BRASS FOUNDRY manufacture
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Process : Forging is a metal forming process used to produce large quantities of identical parts, as in the manufacture of automobiles, and to improve the mechanical properties of the metal being forged, as in aerospace parts or military equipment. The design of forged parts is limited when undercuts or cored sections are required. All cavities must be comparatively straight and largest at the mouth, so that the forging die may be withdrawn. The products of forging may be tiny or massive and can be made of steel (automobile axles), brass (water valves), tungsten (rocket nozzles), aluminum (aircraft structural members), or any other metal. More than two thirds of forging in the United States is concentrated in four general areas: 30 percent in the aerospace industry, 20 percent in automotive and truck manufacture, 10 percent in off-highway vehicles, and 10 percent in military equipment. This process is also used for coining, but with slow continuous pushes.
The forging metal forming process has been practiced since the Bronze Age. Hammering metal by hand can be dated back over 4000 years ago. The purpose, as it still is today, was to change the shape and/or properties of metal into useful tools. Steel was hammered into shape and used mostly for carpentry and farming tools. An ax made easy work of cutting down trees and metal knives were much more efficient than stone cutting tools. Hunters used metal-pointed spears and arrows to catch prey. Blacksmiths used a forge and anvil to create many useful instruments such as horseshoes, nails, wagon tires, and chains.
Militaries used forged weapons to equip their armies, resulting in many territories being won and lost with the use and strength of these weapons. Today, forging is used to create various and sundry things. The operation requires no cutting or shearing, and is merely a reshaping operation that does not change the volume of the material.
Forging : Forging changes the size and shape, but not the volume, of a part. The change is made by force applied to the material so that it stretches beyond the yield point. The force must be strong enough to make the material deform. It must not be so strong, however, that it destroys the material. The yield point is reached when the material will reform into a new shape. The point at which the material would be destroyed is called the fracture point.
In forging, a block of metal is deformed under impact or pressure to form the desired shape. Cold forging, in which the metal is not heated, is generally limited to relatively soft metals. Most metals are hot forged; for example, steel is forged at temperatures between 2,100oF and 2,300oF (1,150oC to 1,260oC). These temperatures cause deformation, in which the grains of the metal elongate and assume a fibrous structure of increased strength along the direction of flow. (See Figure)
Figure - Flow lines in a forged part
Normally this results
in metallurgical soundness and improved mechanical properties. Strength, toughness, and general durability
depend upon the way the grain is placed. Forgings are somewhat stronger and more ductile along the grain
structure than across it. The feature
of greatest importance is that along the grain structure there is a greater
ability to resist shock, wear, and impact than across the grain. Material
properties also depend on the heat-treating process after forging. Slow cooling in air may normalize
workpieces, or they can be quenched in oil and then tempered or reheated to
achieve the desired mechanical properties and to relieve any internal stresses.
Good forging practice makes it possible to control the flow pattern resulting
in maximum strength of the material and the least chances of fatigue
failure. These characteristics of
forging, as well as fewer flaws and hidden defects, make it more desirable than
some other operations (i.e. casting) for products that will undergo high
stresses.
In forging, the dimensional tolerances that can be held vary based on the size of the workpiece. The process is capable of producing shapes of 0.5 to >50.0 cm in thickness and 10 to <100 cm in diameter. The tolerances vary from ± 1/32 in. for small parts to ± ¼ in. for large forgings. Tolerances of 0.010 in. have been held in some precision forgings, but the cost associated with such precision is only justified in exceptional cases, such as some aircraft work.
Forging
is divided into three main methods: hammer, press, and rolled types.
(1) Hammer Forging (Flat Die): Preferred method for individual forgings. The shaping of a metal, or other material, by an instantaneous application of pressure to a relatively small area. A hammer or ram, delivering intermittent blows to the section to be forged, applies this pressure. The hammer is dropped from its maximum height, usually raised by steam or air pressure. Hammer forging can produce a wide variety of shapes and sizes and, if sufficiently reduced, can create a high degree of grain refinement at the same time. The disadvantage to this process is that finish machining is often required, as close dimensional tolerances cannot be obtained.
(2) Press Forging: This process is similar to kneading, where a slow continuous pressure is applied to the area to be forged. The pressure will extend deep into the material and can be completed either cold or hot. A cold press forging is used on a thin, annealed material, and a hot press forging is done on large work such as armor plating, locomotives and heavy machinery. Press Forging is more economical than hammer forging (except when dealing with low production numbers), and closer tolerances can be obtained. A greater proportion of the work done is transmitted to the workpiece, differing from that of the hammer forging operation, where much of the work is absorbed by the machine and foundation. This method can also be used to produce larger forgings, as there is no limitation in the size of the machine.
