Definition of a bolt
A bolt is an externally threaded, headed mechanical device that is inserted through holes in constructed pieces to match with a nut. The nut is typically turned to tighten or loosen the bolt. This design creates a non-permanent (detachable) connection.
Materials used in bolt production
- Carbon and Alloy Steels
- Stainless Steels: Martensitic, ferritic, austenitic.
- Nonferrous Alloys: Copper alloys, silicon bronze, cupro-nickelb brass alloys, nickel alloys, aluminum alloys.
- Nonmetallic Materials: Nylon, vinyl, polycarbonate, acetals, polyethylene, polystyrene, fluorocarbons (TFE), rigid PVC.
Manufacturing Processes of Bolts
1. Raw Material Selection and Wire Preparation:
Bolt production begins with selecting the appropriate raw material, typically carbon steel, alloy steel, stainless steel, or non-ferrous alloys depending on mechanical and corrosion requirements. The selected material is supplied in wire rod or bar form.
Before forming, wire rods undergo descaling, pickling, and lubrication to remove mill scale and ensure smooth metal flow during heading operations. The wire is then drawn to the required diameter with precise tolerances to match the bolt size being produced.
2. Cutting the Wire to Length:
After preparation, the wire is fed into a cutting machine that shears it into individual blanks. Each blank length corresponds to the bolt’s intended length plus allowances for head formation.
The cutting operation must ensure accurate length consistency to maintain dimensional quality during subsequent forming steps.
3. Heading: Cold Forging or Hot Forging
3.1. Cold Forging:
Cold heading is a metal-forming process in which material is forced to flow at ambient temperature into dies, creating thicker sections and complex shapes. This method is carried out on specialized heading machines where wire or bar stock can be upset to a larger diameter in specific areas or, if required, extruded to a smaller diameter than the original wire. Although originally developed for manufacturing bolts, screws, and rivets—and still primarily used for these products—the technique is also suitable for producing many other components with comparable geometries.
Because cold heading requires extremely high forming pressures and consequently heavy-duty equipment, it is typically limited to producing small- and medium-sized parts.
Any metal that retains adequate malleability at room temperature can generally be processed through cold heading. Common materials include carbon and alloy steels, stainless steels, copper and copper alloys, aluminum and aluminum alloys, Monel, and several others, offering a broad selection to meet diverse application requirements. A moderate level of work hardening in the stock material is actually beneficial, as it helps prevent buckling or folding during deformation. However, materials that are too hard may shorten die life or cause shearing and cracking due to internal defects—issues that are less likely with softer materials. Specialty materials such as free-machining screw stocks containing high sulfur levels are typically avoided in cold heading, whereas similar grades without free-cutting additives provide better performance and reliability.
Cold forging method is highly efficient, producing little to no scrap. In many cases, material waste is virtually eliminated compared with machining processes that remove large amounts of metal from solid stock. Additionally, the high production speeds achievable with modern cold heading equipment make this method ideal for mass production, delivering significant reductions in both cost and manufacturing time.
3.2. Hot Forging:
Hot heading is a forming process in which metal is heated and then forced to flow into dies, creating shapes with cross-sectional areas larger than the starting material. While commonly used in fastener production, this method is also suitable for manufacturing a wide range of components with comparable geometries. Any part that can be produced through cold heading can also be manufactured through hot heading; however, the opposite is not always possible. Since metal becomes more formable at elevated temperatures, significantly larger upset volumes can be achieved when metal is hot. Very large upsets or short-length deformations, which may be impractical on conventional headers, can be efficiently produced using vertical upset presses.
During hot forging, the heated metal is soft enough to be directly formed into hexagonal or square head shapes, eliminating the need for separate trimming steps. Nevertheless, a small flash typically forms at the base of the head where excess metal escapes into the clearance between the die and hammer. This flash must be removed in a subsequent shearing operation. In general, cold heading provides tighter dimensional tolerances and smoother surface finishes—particularly on the head—than hot heading. However, additional finishing processes after heating can achieve high levels of precision and surface quality when required.
4. Threading:
4.1. Roll Threading:
Roll threading is a prevalent manufacturing technique where threads are formed on a slightly smaller blank, usually having a diameter close to the final thread pitch diameter. The threads produced by this method generally exhibit superior strength compared to those made by cutting or grinding. During the roll forming process, the material’s grain boundaries flow axially, following the specific contour of the thread, which contributes to increased strength over cut or ground threads. While the surface finish of a roll-formed thread is better than that achieved by cut threading, it is not as fine as a ground thread. However, imperfections in the thread flanks, roots, or crests can occur if the roll thread dies are misaligned or of poor quality. Threading critical, high-strength components after they have undergone the hardening process can improve their thread fatigue strength, although this practice significantly raises the manufacturing expense.
Cut threading is among the oldest techniques employed for thread production. This method involves either using thread cutting chaser sets or employing a turning operation, typically on a lathe with a cutting tool. This approach is considered the least desirable method in terms of both the final thread finish quality and the achieved thread strength. Cut threads are commonly found on fasteners produced in small batches. The threads are produced on blanks that have the full body diameter, meaning the thread’s major diameter is identical to the full body diameter of the blank.
4.3. Ground Threading:
The ground threading method yields a thread of exceptional quality, excelling in three areas: dimensional control, thread strength, and surface finish. This process typically involves creating the thread on fastener blanks that have been previously hardened and contour-ground. Due to the precision and materials involved, ground threading is the most costly of the three techniques, surpassing both roll forming and cut threading in expense.
5. Heat Treatment:
Heat treatment is a critical manufacturing process used to significantly alter the mechanical and physical properties of bolts, primarily focusing on increasing their strength, hardness, and durability. This process is essential for achieving the specific Property Class required for the fastener’s intended application.
The standard heat treatment cycle for medium-carbon steel bolts typically involves three main sequential stages:
5.1. Hardening:
- Austenitizing: The bolt material is heated to a high temperature (above the critical point) and held there for a specific duration. This allows the internal crystal structure of the steel (ferrite and pearlite) to transform entirely into a solid solution known as austenite.
- Quenching: The bolts are then rapidly cooled by immersing them in a medium such as oil, water, or a polymer solution. This rapid cooling locks the carbon atoms within the iron crystal lattice, transforming the ductile austenite into a very hard and brittle phase called martensite.
Tempering is essential to make the hardened bolt usable by reducing brittleness and adjusting strength to the desired Property Class.
- Process: Martensitic bolts are reheated to a temperature below the critical point (often between 300°C and 650°C) and held for a set time, followed by cooling.
- Result: This process precipitates tiny iron carbide particles, transforming the brittle martensite into a structure that has a beneficial balance of high strength and sufficient ductility/toughness.
5.3. Stress Relieving:
For bolts that have undergone cold deformation (like roll threading), or for materials prone to hydrogen embrittlement (especially after electroplating), a lower-temperature stress-relieving bake may be performed. This minimizes internal stresses and the risk of delayed failure.
6. Surface Treatment:
Surface treatment (also known as surface finish or coating) is a crucial step in bolt manufacturing, applied after forming and heat treatment, to provide corrosion resistance, control the friction coefficient (critical for tightening accuracy), and improve aesthetic appeal.