What is Forging?
The ancient art of forging falls into two distinct categories – hot and cold where hot forging has been around for centuries while cold did not begin until the industrial revolution of the 19th Century. Though they are quite different processes, the end results are the same. Forging is a method for deforming metal into a predetermined shape using tools and equipment where deformation is achieved using hot, cold, or warm forging processes. The determination of which process to use depends on the metal and desired shape of the final product.
Hot Forging and How It Works
Hot forging takes place when a piece of metal is superheated beyond its crystallization point. The average temperature at which this happens depends on the type of metal where steel crystallizes at up to 1150o C, aluminum alloys between 360o C to 520o C, and copper alloys between 700o C to 800o C. The required temperature must be maintained above the recrystallization point or the point at which the metal begins to cool. Recrystallization can form microscopic crystals that warp when the metal is reheated causing it to “strain harden”, which makes it unworkable.
Environmental factors can influence the hot forging process where contact with the atmosphere can cause oxidation. To avoid this, forging may be completed in an environmentally controlled chamber or isothermal forging, which is similar to a vacuum.
The main advantage of hot forging is the strength a metal gains. Metals hot forged have a strong ductility, the ability to be significantly deformed and reshaped giving them a stronger resilience to tensile stress. Parts made using hot forging are stronger because the forging process changes the internal grain of the part changing its form and strength characteristics.
Things to Consider When Choosing Hot Forging
Hot forging is used in the production of several different components and parts since the process can produce high precision and complex shapes. It is ideal for metals that have a high formability or can easily be deformed. Metals formed in hot forging gain strength and become more durable. It is a flexible process that is capable of producing customized parts. The excellent surface finishes of hot forged parts allows the use of different forms of finishing and polishing processes.
Some of the unique results from hot forging include the ability to easily produce discrete pieces, low to medium accuracy, the formation of scale on the metal, low metal stress levels with lower work hardening temperatures, grain homogenization, higher ductility levels, and the elimination of chemical incongruities in the metal.
Negative factors of hot forging:
Regardless of how productive hot forging is, it does have certain disadvantages that need to be considered. During the cooling process, there is always the possibility that the metal may warp and become brittle. There are some metals that should not be used in hot forging since it may produce less precise tolerances as well as variances in the grain structure.
Though there are disadvantages and drawbacks to hot forging, it is well suited for producing aerospace products and airplane parts. The softness created in metals makes them easy to shape and produce intricate patterns required by those industries.
The Benefits of Cold Forging or Forming
Cold forging refers to a metal shaping and deforming process that is performed at or slightly above room temperature where the temperature is kept at near three tenths of the recrystallization point of the metal. Squeezing, bending, shearing, and drawing are the most common cold forging methods. Soft metals such as aluminum or copper are ideal for the process.
The method of cold forging uses compressive force to shape the workpiece by hand or machine. In many cases, the metal is pushed into a die shaped in the form of the final product. It is less expensive than hot forging and produces completed products that require little, if any, finishing. With aluminum, after being cold forged, it is heat treated to strengthen the piece, a process known as “tempering”.
The Unique Process of Cold Forging
The most common metals in cold forging are standard or carbon alloy steels. It is used to produce small, high volume products like fasteners such as nails, screws, rivets, and such. The lack of the need for heat makes it inexpensive and efficient. The types of cold forging methods used depends a great deal on the metal and the required shape. Below are descriptions of some common cold forging processes.
Impression die forging
A common form of cold forging is impression-die forging, where the metal is placed into a die attached to an anvil. The metal is struck by a hammer to force it into the die. Depending on the product, the hammer may be dropped several times in rapid succession.
Squeezing, also known as sizing, minimizes the thickness of metal by the application of pressure, which is determined by the sizing area, type of metal, and necessary needed change in the metal’s thickness. It is normally done to give a forged piece dimensional accuracy.
Cold roll forming produces more attractive finished surfaces with closer tolerances in a variety of shapes that can easily be galvanized, painted, or powder coated. Thin sheets of a metal, usually steel, are rolled into shapes such as floor and roof paneling, C, or Z shapes.
Drawing uses tensile forces to pull or stretch a metal to a particular shape or thickness. There are two forms of drawing – sheet metal and wire, bar, or tube. Drawing can only be applied to certain types of metal or materials that have sufficient tensile strength. Though it may seem similar to cold rolling, it differs in the amount and position of the applied force.
Cold forging has other processes designed for shaping metals. The ones listed here are common but only a few of the many that are in use. It is a popular process with auto manufacturers for the production of steering and suspension parts, certain braking systems, axles, and many other automobile parts.
Things to Consider When Choosing Cold Forging
There are several benefits when considering cold forging. First, it is done at room temperature and does not require heating the metal, which helps to keep the cost of individual parts to a minimum. All of the equipment associated with hot forging such as blast and industrial furnaces are not necessary.
Parts produced using cold forging are manufactured quickly and uniformly with superior dimensional control making it appealing to companies that have high volume and require quick turnaround times. With the use of computerization, every manufactured component is exactly the same in every detail and intricacy. Dies used for cold forging last longer since they do not have to endure the stress and wear of hot metals.
The cold forging process is more precise and produces precision parts with high tolerances. The compliance with design specifications increases the uniformity of every part with less likelihood of flaws or errors that would require repeating production runs, and parts have a superior grain structure.
An important factor, in this era of environmental concerns, is the eco-friendliness of cold forging since parts are shaped and formed under high pressure at room temperature. Unlike other processes, cold forging ensures that there will not be air bubbles or other deformities trapped in the workpiece.
Some of the other benefits of cold forging include improved material usage, lower energy costs, and little to no finishing. With the rising costs of energy and the shortage of materials, cold forging is the solution to producing parts efficiently reducing the effect of those factors.