Forging is a metal working process that manipulates, shapes, deforms, and compresses metal to achieve a desired form, configuration, or appearance outlined by a metal processing design or diagram...
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This article takes an in depth look at cold forging.
Cold forging is a metal shaping & manufacturing process in which bar stock is inserted into a die and squeezed into a second closed die. The process, completed is at room temperature or below the metal‘s austenite temperature; to form a metal into a desired shape or configuration.
Cold forging is an efficient and economical metal deforming process for the production of high volumes of parts at low cost. There are three methods of forging that differ according to temperature, which can be cold, warm, or hot and involve the use of hammers, dies, or presses to shape, squeeze, deform, and roll metals. Cold forging should not be confused with machining or casting since the end result is a stronger and higher quality product.
Unlike warm or hot forging, cold forging shapes and deforms bar stock at room temperature using localized, compressive force. Depending on the requirements of the parts design, the workpiece may pass through multiple dies or be struck several times in succession to achieve the proper shape.
The low cost of cold forging is due to the reduction in labor costs and the removal of secondary processing. Parts are rapidly and efficiently produced at rates as high as 1000 per hour, which lowers the per unit cost. Production is simply a matter of feeding a coil and letting the machines quickly and cost effectively do the work.
Finished products from cold forging have perfect surface finishes and improved dimensional stability as can be seen in the image below. The different processes enhance the strength and durability of the workpiece producing long lasting products or parts.
– Prior to forging, the workpiece is treated with a lubricant to prevent it from sticking to the die and to keep it cool during the forming process since deforming can produce temperatures of 250° to 450°. The use of lubricants is dependent on the company and their process and is not generally used.
– The metal coil is fed into the forging machine over the die that has the shape of the final part. The die may have two sections with one attached to the hammer and the other beneath the workpiece. The hammer is the upper portion and striking mechanism that produces the force to deform the metal piece.
– The striking of the workpiece, or stroke, can be produced by three mechanisms – hydraulic, pneumatic, or mechanical. Each of the techniques drives a shaft, with the hammer on it, down at great force onto the workpiece to create the desired shape. This happens within a scale of milliseconds. In some cases, the hammer may have to be dropped several times in succession to produce the exact contour and shape.
– The method of removal of the part depends on the type of process. Most modern manufacturers use automation to remove the part by a conveyor or robotic hand. This is another cost saving measure that removes the need for material handling.
– As with other parts of the process, this can take many forms. For parts that require one die and one stroke, the part is trimmed and sent on to shipping. In the case of parts that have multiple facets, they are moved on to other die processes to have features added. The movement of the die from station to station is normally automated. With larger parts, other mechanized methods are used that may involve a hi-lo, forklift, or crane.
The information below outlines some of the benefits of cold forging and why it is so widely used. Chapter three is a comparison of cold forging to hot forging and the advantages of each.
Cold forging doesn’t necessitate expensive heating equipment and tooling or costly secondary processing. When compared to machining, there is a 70% savings of material. Furnaces, kilns, or electricity are normally used to raise the temperature of a metal above its austenite stage in other metal processing operations. They are expensive to maintain, produce pollutants, and are time consuming.
In cold forging, once a workpiece is processed, it is complete and requires minimal finishing saving on the cost of labor, which is the major cost savings of the cold forging process.
Cold forging is a simple process where the workpiece is placed directly in the forging machine to instantly produce a finished part. Modern producers use automation to load the workpiece and remove it from the press. As you can see in this diagram, the metal is fed into the machine, processed, and moved on. The amount of time between entering and finishing is less than a second.
The working of metal can create a variety of potential problems. Cold forging eliminates some negative effects such as porosity fatigue by increasing the overall strength of the metal and removing the risk of a loss of material integrity. There are certain negative effects that are added in the process as well.
Cold forged parts are capable of handling high stress. When the workpiece is forced beyond its yield or elastic limit, it is still able to retain its altered shape.
Critical and close tolerances of parts is maintained throughout a production run as long as there isn't any die wear. They are reproduced to the minutest detail so that every part is an exact duplicate of the first one.
Unlike other processes, cold forging allows design freedom where intricate contours and shapes can be produced that would require many different and costly secondary methods in other processes.
