Bolts can come in a variety of different sizes and shapes, but the basic production process usually remains the same. The cold-forged steel wire is first made into the correct shape, then heat treated to increase strength and surface treatment to improve durability, before being shipped. However, for more advanced bolt designs, the production process can be expanded with many additional steps. Cold heading STARTS with large steel wires, which are unwound and cut to a certain length. According to the requirements of ISO 898-1, the grades of steel are standardized throughout the industry. Using special tools, the wire is then cold forged into the correct shape. This is basically where the steel is molded, while at room temperature, by forcing it through a series of molds under high pressure. The tool itself can be very complex, containing up to 200 different parts with tolerances of one hundredth of a millimeter. Once perfected, cold forging ensures bolts are produced quickly, in large quantities, and with high uniformity. For more complex bolt designs, contouring cannot be formed by cold forging alone and some additional turning or drilling may be required. Turning involves rotating bolts at high speed while cutting steel to obtain the desired shape and design. Drilling holes can be used to drill holes in the bolts. If desired, some bolts may also have washers attached at this stage of the process.
Heat treatment is a standard process for all bolts, which involves exposing bolts to extreme temperatures in order to harden steel. Threads are usually applied before heat treatment, by rolling or cutting, when the steel is softer. Rolling works in a similar way to cold forging, and requires threading bolts through a mold to form and thread the steel mold. Cutting involves the formation of threads by cutting and removing steel. Since heat treatment changes the properties of the steel to make it harder, it is easier and more cost-effective to pre-apply threads. However, thread processing after heat treatment means better fatigue properties. "Heat treatment can cause heat marks and minor damage to bolts," so some customers require threading after heat treatment, especially for applications such as engine and cylinder head bolts. This is a more expensive process because you need to form hardened steel, but the threads will hold their shape better. For long bolts that are more than ten times the diameter of the bolt, heat treatment can restore the steel to the circular shape of the original steel wire. Therefore, it is usually necessary to apply the straightening process.
The choice of surface treatment depends on the application of the bolt and the requirements of the customer. Usually, the main problem with fasteners is corrosion resistance, so galvanized coating applied by electrolytic treatment is a common solution. This is a process in which the bolt is immersed in a zinc-containing liquid and an electric current is applied to cause zinc to form a coating on the bolt. However, electrolytic treatment does increase the risk of hydrogen embrittlement. Another option is zinc sheets, which offer higher corrosion resistance, although at a higher price. When corrosion resistance is not an issue - such as inside an engine or in applications where oil is frequently exposed - using phosphate is a more cost-effective option. Once the surface treatment has been applied, standard bolts can usually be prepared. However, more advanced designs may require some additional assembly, such as brackets. Other bolts also need some form of patching, whether it's a locking patch or a liquid patch. The locking patch consists of a thick nylon layer on the thread that helps to improve grip. Liquid patches help to improve thread forming torque. Once these steps are complete, the bolt is complete. Now all that remains is some form of quality control to ensure consistency and consistency before the bolts can be packed and shipped.
