pulled out in the form of a single plate. If it had been practicable, this method would have had the advantage of introducing no intermediate stages between the molten metal and the semifinished product. It was not until shortly before World War II that a modification of this technique proved feasible with aluminum. It was later used to cast copper, and is still under development as a tool for casting iron and steel.
MOLDS. Most metallic objects begin their history by being cast in a mold. Mold casting consists of introducing molten metal into a cavity or mold having the desired form and allowing it to solidify. The molding material affects the ease and cost of making the mold, the permanency of the mold, the rate of production, the rate of cooling of the molten metal, the surface roughness, the dimensional tolerances, and the mechanical strength of the molded piece.
Casting techniques that use a mold only once include the following:
Sand-mold casting, which is the oldest process known and is still used for the largest tonnage of castings. A pattern, slightly larger than the desired part to allow for shrinkage, is placed in a flask and molding sand is rammed around it. The pattern is then removed, and the mold is prepared for pouring. The sand used may include bonding agents such as fire clay, bentonite, cereal or liquid binders, and moisture to promote cohesion. Dry sand molds are dried thoroughly before pouring; green sand molds are poured without drying. The type of sand grain, binder, and moisture used depends on the desired results.
Shell mold casting, which uses molds that are thin shells of sand bonded with a thermosetting phenolic resin. The shell is removed from the pattern and baked at 300–400°F (147–202°C) to completely set the resins. Finally the shells are assembled to complete the mold.
Plaster of paris casting, which gives better surface finishes, dimensional accuracy, finer detail, and a more solid structure than sand castings, but it is more expensive. The plaster mold is made by mixing plaster of paris with water, then pouring it around the pattern and allowing it to partially set. The pattern is then removed. Separate parts of the mold assembly are baked separately to complete setting and to drive off moisture.
Precision casting, which differs from sand-mold casting in that the mold consists of a single part. Precision molds are used in the casting of metals and alloys that are difficult to machine. (Cast metals usually require little or no finishing treatment.) Such castings are frequently used in precision engineering, clockmaking, and the manufacture of metal ornaments.
In lost wax casting, the most widely used precision casting method, a model is made of the desired product. The model is used to produce a permanent plaster or glue mold. Wax parts are then made from the mold. The casting mold is produced by pouring a specially bonded sand around the wax pattern and allowing it to harden. The mold is inverted, placed in an oven and baked. The baking hardens the mold and melts the wax, which escapes.
When a large number of parts is needed or when better surface or dimensional control is required, a permanent metal mold may be used. Semipermanent molds consist of metal and sand molds. Metal molds, however, are unsuitable for large castings or for alloys having high melting temperatures. Permanent casting techniques include the following:
Chill casting, which is used to obtain more uniform cooling rates. Thick sections can be made to solidify by chilling them with a metal mold or with pieces of metal close to the section. Thin pieces can be preheated or made from material having poor thermal conductivity.
Pressure die casting, which permits economical production of intricate castings at a rapid rate. In this process molten metal is forced into a mold under considerable pressure. The pressure is maintained until solidification is complete.
Centrifugal casting, which involves pouring a molten metal into a revolving mold. Centrifugal action forces the metal tightly against the mold. The metal solidifies with an outer surface that conforms to the mold's shape and surface of revolution on the inside.
The metal for casting may come from reduced ore, from an open hearth or other remelting furnace, from electroreduction processing, or from remelting and alloying. To obtain a perfect casting, the liquid metal must completely fill every part of the mold before solidifying. Vacuum melting, although expensive, permits higher casting temperatures, better fluidity, and lower surface tension conditions.
As the metal solidifies, impurities that were soluble in the liquid metal become concentrated in the last parts to solidify. This would normally give rise to non-uniform impurity distributions throughout the cast piece. Reservoirs are therefore often incorporated into the casting process to trap the impurities.
In powder metallurgy, articles are produced by agglomeration of fine metallic powder. This technique is used where other methods of shaping such as casting, forging, and machining are impractical. The materials used in powder metallurgy usually consist of a mixture of metallic and nonmetallic powders. The are cold pressed to initially adhere the particles. Then they are heated in compacts in a nonoxidizing atmosphere (sintering) to obtain final cohesion. In isostatic pressing, the powder is pressed in a closed flexible container of rubber or plastic under liquid pressure.
