Bolts are one of the most basic components in engineering and construction, but the production of bolts has become an advanced high-tech program with multiple steps. Let us understand how raw steel becomes a high-precision metal appliance.
Bolts come in many different sizes and shapes, but the basic production process is much the same. The steel wire is first cold-forged into a suitable shape, then the strength is increased by heat treatment, and the durability is improved by surface treatment, and finally packaged and shipped. However, for more advanced bolt designs, the production process may add some extra points.
As one of the leading suppliers of fasteners for the automotive industry, Swedish manufacturer Bulten knows every step and every aspect of bolt production. Henrik Oscarson, technical manager at Bulten's Hallstahammar plant in Sweden, said: "We don't produce catalog parts - everything we produce is tailored to the customer's requirements. There are many different options for producing the right bolts, depending on the fastening. Where is the piece used?"
Before cold forging, the large steel wire needs to be unrolled and cut to a fixed length. According to the requirements of ISO 898‐1, the steel grades of the entire industry are standardized. After that, use a special tool to cold-forge the wire into the correct shape. The steel is substantially stamped and formed in a mold through a series of stampers at room temperature and high pressure. The mold itself can be quite complex, containing up to two hundred different parts with tolerances of only a few hundredths of a millimeter. Once perfect, cold forging ensures fast, high volume, high uniformity in the production of bolts.
For complex bolt designs that cannot be formed by cold forging only, some additional turning or drilling operations may be required. Turning uses high-speed rotating bolts to cut steel to achieve the desired shape and design. Drilling is to punch holes in the bolts. Some bolts may also be fitted with gaskets at this stage, as required by the design.
Heat treatment is the standard procedure for all bolts, and the bolts are placed at high temperatures to harden the steel. Prior to heat treatment, when the steel is soft, the threads are usually machined by rolling or cutting (see sidebar article comparing the two methods). The rolling operation is similar to cold forging, in that the steel is formed by rolling the mold and forming a thread, and the cutting is performed by cutting and stripping to form a thread.
Pre-machining threads is easier and more cost-effective because heat treatment changes the properties of the steel and makes it harder. However, processing the threads after heat treatment means better fatigue resistance.
Henrik Oscarson explained: "The heat treatment may cause hot marks and minor damage to the bolts. For this reason, some customers will require threading after heat treatment, especially for engines and cylinder head bolts. Because of the need to shape harder steel, So the cost of this process is higher, but the thread can better maintain the shape."
For long bolts with bolt lengths exceeding ten times the diameter, heat treatment will return the steel to the original steel wire. Therefore, it is often necessary to carry out the straightening process.
Which surface treatment is used depends on the bolt application and customer requirements. Often, the primary focus of fasteners is corrosion resistance, so adding a galvanized coating by electrolytic treatment is a common solution. In this step, the bolts are immersed in a zinc-containing liquid and then energized to form a coating on the bolts. However, electrolytic treatment increases the risk of hydrogen embrittlement. Another option is the zinc sheet, which gives the bolt a higher corrosion resistance, but at a higher price.
Phosphate is a more cost-effective option if you don't have to consider corrosion resistance—for example, in engines or in applications that often come into contact with oil. After surface treatment, standard bolts usually enter the packaging stage. 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. The locking patch is a thick nylon layer on the thread to increase the grip of the bolt, while the liquid patch helps to increase the torque of the rolling thread.
After completing these steps, the bolts are done. There are only a few quality control measures left to ensure uniformity and consistency of the bolts before they can be packaged for shipment.
production process
Steel wire
Unwind, straighten and cut to a fixed length.
Cold forging
The steel is cast into a suitable shape at room temperature.
3. Bolt head
The steel is pressed into different stampers under high pressure and gradually formed.
4. Thread
The threads are formed by rolling or cutting. See the sidebar article.
5. Heat treatment
The bolt is placed in a high temperature to harden the steel.
6. Surface treatment
It depends on the application. Zinc plating is a common method to improve corrosion resistance.
7. Packaging / Storage
After quality control to ensure uniformity and consistency, pack the bolts.