(3) Die Forging: Open and closed die operations can be used in forging. In
open-die forging the dies are either flat or rounded. Large forgings can be formed by successive applications of force
on different parts of the material. Hydraulic presses and forging machines are
both employed in closed die forging. In closed-die forging the metal is trapped in
recessed impressions, which are machined into the top and bottom dies. As the dies press together, the material is
forced to fill the impressions. Flash,
or excess metal, is squeezed out between the dies. Closed-die forging can
produce parts with more complex shapes than open-die forging. Die forging is the best method, as far as
tolerances that can be met, and also results in a finished part that is
completely filled out and is produced with the least amount of flashing. The
final shape and the improvement in metallurgical properties are dependent on
the skill of the operator. Closer
dimensional tolerances can be held with closed die forgings than with open die
forgings and the operator requires less skill.
The type of machinery to be used depends on the shape, size, material, and number of pieces to be made. Forging hammers apply force by the impact of a large ram. This may be a drop hammer, or weight falling under the force of gravity, or it may be a power hammer, driven by steam or compressed air. Two types of power hammers are: the smith forging hammer and the drop hammer. The largest hammers can provide a total force as high as 80,000 pounds.
Smith Forging Hammer and Board Drop Hammer
· Smith Forging Hammer Heavy workpieces could be processed using a smith-forging hammer, and smaller forgings are die formed in drop hammers. Smith forging hammers are typically steam or air-operated, consisting of a power actuated ram supported by a heavy cast iron frame. The final product is a result of the ram being powered into the dies containing the workpiece.
· Board Drop Hammer A drop hammer differs in that the anvil is an integrated part of the hammer base. It is necessary for the alignment between the forging die elements used. This method is advantageous in that the physical properties of the metal are improved by the severe mechanical working, the operation is rapid, many complicated parts can be forged to shape, a minimum amount of machining is necessary, and internal defects are eliminated. The disadvantages are the cost of machinery and dies, which demands a high quantity of parts to be manufactured in order for the process to be cost effective.
· Forging Press A forging press consists of a hydraulic press, which exerts a force capable of pressing steel or a metal alloy into the shape of the forging die. These machines can be positioned horizontally or vertically. This method can be used to form car wheels, gears, bushings, and other such parts.
· Mechanical Forging Press Mechanical presses have a motor-driven flywheel that stores energy to drive a ram--much lighter than a hammer--through a crank or other mechanical device. The ram in a press moves more slowly than a hammer and squeezes the workpiece. The largest mechanical presses have a total force of 12,000 tons and cannot forge as large or complicated parts as the larger hammers.
· Hydraulic Forging Press Hydraulic presses, in which high-pressure fluid produced by hydraulic pumps drives a ram, are about 100 times slower than hammers. They are used for large or complex die forgings and for extrusion. Presses with a total force of 50,000 tons have been developed in the United States primarily for the forging of large airplane components. Even larger hydraulic presses, up to 78,000 tons, have been introduced in Europe.
Materials can be improved before or after manufacturing by different heat treatment processes. Forging is usually performed to hot metals, allowing for smoother flow and easier deformation. Steel is heated to varying temperatures, usually between 1700oF to 2000oF but can reach as high as 2400oF, depending on the carbon content. Depending on the amount of work required to the piece, it may be necessary to reheat the piece one or more times. The temperature of the metal when completely forged is called the finishing temperature. After forging, the material must be cooled uniformly and protected from moisture or cold air. This is done by placing the material into dry ashes, lime or mica dust in order to retard the rate of cooling.
(1) Preheating: Preheating of materials is done to help prevent cracking or distortion of the material. This is done by placing the metal in a series of furnaces of increasing temperatures instead of throwing it directly into the furnace used to heat the metal for forging, annealing, normalizing or hardening. Another way to achieve this is to start in a cold furnace and slowly bring it to temperature.
(2) Annealing: Annealing should follow forging as soon as possible whenever machining is required. Annealing is the heating and then cooling of metal to make the metal less brittle, or more malleable and ductile. This will soften the steel that was previously hardened and reduce internal stresses. Annealing is done by heating the metal to a temperature beyond the critical temperature and holding it there for a period of time. The metal is then cooled with the furnace and not removed until the furnace is cold. It can also be cooled to a temperature within the furnace that is known to be below the lower critical temperature, at which the annealing is complete. Slower cooling rates are required as carbon content increases in the metal.
(3) Normalizing: Normalizing is done to improve the crystalline structure of the steel, thus obtaining superior properties. Heating the forged part just beyond the critical temperature and then allowing it to air-cool completes normalizing. This allows the grain-size to be refined and, if not held at that temperature too long, will result in a newly formed crystalline structure. The internal stresses, if any, will be relieved, hardened steels will be softened, overheated steels will have a more favorable, normal fine-grained structure, and structural distortion will be removed.
(4) Hardening: Hardening of steels can also be done after forging. The workpiece is heated slowly, to obtain the finest grain-sizes, to its hardening temperature - much higher than annealing temperatures. The metal is kept at this temperature only until uniform heat distribution and completion of the thermal transformation. Prolonged exposure at these elevated temperatures will result in increased grain growth and surface decarbonization, if no protection from oxidation is provided. Oxidation can be avoided by surrounding the metal with some material that will use up the oxygen that is present in the furnace. Once the metal has been uniformly heated to temperature, it is removed from the furnace and placed directly into a quenching tank. This rapidly cools the metal and the metal retains its new qualities.
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