Though there may be variations between cold forging production methods, the high manufacturing speed of cold forging can produce as many as 50 pieces per minute up to over 450 pieces per hour. The only thing to influence the speed of the process is the shape and configuration of the part.
A wide range of metals can be forged, which includes hard metals like carbon steel, alloy steel, and stainless steel as well as soft metals like aluminum, brass, and copper.
This image is a sampling of bolts that have been produced by cold forging. Each of the various bolts has been made from a different metal. Included are ones made of copper, brass, aluminum, and steel.
The difference between hot forging and cold forging is temperature where cold forging strains and stresses metals at room temperature, while hot forging heats metals.
The key to the temperature range is its austenite stage. Cold forging takes place before the austentite while hot forging heats a metal above its austenite stage.
Hot forging is performed at exceptionally high temperatures. The increased temperatures helps to avoid strain and hardening as well as lowers the stress flow and the amount of energy needed to deform and shape metals. Once metals cool, they retain their deformed shape. Hydraulic, pneumatic, and mechanical presses are used in the forming process.
One of the main expenses of hot forging is the cost required to produce the dies and molds that are made of strengthened steel engineered to withstand the extreme temperatures, able to resist fatigue, and have ductility, toughness, and tensile strength.
Though many of the hot forging methods are similar to cold forging ones, other methods include gas forming, die quenching, draw forming, and isothermal.
The type of process dictates the temperature of the metal when it is being shaped. In some processes the metal is completely melted and poured into a mold or pushed through a die. In others, it is annealed and shaped by a die or mold under compression and pressure.
The biggest drawback is the amount of pollutants produced by the heating process, which is dictated by the type of furnace. This has been a major issue since its inception during the first industrial revolution and has been a problem producers have continually confronted over the years.
Parts need to be heated above the austenite stage, which is well over 300° F and can go into the 1000‘s of degrees. Large furnaces or kilns are required to reach the appropriate temperatures.
The complicated nature of hot forging requires time to heat the metal, process it, and cool it. Though the actual stamping, deforming, or shaping takes approximately the same time as cold forging, the heating and cooling processes require careful care and management for extended periods.
The heating and cooling of metals increases their strength, toughness, and ductility but decreases their hardness. During the cooling process, metals can warp and lose shape, which can be accounted for during the engineering phase.
Hot forging can produce oxidation that leads to scaling, which can discolor the metal‘s surface making it difficult to finish.
The temperature at which forging takes place is what separates cold forging from hot forging. For a part to be hot forged, it is heated to a temperature above its austenite stage, which changes its micro-structure. As it is heated, its internal stress and strength is eliminated to make it more ductile. Cold forging does not require heating allowing a metal to retain its strength and micro-structure.
The characteristics of cold forgings are as follows:
Metals are shaped at room temperature below their austenite stage, which has the benefit of lowering costs and avoiding the expense of furnaces.
Speeds range from seven pieces per minute for low volume machines to 400 pieces per minute for high volume ones.
Cold forging is done with machines designed to shape metal and include squeezing, bending, shearing, and drawing. Equipment is available in a wide range of prices depending on the amount of technology and its size.
Cost savings are from material and speed. The material savings comes from the small amount of scrap produced, which is up to 70%. Since parts are produced at a very rapid rate, the cost per unit is significantly low.
There are no emissions or pollutants produced as can be seen in this machine below from Stalcop. Everything is self-contained and enclosed. Carbon and other pollutants are eliminated since there isn‘t any need for heating the metal.
As can be seen in the diagram below, the grain structure of metals is rearranged to follow the flow of the final part eliminating porosity fatigue, increasing shearing strength, and reducing any risk of material integrity. By straining the metal, it becomes stronger and more resilient. The drawback is once a metal is cold forged it loses its ductility and becomes more brittle.
Prior to a metal being forged, it is important to remove any scaling such as rust or corrosion that may develop during storage. If left unremoved, the part will have the same deterioration as is found in hot forging.
There is extremely little finishing required. Once a part is processed, it is ready for use or shipping.
Cold forging isn‘t perfect. Like every method of production, there are limitations and restrictions to cold forging that you will need to consider before choosing it for your next production project.
The development of different technologies and advancements in metal processing has produced several cold metal shaping and forming methods. Each serves a different application, but all are designed to produce products efficiently and quickly without the need of secondary finishing.
