Bolts are one of the most basic components in engineering and construction, but their production has become an advanced, high-tech process involving multiple steps. Here’s how raw steel is transformed into a high-precision piece of metal.
Bolt Sizes
Bolts come in many different sizes and shapes, but the basic production process is very similar. The wire is first cold-forged into the right shape, then heat-treated for strength, surface-treated for durability, and finally packaged for shipment. However, for more advanced bolt designs, the production process may include additional steps.
As one of the leading suppliers of fasteners to the automotive industry, Swedish manufacturer Bulten knows every step and every aspect of bolt production. “We don’t produce catalog parts—everything we produce is tailor-made to the customer’s specifications,” says Henrik Oscarson, technical manager at Bulten’s Hallstahammar plant in Sweden. “There are many different ways to produce the right bolt, depending on what the fastener is going to be used for.”
Before cold forging
The large steel wire needs to be uncoiled and cut to length. Steel grades are standardized across the industry according to ISO 898‑1. Afterwards, the wire is cold-forged into the correct shape using special tools. The steel is essentially stamped into shape in a die at room temperature under high pressure through a series of dies. The die itself can be quite complex, containing up to two hundred different parts with tolerances of just a few hundredths of a millimeter. Once perfected, cold forging ensures that bolts can be produced quickly, in large quantities, and with high uniformity.
For complex bolt designs that cannot be formed by cold forging alone, some additional turning or drilling operations may be required. Turning is the process of cutting the steel to achieve the desired shape and design by spinning the bolt at high speed. Drilling is the process of punching holes into the bolt. Some bolts may also have washers added to them at this stage if the design requires it.
Heat Treatment
Heat treatment is a standard process for all bolts, where the bolt is subjected to high temperatures to harden the steel. Prior to heat treatment, when the steel is soft, the threads are usually formed by rolling or cutting (see the sidebar article comparing the two methods). Rolling is similar to cold forging, in that the steel is formed and threads are formed by rolling through a die, while cutting forms threads by cutting and skiving.
Since heat treatment changes the properties of the steel and makes it harder, it is easier and more cost-effective to pre-thread. However, threading after heat treatment means better fatigue resistance.
Henrik Oscarson explains: “Heat treatment can cause heat marks and slight damage to the bolts. For this reason, some customers ask for threading after heat treatment, especially for applications such as engine and cylinder head bolts. This process is more expensive because a harder steel needs to be formed, but the threads retain their shape better.”
For long bolts, where the length is more than ten times the diameter, heat treatment will return the steel to the round shape of the original wire. Therefore, a straightening process is often required.
The surface treatment used depends on the bolt application and customer requirements. Usually, the main concern for fasteners is corrosion resistance, so adding a zinc coating through electrolytic treatment is a common solution. In this step, the bolt is immersed in a zinc-containing liquid and then an electric current is passed to form the zinc coating on the bolt. However, electrolytic treatment increases the risk of hydrogen embrittlement. Another option is zinc flakes, which can make the bolts more corrosion-resistant, but are also more expensive.
Corrosion Resistance
If corrosion resistance is not a concern, such as in an engine or in an application where oil is frequently in contact, using phosphates is a more cost-effective option. After surface treatment, standard bolts are usually ready for packaging. However, more advanced designs may require some additional assembly, such as brackets. Other bolts will also require some form of patch, such as a locking patch or a liquid patch. A locking patch is a thick layer of nylon added to the threads to improve the grip of the bolt, while a liquid patch helps increase the torque of the rolled threads.
Once these steps are completed, the bolt is ready. Now it’s just a few quality control measures to ensure the uniformity and consistency of the bolts before they can be packaged for shipment.
Production Process
Packaging/Storage
After quality control to ensure uniformity and consistency, the bolts are packaged.
Wire
Uncoiled, straightened and cut to length.
Cold Forging
The steel is cast into the desired shape at room temperature.
Bolt Head
The steel is pressed into different dies under high pressure and gradually shaped.
Threading
The threads are formed by rolling or cutting. See sidebar article.
Heat Treatment
The bolts are put into high temperatures to harden the steel.
Surface Treatment
Depends on the application. Galvanizing is a common method to improve corrosion resistance.
