Forging

Forging, process of shaping iron and other malleable metals by hammering or pressing them after making them plastic by application of heat. Forging techniques are useful in the working of metal because the metal can be given the desired form, and the process improves the structure of the metal, particularly by refining the grain size of the metal. Forged metal is stronger and more ductile than cast metal and exhibits greater resistance to fatigue and impact.

Sometimes called smithing, or blacksmithing, hand forging is the simplest form of forging and it is one of the methods by which metal was first worked. The metal to be forged is first heated to red heat in the fire of a forge, and then is beaten into shape on a metal anvil with sledges or hammers. The forge consists of an open hearth, made of some durable, refractory substance such as firebrick, which is provided with a number of air openings, or tuyeres, through which air is forced by a bellows or blower fan. Charcoal, coke, and coal are used as fuels in the forge. Hammers and other tools are employed by the blacksmith in the various forging operations.

In general, six basic types of forging exist: upsetting, or decreasing the length and increasing the diameter of the metal; swaging, decreasing the diameter of the metal; bending; welding, joining two pieces of metal together by semifusion; punching, the forming of small openings in the metal; and cutting out, the forming of large holes in the metal.

A piece of metal, called the work, is upset when it is struck along the longest dimension (for example, the end of a rod or bar), which shortens and thickens it. Swaging is accomplished by hammering the metal stock while it is held on the anvil within any one of various concave tools called swages. Bending is accomplished either by hammering the work around a form or by leveraging it against a supporting fulcrum. In forge welding of iron, a flux such as borax is first applied to the heated metal to remove any oxides from the surfaces of the two pieces, and the pieces are then joined by hammering them together at high temperature; a welded joint of this kind, when properly made, is entirely homogeneous and is as strong, that is, uniform, as the parent metal. To punch small holes, the work is supported on a ring-shaped piece of metal atop the anvil, and a punch of the proper shape is driven through the work by hammer blows. Larger holes are cut out, and portions of the work are cut off with heavy, sharp chisels similar to cold chisels used to cut cold metal. Combinations of several of these operations can produce forgings of a wide variety of shapes.

The chief difference between hand forging and machine forging is that in the latter technique various types of machine-powered hammers or presses are used instead of hand sledges. These machines enable the operator to strike heavy blows with great rapidity and thus to produce forgings of large size and high quality as swiftly as required by modern production-line methods. Another advantage of machine forging is that the heavier the blows struck during forging, the greater the improvement in the quality of metallic structure. Fine-grain size in the forging, which is particularly desirable for maximum impact resistance, is obtained by working the entire piece. With large, hand-forged metal, only the surface is deformed, whereas the machine hammer or press will deform the metal throughout the entire piece.

A special type of machine forging is drop forging, also called impact-die forging. Drop forging consists of placing soft, hot metal between two shaping dies (see Die). The upper one of these dies is hammered, or dropped, on the lower die, forcing the heated metal into the shaped die cavities, as in coin-making operations.

For reducing part of a piece of metal stock to a predetermined size, forging rolls are sometimes employed. These consist of a pair of grooved, cam-shaped rollers through which the metal is passed. The rollers touch each other and work on the metal during only part of each rotation and therefore reduce only part of the stock that is fed to them.

Machine-forging operations are frequently accomplished by use of a series of dies mounted on the same press or hammer. The dies are arranged in sequence so as to form the finished forging in a series of steps. After the piece has been partially formed by one stroke, it is moved to the next die for further shaping on the next stroke.