As I will discuss at the end of this chapter, there are more processes than just the ones listed. The descriptions below provide you with a baseline of data for an initial understanding of the various methods.
Also, a very important aspect of the cold forging process is the type of lubricant used. The two most common are zinc phosphate or some form of polymer coating. Though cold forging is done at room temperature, the bending and shaping process increases the temperature of metals. The lubricant can prevent errors and keep the workpiece from sticking to the die and extend the lifetime of tools.
Lubricant manufacturers offer a variety of products to fit each type of cold forging process. Pictured are graphite lubricants, but graphite free types are available as well as calcium aluminate, aluminum fluoride, and phosphate coatings.
Bending is performed using a press and a die where the workpiece is forced against the shaping tool. It is also referred to as pyramid rolling and is sometimes used to prepare a piece for another cold forging process. The workpiece is strained along a single axis to form an angle.
Rolling is a forming process where metal is passed through a pair of rotating rollers for plastic deformation caused by compressive force. The compressive stresses produce friction between the rolls and the metal stock‘s surface. It is commonly used for the processing of steel.
In closed die forging, the workpiece is shaped by successive mechanical blows after it has been placed between two die halves. Since the hammer strikes the workpiece multiple times, some producers refer to closed die forging as drop forging. As the metal is struck, it flows into the cavities of the die changing it to the shape of the die.
Drawing is pulling the workpiece through a die by the use of tensile strength applied at the exit of the die. As the workpiece is pulled through, there is a reduction in the cross sectional area with an increase in its length. The formed metals have closer dimensional tolerance than is produced by rolling.
The billet or slug is forced through a die, below a compressive force, that has the profile of the final part. Once it passes through, it is cut to the required length, prepared for shipping, or sent on for further processing. The force applied in cold extrusion can reach as high as 20,000 kN or 2007 tons. Extrusion can be done forward, backward, or in both directions.
Forward extrusion – the metal is pushed forward through the die.
Backward extrusion – the metal enters the die backward to form holes or cups making the bottom thicker than its sides.
Lateral extrusion – force is applied laterally, sideways, to the direction of the extrusion to add a second feature to the profile.
Open die forging involves two flat dies without a pre-cut profile. The workpiece is progressively shaped using several processes allowing a broad range of shapes and sizes to be produced. It is mostly used with designs that incorporate large metal components that require the highest structural integrity. Deformation is achieved by repositioning of the workpiece.
Squeezing, also known as sizing, is a form of open die processing where force is applied over a short distance producing an accurate dimensional finish.
With ring forging, a circular workpiece punched in the middle to produce a donut shape. As the pierced piece is rotated, it is hammered and squeezed. The process produces seamless rings with perfect diameters and strength.
Swaging, or radial forging, is deformation to a workpiece so that two parts will fit together. It is automated and highly reliable. The two types of swaging are tube and radial. Tube swaging is like extrusion where the workpiece is forced through a die. With radial swaging, a hammer forces the workpiece through two or more dies.
When you begin your search for a cold forging manufacturer, you are going to find more methods than the seven described here. Having a basic understanding of a few of the possible methods will help you to speak authoritatively and intelligently to producers as well as be able to interpret their lingo.
As with any modern production method, you will find that cold forging is ever growing as new techniques and methods develop. An important, and increasing factor, is the addition of automation and robotics, which is rapidly changing the face of the industry. A forging specialist can point you in the right direction to find the process that best fits your needs.
Cold forging equipment and machines come in three varieties – hydraulic, pneumatic, and mechanical. While some suppliers specialize in just one type, you will find the majority have several choices depending on price and type of operation. Technology and automation are common as you can see in the picture below, which is a servo cold forging press from Marvel Machinery. Part design renderings are created using engineering software such as CAD.
The hydraulic die forging hammer is designed to provide the maximum of force with the lowest investment. It can produce a wide assortment of impressions from a variety of dies. It operates using the engineering concepts of hydraulics where an incompressible liquid is contained in a cylinder. When the liquid is compressed by a piston, a shaft, with the die attached, is driven down onto the workpiece.
The screw press is used for large deformation because of its slow press speed. It can be used for flashless dies and long rod forgings. The configuration of the screw press allows it to be used for single slot dies that include bending and final forgings. An electrical motor provides the power to turn the screw that pushes the die onto the workpiece.
