Forging is a manufacturing process involving the shaping of metal using localized compressiveforces. The blows are delivered with a hammer(often a power hammer) or a die. Forging is often classified according to the temperature at which it is performed: cold forging (a type of cold working), warm forging, or hot forging (a type of hot working). For the latter two, the metal is heated, usually in a forge. Forged parts can range in weight from less than a kilogram to hundreds of metric tons. Forging has been done by smiths for millennia; the traditional products were kitchenware, hardware, hand tools, edged weapons, cymbals, and jewellery. Since the Industrial Revolution, forged parts are widely used in mechanisms and machineswherever a component requires high strength; such forgings usually require further processing (such as machining) to achieve a finished part. Today, forging is a major worldwide industry.
Advantages & Disadvantages
▪ Advantages :-
Forging can produce a piece that is stronger than an equivalent cast or machined part. As the metal is shaped during the forging process, its internal grain deforms to follow the general shape of the part. As a result, the grain is continuous throughout the part, giving rise to a piece with improved strength characteristics. Additionally, forgings can target a lower total cost when compared to a casting or fabrication. When you consider all the costs that are involved in a product’s lifecycle from procurement to lead time to rework, then factor in the costs of scrap, downtime and further quality issues, the long-term benefits of forgings can outweigh the short-term cost-savings that castings or fabrications might offer.
▪ Disadvantages :-
Production forging involves significant capital expenditure for machinery, tooling, facilities and personnel. In the case of hot forging, a high-temperature furnace (sometimes referred to as the forge) is required to heat ingots or billets. Owing to the size of the massive forging hammers and presses and the parts they can produce, as well as the dangers inherent in working with hot metal, a special building is frequently required to house the operation. In the case of drop forging operations, provisions must be made to absorb the shock and vibration generated by the hammer. Most forging operations use metal-forming dies, which must be precisely machined and carefully heat-treated to correctly shape the workpiece, as well as to withstand the tremendous forces involved.
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Processes
There are many different kinds of forging processes available; however, they can be grouped into three main classes:
1) Drawn out: length increases, cross-section decreases.
2) Upset: length decreases, cross-section increases.
3) Squeezed in closed compression dies: produces multidirectional flow.
Common forging processes include:
Roll forging, Swaging, Cogging, Open-die forging, Impression-die forging, Press forging, Automatic hot forging and upsetting.
Materials and Applications
1) Forging of Steel
Depending on the forming temperature steel forging can be divided into:
a) Hot forging of steel
▪ Forging temperatures above the recrystallization temperature between 950 - 1250 °C
▪ Good formability
▪ Low forming forces
▪ Constant tensile strength of the workpieces
b) Warm forging of steel
▪ Forging temperatures between 750 – 950 °C
▪ Less or no scaling at the workpiece surface
▪ Narrower tolerances achievable than in hot forging
▪ Limited formability and higher forming forces than for hot forging
▪ Lower forming forces than in cold forming
c) Cold forging of steel
Forging temperatures at room conditions, self-heating up to 150 °C due to the forming energy. Narrowest tolerances achievable, no scaling at workpiece surface increase of strength and decrease of ductility due to strain hardening. Low formability and high forming forces are necessary.
For industrial processes steel alloys are primarily forged in hot condition. Brass, bronze, copper, precious metals and their alloys are manufactured by cold forging processes, while each metal requires a different forging temperature
2) Forging of Aluminium
Aluminium forging is performed at a temperature range between 350 and 550 °C.
Forging temperatures above 550 °C are too close to the solidus temperature of the alloys and lead in conjunction with varying effective strains to unfavorable workpiece surfaces and potentially to a partial melting as well as fold formation.
Forging temperatures below 350 °C reduce formability by increasing the yield stress, which can lead to unfilled dies, cracking at the workpiece surface and increased die forces.
Due to the narrow temperature range and high thermal conductivity, aluminium forging can only be realized in a particular process window.
Equipments
The most common type of forging equipment is the hammer and anvil. Principles behind the hammer and anvil are still used today in drop-hammerequipment. The principle behind the machine is simple: raise the hammer and drop it or propel it into the workpiece, which rests on the anvil. The main variations between drop-hammers are in the way the hammer is powered; the most common being air and steam hammers. Drop-hammers usually operate in a vertical position. The main reason for this is excess energy (energy that isn't used to deform the workpiece) that isn't released as heat or sound needs to be transmitted to the foundation. Moreover, a large machine base is needed to absorb the impacts.
To overcome some shortcomings of the drop-hammer, the counterblow machine or impactor is used. In a counterblow machine both the hammer and anvil move and the workpiece is held between them. Here excess energy becomes recoil. This allows the machine to work horizontally and have a smaller base.
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