One of the most important properties of metals is their malleability, i.e., their ability to be mechanically deformed by forging, rolling, extrusion, etc., without rupture and without significant resistance to deformation. If metals can be mechanically deformed when cold, the material is said to be ductile. In the course of such deformation, most metals undergo work hardening (strain hardening). Metals that undergo work hardening are processed at room temperature. Those that are first heated above certain temperatures to make them malleable are hot formed. Forging is an important hot-forming process. In the process, the metal flows in the direction of least resistance. The most important forged metals are steel and steel alloys.
In cold extrusion, the metal is made to flow while cold by the application of high pressure. The process is used with any cold workable material, e.g., tin, zinc, copper and its alloys, aluminum and its alloys, and low-carbon soft-annealed steels.
Hot extrusion is a hot-working process that makes use of the deformability of heated metallic materials to shape them. The process is sited for producing barlike and tubular objects. Most metals and alloys can be extruded.
Cutting and machining
Forging and extrusion do not involve the removal of metal by means of cutting tools. Many important shaping processes are based on cutting operations. Cutting tools are made of special steels (tool steels), hard metals, oxide ceramics, and diamond.
Welding is the joining of metals by the application of heat and/or pressure, with or without the addition of a filler metal. Welding is used to form joints and connections, or to protect components against corrosion or wear by the application of an armoring layer of a more resistant metal.
In pressure welding, the parts to be joined are locally heated at the place where the joint is to be formed. The parts are then pressed together in the plastic state so that they are joined. Usually no filler is employed. Cold pressure welding makes use of high pressure, without the aid of heat, to unite parts. Ultrasonic and explosion welding are variations of this technique.
In fusion welding, metals are heated to the temperature at which they melt, and are then joined without hammering or the application of pressure. Although the joint can be formed without using a filler material, a filler is usually employed. The source of heat may be gas, electricity, chemical reactions, etc. Gas welding uses a flame produced by burning acetylene in oxygen or sometimes another fuel gas. This is a widely employed method of welding iron, steel, cast iron, and copper. The flame is applied to the edges of the joint and to a wire of filler material, which is melted and runs into the joint.
Soldering is the process of joining metal parts by means of a molten filler metal (solder) whose melting point is lower than that of the metals to be joined. The metals to be joined are wetted by the solder without themselves being melted (as in the case of welding). Unlike the case of welding, two different metals can be joined by soldering.
There are two types of solders: soft and hard. Soft solders usually consist of a mixture of lead and tin; and the heat required to melt them is supplied by a soldering iron. Hard solders include brass (copper-zinc alloys) solders, silver solders, copper solders, nickel-silver solders, and solders for light alloys; the heat to melt them is usually supplied by a blow torch.
Sheet metal can be formed into a wide variety of hollow shapes and sections. The equipment required to work sheet metal ranges from simple hand tools to highly automated machinery. The process usually begins with basic shearing operations such as cutting, slitting, and perforating. This is followed by shaping operations, i.e., folding and bending.
Zinc plays an important role in protecting iron and steel from corrosion. The process of applying the zinc coating is called galvanizing. In hot-dip galvanizing, the zinc coating is applied by dipping the object to be coated into a bath of molten zinc; the zinc combines with the iron to form a coating of iron-and-zinc crystals. Other galvanizing techniques include electrogalvanizing, metallizing, and sherardizing (forming intermetallic compounds of iron and zinc on a steel surface by heating in the presence of zinc dust below the dust's melting point).
Metallizing is a process for applying protective coatings to iron and steel. It consists of spraying particles of molten metal to the surface to be treated, and can be used with most common metals including aluminum, copper, lead, nickel, tin, zinc, and various alloys. Coatings of lead, aluminum, silver or stainless steel are sometimes used for protection against corrosion in the chemical and food industries. Steel or hard alloy coatings are used as wearing surfaces. In the electronics industries, metallic coatings are applied to nonmetallic materials to make them electrically conductive.
Electroplating is the process of producing a metallic coating on a surface by electrodeposition involving an electric current. In electroplating, the coating material is deposited from an aqueous acid or alkaline solution (electrolyte) onto the metal surface to be coated. Such coatings may have protective and/or decorative functions.
See also Metal production.
Lyman, Taylor. Metals Handbook. Metals Park, OH: American Society for Metals, 1961.
Macaulay, David. The New Way Things Work. Boston: Houghton Mifflin Company, 1998.
Smith, Charles O. The Science of Engineering Materials. Englewood Cliffs, NJ: Prentice-Hall, Inc, 1969.
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