Fatigue (materials)

Fatigue (materials), in metals, progressive deterioration, that ultimately results in the breaking of the metal. Fatigue is caused by repeated application of stress to the metal, and the deformation of a material or object as a result of the stress is known as creep. The fatigue strength of a typical steel alloy is about 50 percent of the ultimate strength and 75 percent of the elastic strength but may be considerably lower, particularly for the strongest heat-treated steels. If the elastic strength of a steel beam is about 45,000 kg (about 100,000 lb), it could withstand a continuous stress of about 41,000 kg (about 90,000 lb) for centuries, with no measurable yielding. A stress of about 36,000 kg (about 80,000 lb) alternately applied and withdrawn, however, would probably cause fatigue failure after a few million applications. Fatigue is not important in civil engineering structures, in which stress is generally continuous, but in an engine turning at 3000 rpm, any stress to which an engine part is subjected will often be applied millions of times within a few hours of operation. Fatigue failures account for an overwhelming majority of all structural failures in cyclic devices such as engines, and design engineers must consider fatigue strength, rather than elastic strength or ultimate strength, in their calculations.

The problem of metal fatigue has gained great importance in the field of air transport since the end of World War II. The increased stresses of high-speed flight with heavy loads at high altitudes have posed difficult problems for structural engineers, especially in wing and engine design. The exact structural changes that occur as a result of fatigue are not known. The failure usually starts at a point of stress concentration and proceeds along the intercrystalline planes of the metal. The break often shows a characteristic coarsely crystalline structure except where the surfaces are worn smooth by rubbing against one another after the break has started. The term fatigue is not an entirely appropriate one, because no amount of rest between stress applications has any measurable effect upon the ultimate failure. See Materials Science and Technology; Metallography; Metals; Tensile Strength.

Flame

Flame, glowing body of mixed gases undergoing the process of combustion. Flames generally consist of a mixture of oxygen (or air) and another gas, usually such combustible substances as hydrogen, carbon monoxide, or hydrocarbon.

A typical flame is that of a burning candle. When the candle is lighted, the heat of the match melts the wax, which is carried up the wick and then vaporized by the heat. The vaporized wax is then broken down by the heat and, finally, combines with the oxygen of the surrounding air, producing a flame and generating heat and light. The candle flame consists of three zones that are easily distinguished. The innermost zone, a nonluminous cone, is composed of a gas-air mixture at a comparatively low temperature. In the second, or luminous, cone, hydrogen and carbon monoxide are produced by decomposition and begin to react with oxygen to form water and carbon dioxide, respectively. In this cone the temperature of the flame—about 590° to 680° C (about 1090° to 1250° F)—is great enough to dissociate the gases in the flame and produce free particles of carbon, which are heated to incandescence and then consumed. The incandescent carbon produces the characteristic yellow light of this portion of the flame. Outside the luminous cone is a third, invisible cone in which the remaining carbon monoxide and hydrogen are finally consumed.

If a cold object is introduced into the outer portions of a flame, the temperature of that part of the flame will be lowered below the point of combustion, and unburned carbon and carbon monoxide will be given off. Thus, if a porcelain dish is passed through a candle flame, it will receive a deposit of carbon in the form of soot. Operation of any kind of flame-producing stove in a room that is unventilated is dangerous because of the production of carbon monoxide, which is poisonous.

All combustible substances require a definite proportion of oxygen for complete burning. (A flame can be sustained in an atmosphere of pure chlorine, although combustion is not complete.) In the burning of a candle, or of solids such as wood or coal, this oxygen is supplied by the surrounding atmosphere. In blowpipes and various types of gas burners, air or pure oxygen is mixed with the gas at the base of the burner so that the carbon is consumed almost instantaneously at the mouth of the burner. For this reason such flames are nonluminous. They also occupy a smaller volume and are proportionately hotter than a simple candle flame. The hottest portion of the flame of a Bunsen burner has a temperature of about 1600° C (about 2910° F). The hottest portion of the oxygen-acetylene flames used for welding metals reaches 3500° C (6330° F); such flames have a bluish-green cone in place of the luminous cone. If the oxygen supply is reduced, such flames have four cones: nonluminous, bluish-green, luminous, and invisible.

The blue-green cone of any flame is often called the reducing cone, because it is insufficiently supplied with oxygen and will take up oxygen from substances placed within it. Similarly, the outermost cone, which has an excess of oxygen, is called the oxidizing cone. Intensive studies of the molecular processes taking place in various regions of flames are now possible through the techniques of laser spectroscopy.

Date: 2015-01-02; view: 886