The C frame design is suitable for forming, punching, bending, and multi-pressing operations. They come in single or double crank designs with a punch force of 110 to 400 tons and a slide stroke of 110 to 280 mm. They can accept die heights of 435 to 600 mm and can be used to produce small parts.
The pneumatic powered press can cover the full range of die pressing functions. They come with a pneumatically powered operated friction clutch and brake. The power for the press is produced by an air compressor that forces the forging hammer down onto the workpiece by air pressure in a cylinder with a piston.
Of the varieties of cold forging machines, the mechanical version is becoming the least used since they need to be very large to supply the necessary force. They have a flywheel that stores energy from a motor. When the flywheel is activated, it drives the hammer or ram down onto the die. It can provide power over several rotations but has to idle in order to regain power from its motor before it can continue its cycles.
Of the many varieties of cold forging equipment, the most common types are hydraulic and pneumatic, because they take up less room, can supply varied amounts of force, and are programmable. When you are examining your choices for production, it is best to know the types of equipment a producer has since up to date equipment is more likely to produce better quality parts.
Cold forging offers a wide array of choices when you are choosing a metal for a project. The different varieties include hard metals such as carbon steel, alloy steel, and stainless steel. Aluminum, brass, copper, silicon, and magnesium are soft metals that can be used. The one requirement for any metal is that it has a hardness of 44 HRC or lower on the Rockwell scale.
Copper is an excellent metal for cold forging since it is very ductile and malleable. It can be shaped, bent, or pulled with little force and produces parts that are corrosion and rust resistant.
Aluminum is a non-ferrous metal that is extremely light with a low density. It has a melting temperature of 1220° F and is malleable as well as rust and corrosion resistant.
Carbon steel is an alloy of iron and carbon. The various grades depend on the amount of carbon that is mixed with iron. It has exceptional strength and ductility.
Stainless steel has become one of the most used metals for its corrosion resistance, appearance, and strength. Though the term stainless steel is generally used to describe any steel that has its characteristics, stainless steel comes in a variety of grades depending on its alloy content.
Low carbon steels with a carbon content of 0.1% to 0.25%. Cold forging improves strain hardening in steel removing the need for austenitization, quenching, or annealing.
|Suitable Alloy||Cold Formability Characteristic|
|Gold, Silver and most of their Alloys||Excellent|
|Brass- Cartridge Brass||Good|
|Platinum, palladium, tantalum, and their alloys||Most are cold formable.|
|Titanium and its alloys||Pure Ti and alloys with high ductility, yes, but alloys like 6-4 are only hot head able.|
|Nickel and its alloys||Pure Ni yes, alloys with room temperature elongation of 20% or more, yes.|
|Iron and steels||Pure iron, yes. Steels, depends on the steel. Many are cold formable.|
The most logical and important thing you can do when you are deciding on a metal for a project is to do research. Each type of metal reacts to plasticizing and deforming in different ways. The metal you choose has to fit its final use and the strength that you require. If you consult with a forging expert, you can gain valuable information that can guide you to making the right choice.
Cold forging produces shapes of any size with a high degree of dimensional accuracy and structural exactness. The economic efficiency and speed of cold forging has made it the most popular production alternative.
The high strength, reliability, quality, and affordable pricing of cold forging has made it very attractive to automobile manufacturing. Cold forged parts are installed at high stress points because of their excellent shock resistance. Components such as drive trains, drive shafts, and struts or shocks are cold forged. The diagram below is a display of some of the parts of the undercarriage of a car that are produced by cold forging.
Connectors such as nails, bolts, rivets, and nuts have been produced by cold forging for years. The low tolerances and excellent dimensional accuracy is why producers of hand tools prefer cold forging over other methods such as machining.
The military has very strict stipulations regarding military hardware and choose cold forging for the manufacture of shell casings, bullets, and other military hardware. Parts have high reliability and performance during times of crisis. The low tolerances and strength of parts makes them perfect for military weapons.
Cold forging is used for gear production because it eliminates the need of cutting in gear shaping. Gears can be produced from billets that are less than 50 mm or shaped using coiled wire. Some annealing may be necessary to remove residual stress and work hardening. An important benefit of cold forged gears is how smoothly and quietly they intermesh.
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