Cold forging is an important process used to shape and strengthen metals. In order to best understand how it works, it’s important to first understand what forging is, and what each type of forging offers.
What is Forging?
Forging is a manufacturing process through which a solid metal workpiece is deformed and then re-shaped using compression. Unlike other methods of shaping metal, forging gives the creator more control over the final result because the grain of the metal deforms to follow the new shape. This means that the forger can decide which parts of the new metal object will be the strongest. As a result, a forged piece is stronger than the same piece created through casting or machining.
Different tools are used to accomplish forging, including the more traditional hammer and anvil, as well as the industrial use of hammers powered by electricity, steam, or hydraulics. Today, forging is largely done by machines at the industrial level and is a worldwide industry.
Forging is done either, ‘hot,’ ‘warm,’ or ‘cold.’ No matter the temperature, the method and machines used can be classified as one of the following:
- Drop Forging: Use of forging hammers and screw presses
- Pressure Forging (rotational motion): Use of hydraulic and mechanical machines
- Pressure Forging (translational motion): Use of rolling mills
- Pressure Forging (a combination of translational and rotational motion): Flospinning and orbital forging
What is Cold Forging?
Also called cold forming, cold forging is a process that takes place near room temperature, rather than at higher temperatures like warm and hot forging. It’s done by placing the workpiece in-between two dies, and pounding the dies until the metal assumes their shape. Because of the friction created by the process, the temperature of the metal being forged can actually rise to 250° C or 482° F. Cold forging has its advantages and disadvantages.
Advantages of Cold Forging
Because this process takes place without heating the metal beforehand, it provides many advantages and is often used in the automotive industry. For example, it’s often more economical than other processes, as the final product doesn’t require much finishing if any at all. There’s also little to no excess material left afterward, as the net weight of the original metal is about the same as the weight of the finished product. In addition to having no excess material, the dies used in cold forging last longer than in hotter processes, meaning fewer replacements.
Another advantage is that although the ductility of the metal decreases during the cold forging process, the metal sees a great increase in both yield and tensile strength. Because it’s not heated, the grain of the metal retains its size and changes direction to match the change in the shape of the piece itself, resulting in this increased strength.
Disadvantages of Cold Forging
The main disadvantage of cold forging is that it can’t be used for every metal because certain types are much more likely to crack or break during the process. For example, while some steel can be forged at room temperature, steel types with a carbon content of .5% or above cannot.
Another disadvantage is that cold forging can only create certain shapes. Those shapes are usually more basic and mass-produced. If you’re looking for a custom metal piece, a different process will probably work better for your desired outcome.
How is it Different From Hot Forging?
The most obvious difference between cold and hot forging is the temperature. Unlike cold forging that starts at or near room temperature, hot forging happens when metal is heated above its recrystallization temperature. This is usually an extremely high temperature, for example, steel is heated to 1150°C or 2202 °F, and copper alloys to 700-800°C or 1300-1470°F, which equals about 75% of the melting temperature. These high temperatures are necessary to keep the metal from strain hardening during the forging process. At these temperatures, the metal is in a plastic-like state, still technically a solid, but much more malleable.
Hot forging is usually done through a process called stamping, which means the heated metal is put into a press, and then squeezed in between a die and a tool.
Advantages of Hot Forging
Because the forging process occurs at high temperatures, the plastic metal is ductile and easily molded. Intricate shapes and designs can be created during hot forging, unlike cold forging where this can be extremely difficult. If you’re looking to manufacture customized metal pieces, hot forging is the best option because of its malleable state before hardening. This method is ideal for creating 3-D and complex shapes.
Unlike cold forging, which sometimes requires the extra step of heating the outside of the metal afterward to give it the right properties, the surface of the metal that has been forged through high temperatures is ideal for most types of finishing work.
Hot forging is also ideal for metals with a high formability ratio, because the integrity of the metal isn’t compromised by the high heat. These metals can be easily shaped without seeing any defects in the metal, making them both strong and ductile.
Disadvantages of Hot Forging
A disadvantage of hot forging, when compared to cold forging, is that some metals can be warped if not carefully watched when heated, forged, and then cooled. This means less precise tolerances than metal forged through cold forming.
Hot forging is also often more expensive than cold forging because of the heat treatment necessary to start the forging process, as well as the cooling process that prevents warping. This heat treatment is best done when automated, especially on the industrial scale, which may result in additional up-front costs to purchase tools.
Ultimately, the process you choose is dependent on the product you need and the type of metal you’ll be using. Both forging methods can be effective ways to strengthen and form your metal workpieces into the final product you need.